Mopar electrical weirdness. Temp gauge, Gas gauge, lights

[Dsertdog]

My sweet little '66 Polara ragtop has what I consider a typical Mopar electrical system.

When you turn on the lights, the temp gauge goes up (goes back down when you turn them off), and the fuel gauge goes up.

My sense tells me this is a grounding issue but where would one start? I have an FSM but the issue isn't listed.

Temp gauge doesn't fluctuate with the fuel level gauge and vice versa

The gas tank and sender are new, and the temp sender tests good.

I'm running the alternator bypass wire to keep from frying the bulkhead connector.

 

[HOT FURY]

I would check all of your connections at your bulkhead and everywhere else to make sure they're clean/corrosion free then add a light coat of dielectric grease to all. Then check and possibly add some grounds. Battery should go to your engine block and I added one from my block to chassis and frame. Make sure these are clean with good contacts and a thin layer of dielectric grease helps there too against corrosion or rust forming that would cause a bad connection later.

 

[3175375]

Instrument voltage regulator…

 

[Dsertdog]

I've basically bypassed the bulkhead with the jumper from the alternator to the starter relay.

The negative cable runs to the intake manifold bolt (stock position) and it's clean. I'm going to add an extra ground to that and try to find the other chassis grounds for the lights and clean them.

Troubleshooting the problem with the FSM, it suggested that one gauge or the other can be the cause of a dash voltage regulator failure but not both.

 

[Dsertdog]

Update:
I fabricated new ground wires for the engine to firewall and negative cable and cleaned a ground cable I found in the headlight wiring
harness. No change.

Next, I conducted an in-vehicle test of the voltage limiter as found in the FSM using the temp sender. The VOM indicated fluctuating voltage which the
FSM says is correct. That should eliminate the limiter.

 

[Gerald Morris]

Instrument voltage regulator…

Very likely. Damn things get old, short out, and the Fun starts. A solid state one or any other 5V DC converter of sufficient ampacity would improve things. Suffice to say, the OP should check that bulkhead connector first, and possibly the circuit board on the instrument panel as well. THAT can also lose metal to oxidation. If it happens, copper foil tape can save one much $$ and grief.

 

[Dsertdog]

Very likely. Damn things get old, short out, and the Fun starts. A solid state one or any other 5V DC converter of sufficient ampacity would improve things. Suffice to say, the OP should check that bulkhead connector first, and possibly the circuit board on the instrument panel as well. THAT can also lose metal to oxidation. If it happens, copper foil tape can save one much $$ and grief.

I tested the limiter in the car per the FSM and it passed.

The limiter isn't a plug and play on a 1966, it's part of the temp gauge.

I have thought about the circuit board and corrosion, although I'd attempted to clean all that when I first received the car.


I was hoping @Big_John might comment as well.

 

[Big_John]

I was hoping @Big_John might comment as well.

Sounds like it's broken.








Sorry... Had to.

I think it's a ground issue, but I'm not real sure where to start either. I doubt seriously if it's the voltage limiter as that does not tie into the lights in any manner.

What I think is happening is you don't have a ground for something, possibly dash lights (just spit balling that one) and it's getting ground by going through the gauges.

So, here's what I would do if it were my own car. First, I would disconnect the "bypass" as that's apparently new and test. Not saying that's the problem (probably not), just eliminating any changes first before getting serious. Then, I would take a piece of wire and attach one end to a good known ground point, like a battery terminal and start touching the other end to various points. I'd start with the dash itself then I'd try the front and rear light housings.

Questions... Does this happen with just headlights or does it do it the parking lights also? Have you changed any other wiring? Do the lights work? Do the dash lights work?

Ground problems are frustrating because they manifest themselves in odd ways. The thing to remember is that if you don't have a good ground, the system will find ground somewhere and it will be the path of least resistance. What you want to do is make sure that ground path is where it was designed to be.

 

[Gerald Morris]

I tested the limiter in the car per the FSM and it passed.

The limiter isn't a plug and play on a 1966, it's part of the temp gauge.

I have thought about the circuit board and corrosion, although I'd attempted to clean all that when I first received the car.


