Question about ignition switch"start" and "run" outputs

[Big_John]

We are not in disagreement. The ECU should be wired into the RUN side of the BR (blue wire). I wrote: "From the OPs description, he has correctly wired his ECU to the RUN side of the BR, but he says that during cranking his ECU is not getting enough voltage to operate."

The RUN wire is not powered from the ignition switch during cranking, only the START wire is powered. Therefore during cranking the ECU will be powered through the BR.

OK, I'm following you now.

 

[Big_John]

Now that I give this more thought... And that is sometimes really scary... You twist the key to start the car and what happens? The radio and heater fan shut off. Release it to "run" position and they come back on.

Proof right there that start side of the ignition switch doesn't supply current to anything but the "start" wire.

 

[Mike66Chryslers]

Now that I give this more thought... And that is sometimes really scary... You twist the key to start the car and what happens? The radio and heater fan shut off. Release it to "run" position and they come back on.

Proof right there that start side of the ignition switch doesn't supply current to anything but the "start" wire.

From my own observations one one of my Chryslers, I am sure that there is no voltage from the ignition switch to the RUN wire during START. However, your radio etc. observations are inconclusive. They are on a separate circuit from the ignition switch which is on for RUN and ACC but not START. The RUN wire to the BR is not powered when the key is in ACC. That's good because you really don't want your ignition system to be live when you're sitting listening to the radio with the engine off.

 

[Big_John]

From my own observations one one of my Chryslers, I am sure that there is no voltage from the ignition switch to the RUN wire during START. However, your radio etc. observations are inconclusive. They are on a separate circuit from the ignition switch which is on for RUN and ACC but not START. The RUN wire to the BR is not powered when the key is in ACC. That's good because you really don't want your ignition system to be live when you're sitting listening to the radio with the engine off.

Good point. I knew I shouldn't have done any thinking on this.

 

[WissaMan]

I have to wonder, why wouldn't the ECU get connected to the START side of the BR? That way it would get direct power when cranking, then get power through the BR once the engine is running and the alternator is providing charge to the system.

 

[Mike66Chryslers]

I have to wonder, why wouldn't the ECU get connected to the START side of the BR? That way it would get direct power when cranking, then get power through the BR once the engine is running and the alternator is providing charge to the system.

While running, the ignition coil will also be connected to the RUN side of the BR and it draws a lot of current, so the voltage at that node will be reduced. A typical coil for use with the Mopar ECU system will have ~1.5 ohms resistance through the primary winding. If the BR is also ~1.5 ohms, this forms a 50% voltage divider so the actual voltage available to the ECU would be half of the system voltage. At idle when the charging system isn't doing much, the ECU would only get 6V.

 

[WissaMan]

My brother pointed out to me that one thing we didn't do was check the pickup/reluctor gap on the distributor before installing it, so that is something we are going to check next time we get to work on the car. But could that possibly be causing this issue? Would low power to the ECU cause it to be more sensitive to there being too wide of a gap?

Regarding some of the other suggestions, we did remove the bulkhead connectors and sprayed them generously with deoxit (they were kind of cruddy) but that didn't make much, if any, difference.

I am tempted to just install 2 relays to feed 12v directly from the battery to the ECU (w/ a fusable link) and connect the coil from one to the Run and the other to the Start. I know I wouldn't be actually figuring out what the root issue is, but I'd feel better about the ECU not having to get power through the BR during cranking. Seems like that'd be a better setup in general.

As for just removing the BR only if the coil has internal resistance, this is the type of coil on the engine: ACCEL 8140C Ignition Coil - Chrome - 42000v 1.4 ohm primary - Points - good up to 6500 RPM

and these are the specs:
Ohms Secondary Resistance 9.2 k
Ohms Turns Ratio 94:1
Primary Resistance 1.4

I don't know if that's what you were referring to though by saying "internal resistance"

 

[Mike66Chryslers]

My brother pointed out to me that one thing we didn't do was check the pickup/reluctor gap on the distributor before installing it, so that is something we are going to check next time we get to work on the car. But could that possibly be causing this issue? Would low power to the ECU cause it to be more sensitive to there being too wide of a gap?

Regarding some of the other suggestions, we did remove the bulkhead connectors and sprayed them generously with deoxit (they were kind of cruddy) but that didn't make much, if any, difference.

I am tempted to just install 2 relays to feed 12v directly from the battery to the ECU (w/ a fusable link) and connect the coil from one to the Run and the other to the Start. I know I wouldn't be actually figuring out what the root issue is, but I'd feel better about the ECU not having to get power through the BR during cranking. Seems like that'd be a better setup in general.

As for just removing the BR only if the coil has internal resistance, this is the type of coil on the engine: ACCEL 8140C Ignition Coil - Chrome - 42000v 1.4 ohm primary - Points - good up to 6500 RPM

and these are the specs:
Ohms Secondary Resistance 9.2 k
Ohms Turns Ratio 94:1
Primary Resistance 1.4

I don't know if that's what you were referring to though by saying "internal resistance"

Some people install a coil like Pertronix Flamethrower #40501 or #40511, which has 3.0 ohms primary resistance, then get rid of the BR and short the two BR wires together. I don't recommend it. This is a separate discussion and I did a big explanation why in a thread here before.

 

[WissaMan]

We're going to try and get some time on it this weekend so I'll let everyone know how we make out

 

[WissaMan]

We checked the gap between the reluctor and pickup and it was much wider than the recommended .008" so we adjusted that and set base timing to 12.5º. Car started good, but it was in the 40's when we did this so we let the car sit for a week and tried it yesterday when the temperature was in the 20's and it started right up. So while the things I posted regarding the voltages are still as they were, it seems likely that the wide pickup/reluctor gap was the reason it wasn't starting while cranking.

Blame that one on me for not reading the install instructions close enough. Lesson learned (hopefully for the last time)

Thanks to everyone though who took the time to try and help me troubleshoot...

 

[WissaMan]

Oh, and I'll mention that the Chrysler master tech videos on Youtube are very helpful while at the same time entertaining. Most folks here probably already know about them but for those that don't, check them out