Old ignition module - how to clean up, and will they still work (well) ?

[MoPar~Man]

Is there a lot of variability in these modules? I mean from an operational POV. As in, part numbers really do (or don't) matter ?

And how would you go about cleaning this up, and do they usually still work afterwards?

 

[CBODY67]

How intact is the potting compound on the backside of things? The potting compound is available, if needed.

There is a way to electrically test the components. Usually with a quality meter or a testing device.

Part numbers might not matter as much as suspected as each part number change could mean a different supplier to Chrysler for the part. Seems like there is a chart of part numbers, applications, and their differences?

I suspect that @halifaxhops can hare information in these areas.

Just some thoughts,
CBODY67

 

[halifaxhops]

They are all rated for different rpm ranges usually. Tons of testers out there for them Here is the one I use. Biggest thing is to match the componets properly. Just one example of a chart from MP

 

[MoPar~Man]

The potting is in great shape. The case is super-rusty, I assume there's supposed to be P/N stamped or painted on it but there's no way it survived. It does have the Chrysler sticker on it, I peel it back a little and it looks like the case was painted black. Maybe the backside can help ID the year and application. It does have a 5-pin connector.

 

[Toolmanmike]

The potting is in great shape. The case is super-rusty, I assume there's supposed to be P/N stamped or painted on it but there's no way it survived. It does have the Chrysler sticker on it, I peel it back a little and it looks like the case was painted black. Maybe the backside can help ID the year and application. It does have a 5-pin connector.

View attachment 730375

Clean it mechaniically (not chemically) like you would any other metal part. Sandpaper and or a wire brush. You probably won't be able to save the label. It may work it may not. If it does, it might last for a long time and may be it won't. If you do repaint it, make sure you use toothed washers or remove the paint around the bolt holes so you get a good ground. I would contact Halifaxhops and buy a known good one and keep this one as a spare if it works. Just in case. Good luck.

 

[Vaanth]

Is there a lot of variability in these modules? I mean from an operational POV. As in, part numbers really do (or don't) matter ?

And how would you go about cleaning this up, and do they usually still work afterwards?

View attachment 730320

The potting is in great shape. The case is super-rusty, I assume there's supposed to be P/N stamped or painted on it but there's no way it survived. It does have the Chrysler sticker on it, I peel it back a little and it looks like the case was painted black. Maybe the backside can help ID the year and application. It does have a 5-pin connector.

View attachment 730375

The pictured electronic control unit (ECU) appears to be Chrysler part number 3874020 or 3755550. Following is a summary of the Chrysler production ECUs:

Part Number	Usage	Pins	Notes
3438850	1971-1972	5	1971 - 340 manual transmission only. Black case, gold heat sink.
3656127	1972	5	400 & 440 HP manual transmission, engine speed limited. Black case, red heat sink.
3656128	1972	5	340 manual transmission, engine speed limited. Black case, blue heat sink.
3656900	1973	5	Improved cold starting. Black case with white dot, gold heat sink.
3755550	1974	5	Added radio suppression capacitor. Black case, gold heat sink.
3874020	1974 - 1979	5	Added voltage spike protection. Black case, gold heat sink.
4111850	1980 - 1991	4	Four pins, two terminal external ballast resistor. Black or blue case, gold heat sink.

The five pin units are essentially interchangeable for basic function. A four pin ECU can replace a five pin ECU, but a five pin ECU cannot directly replace a four pin ECU unless the associated ballast resistor wiring is backdated to the four terminal ballast resistor. Note that some aftermarket replacement four pin ECUs might have a "dummy" fifth pin.

The simplest, and best, test of the ECU is to put it on a good running vehicle and see if it works, and if it performs okay. Relatively full bench testing can be done with specific test equipment such as the NAPA Echlin 4643 Ignition Module Tester pictured in post #3. This is a NAPA branded version of the Zetron IMT-1000. This unit is handy for testing multiple brands and types of ignitions.

Chrysler provided their own tester from Miller Tools (now OTC) with the C-4166 and C-4166-A. The C-4166-A can test ECUs on the bench and on the vehicle in conjunction with the vehicle wiring, ballast resistor, and distributor pickup. The following image shows my own tester checking an ECU:

Chrysler also provided a more universal ignition tester, the 1-3500 Uni-Tester, to test their own and other brand ignition systems. The following image shows a Uni-Tester:

The only real bench test with a multimeter or ohm meter is checking pin 5 to case ground. The reading should be essentially zero ohms. The following image shows me checking pin 5 which, neglecting test leads resistance of ~0.2Ω, is close to 0.1Ω:

A bad reading indicates the ECU will not work. A good reading only indicates the internal ground is good. The unit might still not work.