I was hoping @Big_John might comment as well.

Having a '66 which we drove 5.5 yrs, I'm sufficiently familiar with those for the '66, and the different one for my current '68. So long as it isn't introducing any random current to the LIGHT bus on your instrument cluster, well and good.

Those old boards certainly oxidize.

If, as seems likely, something isn't properly grounded, then the current introduced will seek path to ground through odd things like your instruments. Another possibility is the odd nicked conductor bleeding current in.

One thing to be said for LED bulbs, current will go only ONE WAY through them, thus eliminating backfeed from instrument lights IFF you have them in that board. They produce better light for much less current also.

Bench test the instrument board, then check the lights, after inspecting and cleaning the bulkhead connector. I repaired a broken conductive runner to a turn signal 2 yrs ago, to good effect on my then newly purchased '68.... When the blinker relay pulsed, it would pulse the fuel gauge, until I sent the current where it was supposed to go.

 

[Dsertdog]

@Big_John

I dunno about broken but that made me chuckle. Some would call it attitude or "character", and this car has that for sure.

I followed your suggestions.

All the lights and dash lights work. When I got the car, some of the dash lights were burnt out. I pulled the gauge pods and replaced all the bulbs. All the headlights were replaced, and I just tried to clean the terminals again.

Removing the bypass didn't change anything.

I unhooked the aftermarket tach wiring on a whim. No change. Gauges still move when lights are turned on.

The gauges move when the parking lights are turned on without the headlights. The parking light bulbs ground through the housing, right? I could see where rust could possibly create an issue

I'll have to make a longer test wire to check the rear housings. There is wiring from a trailer hitch I'll have to look at. It was neatly installed and even has little tags tied with string to the wiring with neatly written notes on them! The ground runs to one of the bolts on the trunk lid.

There sure is a lack of grounds in the wiring harness. I'm not used to that! Where do the dash lights ground? Is there a stud somewhere?


@Gerald Morris

Didn't know you'd owned a 66. I thought all 60's Mopars had plug in regulators until recently. I had a '69 Roadrunner with a damaged circuit board and had the same problem with a pulsing gas gauge when the turn signals were used.

A limiter didn't fix that issue either

 

[Big_John]

I dunno about broken but that made me chuckle. Some would call it attitude or "character", and this car has that for sure.

I followed your suggestions.

All the lights and dash lights work. When I got the car, some of the dash lights were burnt out. I pulled the gauge pods and replaced all the bulbs. All the headlights were replaced, and I just tried to clean the terminals again.

Removing the bypass didn't change anything.

I unhooked the aftermarket tach wiring on a whim. No change. Gauges still move when lights are turned on.

The gauges move when the parking lights are turned on without the headlights. The parking light bulbs ground through the housing, right? I could see where rust could possibly create an issue

I'll have to make a longer test wire to check the rear housings. There is wiring from a trailer hitch I'll have to look at. It was neatly installed and even has little tags tied with string to the wiring with neatly written notes on them! The ground runs to one of the bolts on the trunk lid.

There sure is a lack of grounds in the wiring harness. I'm not used to that! Where do the dash lights ground? Is there a stud somewhere?

"Sounds like it's broken" is a running joke with me. Long story....

I'm a bit at a loss here. It still sounds like a ground problem. It's hard to help from across the country and I haven't been in Oklahoma (Muskogee) since my father in law died 11 years ago, or I'd offer to look at it.

 

[Dsertdog]

"Sounds like it's broken" is a running joke with me. Long story....

I'm a bit at a loss here. It still sounds like a ground problem. It's hard to help from across the country and I haven't been in Oklahoma (Muskogee) since my father in law died 11 years ago, or I'd offer to look at it.

I appreciate that.

Silly questions.... I'm looking at the electrical schematic for the car in the FSM.

I don't see any grounds for the dash. How does that work?

Does everything ground to the instrument pods and touch the dash frame?

I see a ground for the cigar lighter. Actually, the outer housing of the lighter socket looks like it grounds to the dash?

Newer cars have all kinds of dedicated grounds, probably because of all the plastic. I'm sure that is why I'm having an issue.