Alternatively, without a specific tester, a more full bench test could be set up with a power supply, pulse or signal generator, and oscilloscope, fast capture volt meter or perhaps a frequency counter (with attenuator).

The ECUs can be cleaned and stripped and still work if the worked before, but the process can be tedious and tricky, and component damage is a strong possibility, especially if dealing with potting removal. Components can be replaced to an extent.

Your pictured unit is heavily rusted, but might be cleaned with careful wire brushing and hand brushing of the case, although making it look new will be a task. It could partially be cleaned first by masking the connector and transistor/heatsink, plus cover the back, and carefully bead blasting it. The heatsink is anodized aluminum and might be buffed clean if the anodizing is still intact and the aluminum is not corroding. If it is corroding, heavier brushing might be used, but care should be exercised around the insulating pad. The transistor case is nickel plated steel and it can be lightly hand brushed, but any remnants of marking will be gone, and due to its shape, full cleaning of corrosion will be tricky.

The following image shows a case that was rusted, though not as bad as yours, in the process of being cleaned. The rust removal so for has been with a wire brush by machine:

Evaporust can be used in areas that can't be reached with brushing or to get into pits. Some light chemical etching of the external part of case with phosphoric acid is possible. Don't use the phosphoric acid on the heatsink or transistor. The connector pins can be cleaned with a contact cleaner such as CRC or DeOxit brands. Lacquer thinner can be used for some clean up, especially for any potting on the outside.

Your potting does look in decent shape but it can be removed if ever needed. The following image shows some ECUs that I have removed the potting and will be re-potted when any component repair, further case work, and painting is complete:

The cases can be repainted after cleanup and new labels are available if desired. The following image shows a couple I have repainted, a P4120505 and an aftermarket version of 4111850. The paint is not the exact color, or sheen, but it's close enough for my use:

When repainting, don't paint the heatsink, transistor, or connector. I have used a light coat of VHT high temperature clear on the transistor cap to preserve the marking.

In general, for one as bad as the one pictured, I likely would not try too hard to refurbish it. A refurbished ECU might still fail from damage during the process of refurbishment, or just from age like any. Although I rarely have had an original ECU fail, I only use original Chrysler units or good aftermarket replacement units made over 25 years ago in the USA, when needed. I keep a few on hand:

 

[MoPar~Man]

@Vaanth that is a lot of great information, thanks for that. I have another ECU, from my '73 Satellite (which I disassembled and junked back in April) it looks exactly the same as the one I posted, same sticker, same amount of rust, same appearance on the back side, 5 pins also. I'm cleaning up the distributor from the Satellite, gonna jury rig it into my '67 Monaco, see if the 318 likes it.

I've looked at some video's of the various GM modules (4 pin through 7 pin). I will probably eventually go with one of those.

 

[Toolmanmike]

The pictured electronic control unit (ECU) appears to be Chrysler part number 3874020 or 3755550. Following is a summary of the Chrysler production ECUs:

Part Number	Usage	Pins	Notes
3438850	1971-1972	5	1971 - 340 manual transmission only. Black case, gold heat sink.
3656127	1972	5	400 & 440 HP manual transmission, engine speed limited. Black case, red heat sink.
3656128	1972	5	340 manual transmission, engine speed limited. Black case, blue heat sink.
3656900	1973	5	Improved cold starting. Black case with white dot, gold heat sink.
3755550	1974	5	Added radio suppression capacitor. Black case, gold heat sink.
3874020	1974 - 1979	5	Added voltage spike protection. Black case, gold heat sink.
4111850	1980 - 1991	4	Four pins, two terminal external ballast resistor. Black or blue case, gold heat sink.

The five pin units are essentially interchangeable for basic function. A four pin ECU can replace a five pin ECU, but a five pin ECU cannot directly replace a four pin ECU unless the associated ballast resistor wiring is backdated to the four terminal ballast resistor. Note that some aftermarket replacement four pin ECUs might have a "dummy" fifth pin.

The simplest, and best, test of the ECU is to put it on a good running vehicle and see if it works, and if it performs okay. Relatively full bench testing can be done with specific test equipment such as the NAPA Echlin 4643 Ignition Module Tester pictured in post #3. This is a NAPA branded version of the Zetron IMT-1000. This unit is handy for testing multiple brands and types of ignitions.

Chrysler provided their own tester from Miller Tools (now OTC) with the C-4166 and C-4166-A. The C-4166-A can test ECUs on the bench and on the vehicle in conjunction with the vehicle wiring, ballast resistor, and distributor pickup. The following image shows my own tester checking an ECU:
View attachment 730398

Chrysler also provided a more universal ignition tester, the 1-3500 Uni-Tester, to test their own and other brand ignition systems. The following image shows a Uni-Tester:
View attachment 730395

The only real bench test with a multimeter or ohm meter is checking pin 5 to case ground. The reading should be essentially zero ohms. The following image shows me checking pin 5 which, neglecting test leads resistance of ~0.2Ω, is close to 0.1Ω:
View attachment 730399
A bad reading indicates the ECU will not work. A good reading only indicates the internal ground is good. The unit might still not work.

Alternatively, without a specific tester, a more full bench test could be set up with a power supply, pulse or signal generator, and oscilloscope, fast capture volt meter or perhaps a frequency counter (with attenuator).

The ECUs can be cleaned and stripped and still work if the worked before, but the process can be tedious and tricky, and component damage is a strong possibility, especially if dealing with potting removal. Components can be replaced to an extent.

Your pictured unit is heavily rusted, but might be cleaned with careful wire brushing and hand brushing of the case, although making it look new will be a task. It could partially be cleaned first by masking the connector and transistor/heatsink, plus cover the back, and carefully bead blasting it. The heatsink is anodized aluminum and might be buffed clean if the anodizing is still intact and the aluminum is not corroding. If it is corroding, heavier brushing might be used, but care should be exercised around the insulating pad. The transistor case is nickel plated steel and it can be lightly hand brushed, but any remnants of marking will be gone, and due to its shape, full cleaning of corrosion will be tricky.

The following image shows a case that was rusted, though not as bad as yours, in the process of being cleaned. The rust removal so for has been with a wire brush by machine:
View attachment 730401

Evaporust can be used in areas that can't be reached with brushing or to get into pits. Some light chemical etching of the external part of case with phosphoric acid is possible. Don't use the phosphoric acid on the heatsink or transistor. The connector pins can be cleaned with a contact cleaner such as CRC or DeOxit brands. Lacquer thinner can be used for some clean up, especially for any potting on the outside.

Your potting does look in decent shape but it can be removed if ever needed. The following image shows some ECUs that I have removed the potting and will be re-potted when any component repair, further case work, and painting is complete:
View attachment 730400

The cases can be repainted after cleanup and new labels are available if desired. The following image shows a couple I have repainted, a P4120505 and an aftermarket version of 4111850. The paint is not the exact color, or sheen, but it's close enough for my use:
View attachment 730404
When repainting, don't paint the heatsink, transistor, or connector. I have used a light coat of VHT high temperature clear on the transistor cap to preserve the marking.

In general, for one as bad as the one pictured, I likely would not try too hard to refurbish it. A refurbished ECU might still fail from damage during the process of refurbishment, or just from age like any. Although I rarely have had an original ECU fail, I only use original Chrysler units or good aftermarket replacement units made over 25 years ago in the USA, when needed. I keep a few on hand:
View attachment 730402

That's a nice collection. I sold Filko brand at the parts store I managed so I am a fan of their quality. I bought one from Hoppy to make my 64 Polara start and run. No disappointment here.

 

[Hyper_pak]

You can clean them up, if it will work?
That is the 64 K Question!

 

[MoPar~Man]

I have cleaned (wire wheel) the ignition module case, and have taken apart, cleaned up, the electronic distributor salvaged from my '73 Satellite, along with it's intact wire harness and dual-resistor ballast, and have connected the harness run and start wires to the wires going to my existing ballast resistor. I connected the ECM case to ground using a wire bolted to the case (I haven't mounted the ECM).

And success! The engine started and ran without much trouble. I think it idles better vs the points. I was looking to see where I would mount the ECM and use the '73 wire harness and keep it intact. The harness includes the alternator wires (for a 2-wire alternator) and electronic voltage regulator, which I could use, but it's a matter of figuring out where to mount everything within the confines of the cable.

But all that would be plan B. Plan A would be to obtain a GM 7-wire ECM. These are similar to the 4-wire but has an output that I could use to drive a tachometer. These modules can be mounted to the underside of the distributor with an adapter plate.

 

[CBODY67]

The GM HEI module is not the GM ECM.

The GM HEI 4-terminal module is the basic one. The "more terminals" version is designed to use a spark knock sensor and a separate-related module to retard the spark timing when needed. In those engines, the detonation limiter sensor was screwed into the rh side block drain plug hole. One wire went to the sensor. The detonation limiter module was mounted behind the glove box on pickup truck vehicles. The light-duty pickups with that system had "Electronic Spark Control" badging on the tailgate.

The way to check the system operation is to take a wrench and tap the lh exhaust manifold with it. The timing will retard. On an engine with a knocking rod or main bearing, the engine will start and after the knocking starts, the spark retard will kill the engine. We had a few engines like that come through the shop back then, in the middle 1980s.

This detonation limiter system is a free-standing system from the normal GM Engine Control Module. Should be easy to replicate, provided a working detonation limited control module can be found in a salvage yard. It kind of surprised me that the hot rod market did not pickup on this system as it would have worked well with the decreasing fuel octane levels of back then. BTAIM

Enjoy!
CBODY67

 

[MoPar~Man]

I wouldn't do anything with the pins used to retard timing or sense knock. Just not connect anything to them. Maybe they need to be tied to ground or +5V, so ok I'd do that. One of the pins provides a buffered / rectified signal from the reluctor. I would use that as a tach input. I thought an HEI system included a (special) coil? Otherwise it's just an ECU (a box that switches current through your existing old-school coil).

 

[Big_John]

But all that would be plan B. Plan A would be to obtain a GM 7-wire ECM. These are similar to the 4-wire but has an output that I could use to drive a tachometer. These modules can be mounted to the underside of the distributor with an adapter plate.

Some reading on the HEI conversions. Note the link to another thread on the subject as they get pretty deep into it too. Links in the other threads too.

As with other forum conversations, take some things with a grain of salt and apply common sense as needed. You may need to join to see all the pics etc.

HEI Electronic Ignition Retrofit How-To - Slant Six Forum

 

[CBODY67]

I wouldn't do anything with the pins used to retard timing or sense knock. Just not connect anything to them. Maybe they need to be tied to ground or +5V, so ok I'd do that. One of the pins provides a buffered / rectified signal from the reluctor. I would use that as a tach input. I thought an HEI system included a (special) coil? Otherwise it's just an ECU (a box that switches current through your existing old-school coil).

The HEI coil is specific to that application, as it fits on top of the distributor cap, except on some inline 6-cyl applications where it is separate from the cap. Allegedly, one of the main design criteria was the capability to fire a .1000" gap, as many OEM Oldsmobile applications spec'd plug gaps of .080", which were later dropped to .060".

There is an episode on the YouTube channel, "Dead Dodge Garage" where he somewhat crudely replaced a Chrysler ign control module with a GM HEI V-8 module.

The SAME can be done with the Ford ign control modules, as long as you mount it AWAY from the engine, on a body part. A thread on that in the AACA Riviera Forum, a few years ago. That particular poster was getting his modules from the salvage yard and wiring them in in place of the normal ignition points. The same module that was so troublesome for Fords of an earlier decade, until Ford put out a TSB about isolating the module from engine heat, by locating it away from the engine. Being cooler, it worked fine.

The GM HEI detonation limiter circuit is a "ground circuit". Tach circuit probably is, too. Which can be indicated by their "one wire" hookups. Plus one power wire to the distributor mechanism.

Davis Unified Ignition, now DUI, got their start by using the 4-pin GM HEI module to replace the OEM ign modules on OEM distributors. Mounting the HEI modules on the outside of the OEM distributors. Looked kind of different, but worked well. There allegedly are some "magical powers" inside of the GM HEI module. Like variable dwell, and such, but I have never seen that mentioned in GM service literature. I just know they work well as long as they have the heat sink compound between them and the distributor housing they mount to, inside the cap.

Enjoy!
CBODY67

 

[MoPar~Man]

I don't know what Dan means by this:

"Your stock coil might work OK for awhile, but really isn't a good choice. At the very least, replace it with a coil meant for use with electronic ignition."

Dan's post was from 2006. Does he still stand by those words?

Replacing this:

With this (7-pin, or 4-pin varient) which I call an ECU:

Makes no difference to this guy:

But Dan seems to think there is a difference? Were the coils different between 1967 points setup vs 1973 Mopar ECU setup?

Using the 4-pin / 7-pin ECU's to drive an HEI coil is fine, of course. I likely will explore that with time for my 318.

 

[Toolmanmike]

Is there a lot of variability in these modules? I mean from an operational POV. As in, part numbers really do (or don't) matter ?

And how would you go about cleaning this up, and do they usually still work afterwards?

View attachment 730320

I looked at your original photo again. That's pretty crusty rusty and the heat sink is quite corroded. If you can remove the rust you will need some body putty or high build primer to make it smooth and look like new again.

 

[Big_John]

But Dan seems to think there is a difference? Were the coils different between 1967 points setup vs 1973 Mopar ECU setup?

I grabbed this from my '65 Chrysler FSM and '73 Dodge Chassis FSM. I used the 383 automatic from 1965 and the 400 automatic from 1973. I first thought about comparing specs, but check out the part numbers! I didn't think there was much difference, but was not expecting to see the same part numbers for the Prestolite coils over 8 years.

1965

1973

As I said, take everything in those threads with a grain of salt. I think there are some assumptions made here and there. It's worth reading the threads he attached and probably even farther down the rabbit hole with other attached threads.

I also think the thing to do might be to look at specs for the GM HEI coil if they are available. I went to the Standard site and was surprised and disappointed to not see specs, like primary resistance, listed for the recommended coils.

Dan is a member here, but I don't think he comes on here very often and hasn't been here in a while.

 

[CBODY67]

In the earlier days of electronic ignitions, like the late 1970s and such, there were some comments I saw which claimed the coils used had to be the ones from the conversion kit's manufacturer. Not unlike what Pertronix does in more recent times. Not sure if it was due to the internal resistance or related spark energy the customer might desire. OR if it was due to "warranty considerations" for the conversion kit.

I DO know that the GM HEI coil MUST be grounded to the distributor housing. There is a specific grounding strap in the parts book, which is either a formed steel strap or an insulated (black) wire that attaches to one corner of the metal housing and then attaches as something of a "hold-down" item (the metal strap). People who discarded that item usually ended up walking later on.

When the 1985 Camaro Z/28 L69 HO 305 appeared, it had a different coil in the HEI distributor. Allegedly for higher output as that engine was raced in SCCA and similar, back then. Different insulation colors on the two hook-up wires designated it from the normal V-8 HEI coil. Never did see any specs on it, though.

IF you read the advertising documents, almost ALL of the aftermarket electronic ign kits mentioned "hotter sparks" compared to the normal OEM systems of the time. That would also mean with their recommended ign coils, I suspect. All Chrysler was desiring to do was to have more consistent and consistently-spaced sparks rather than "spark plug frying" sparks. So no need for 50KV coils!

On the "50KV coil" issue, a Holley field engineer noted that the coil will only produce enough spark to fire the mixture, no more, no less. No matter the POTENTIAL spark it can produce. Which deflated any suspicions that with a 50KV coil, EVERY spark was 50KV. With the wider spark plug gaps, the coil would naturally have to build more voltage to jump the wider plug gap. From my experiences, a stock Chrysler coil can go up to about .045" gap. I had no issues with .040" gaps on my 383s, in the earlier 1970s.

In later times, after "hot sparks" seemed to fall from grace, most OEMs ended up with .060" gaps on Iridium plugs as combustion dynamics were better understood and finessed. Which ALSO relates to BORE SIZE!

Enjoy!
CBODY67

 

[Vaanth]

Were the coils different between 1967 points setup vs 1973 Mopar ECU setup?

The same type of coil, Chrysler part number 2495531 (Mopar #CH412) as used with the 1964 through 1972 contact points systems (except early 426 Hemi and Prestolite point triggered transistor ignition systems) continued to be used in production with the Chrysler electronic ignition system. 2495531 replaced the earlier 2084847 used from 1960 - mid 1963.

The 2495531 negative (-) terminal was relabeled from "DIST." to "ECU/DIST.". The 2495531 coil primary resistance at 70-80 degrees Fahrenheit measures 1.6 - 1.79 Ω for the Prestolite 2444242 type coil and 1.34 - 1.55 Ω for the Essex/Echlin 2444241 type coil. The interchangeable coil type reference numbers specify the coil manufacturer.

The 2495531 coil continued to be used through 1979. In 1980, with other revisions in the electronic ignition system, Chrysler released the 4176009 (Mopar #CH419) coil with a primary resistance of 2 Ω.

 

[MoPar~Man]

@Vaanth based on your write-up I'm not sure if the 1.8 ohm coils (that were the standard coil for points) was also the same coils for the early ECU setup.

I've seen an 8-page tech writeup by Chrysler where they explain the ECU and show the 3 ballast resistors used. Two of them were dual-resistor. Of those, the coil resistor was different. One was an exposed wire-wound with a value of 0.5 ohms. The other was potted with a value of 1.25 ohms. I'm curious why there were 2 types. The points setup used the single 0.5 ohm resistor I think. Either they went with the 1.25 ohm resistor to pair it with a different coil (with a resistance less than 1.8 ohms) or they wanted to limit the current through the 1.8 ohm coil (maybe for the benefit of the ECU).