Erratic Gauges

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carfreak6970

carfreak6970

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Hello Folks,

I have a funky gauge issue on a 70 charger that I am hoping to get some insight on.

So the car has brand new wiring and a new instrument cluster circuit board. All gauges worked as well as these factory gauges do but a couple months ago I removed the cluster to add in this gauge light surround to the lens to keep the dash lights from bleeding around the gauges and blinding you at night. In the process I completely disassembled the cluster to clean the housing and put in new blue light lenses. I reassembled the cluster and added in a ground wire that went from a screw on the wiper switch to a couple of the hold down screws that ground the circuit board to the cluster housing. that wire then went to one of the hold down screws for the tic toc tac. I then had a wire from from that same tic toc tac screw go to a screw on the underside of the dash.

When I installed the cluster the dash lights work, tac works, fuel gauge works but the oil and temp gauges continually bounce. According to the FSM it says erratic gauge behavior is due to lose or dirty connections. Well the sending units are new as is the wiring (maybe about 500 miles on everything at this point) so I dont believe dirt has anything to do with it. At first I thought it was the connector that plugged onto the prongs on the circuit board. I saw that the wires werent completely secured in that connector so I played with it a bit to make sure those connections were good and added dielectric grease. This did not change the gauge bounce situation. The connections at the bulk head connector did not move when I tried to push them in further.

I then ran a jumper wire from that wire that connected to the dash straight to the negative battery terminal and it changed nothing. I added a jumper wire from the ground hardness I made to the negative terminal on the battery, still no change. I added a jumper wire from the cluster housing itself to the negative battery terminal and still no change.

However, when I ground the sending unit wires to the body via a jumper cable the gauges go to full peg.

Any ideas on what this issue is?

I have spare gauges that I could wire up to see if it is indeed a gauge issue...

The only idea I have came up with for this issue is this:
The Fuel gauge is grounded through the sending unit, to the fuel line to the body to the battery. Where as the oil and temp gauges are grounded through the engine to the battery. Could it be possible that the negative battery cable has just enough resistance that it would give the erratic readings at the lower voltage of gauge reading? The negative battery cable is an original one, that was the only cable that was not replaced. I could test this theory by adding an additional wire from the engine to the battery. Does this sound too far off?

is there anything else I am missing?
 
Don't know but is the new light led? If so turn it around as it will only work one way (Diode is a one way check valve). Also maybe a ground , is there a ground from the Dash to the steering column ? Probably means nothing just suggestions.
 
No LED's in the dash at this time.

there is a ground from the dash to the steering column
 
carfreak6970 said:
No LED's in the dash at this time.

there is a ground from the dash to the steering column
Click to expand...

Just for kicks disconnect the ground wire from the wiper switch and see what happens.
A lot of the wiper switches are the wiper motor ground.
A bad ground between your wiper switch and dash metal may be trying to back feed power into your gauges.
Only thing I can think of off the top.
 
TrailBeast said:
Just for kicks disconnect the ground wire from the wiper switch and see what happens.
A lot of the wiper switches are the wiper motor ground.
A bad ground between your wiper switch and dash metal may be trying to back feed power into your gauges.
Only thing I can think of off the top.
Click to expand...

Thank you for that!! thats something I would not have thought of. Ill give that a go.

anything else to try?
 
I second the disconnect of the wiper ground, I had a similar experience with wiper ground.
 
The needles pegging when grounding sender wires is normal
If the battery ground had much resistance, the engine would not crank, and if the battery to body jumper had much resistance, the lights would be going nuts.

My guess is there's trouble right in the cluster, maybe the ground plane is not actually grounding for some reason, I don't know.

What I would do is this:

"Plan" on removing the cluster, but before you do, obtain test resistors--or use an old gas tank sender to mimic the other senders. Connect the test resisntance to each sender wire one at a time, test with key "in run" and see what happens.

The old mechanical voltage limiter DOES pulse, but the gauges are so slow to react you don't see them do so.

Does this happen only when running, or both runniing and not running, etc?

Test resistances

c-3826-jpg-jpg.jpg
 
The pulsing happens when the engine is running on them temp and oil. it happens to the temp gauge when the engine is off, dont know about the oil gauge due to no pressure when the engine is off. The fuel gauge reads as it should in both conditions.
 
If it happens running it might be a charging system/ voltage drop / harness issue.

You might get a solid state gauge regulator from someone like RTE and see if that helps
 
carfreak6970 said:
Thank you for that!! thats something I would not have thought of. Ill give that a go.

anything else to try?
Click to expand...

67Dart273 said:
If it happens running it might be a charging system/ voltage drop / harness issue.

You might get a solid state gauge regulator from someone like RTE and see if that helps
Click to expand...

Dang Del, great explanation.
I do diags a little different and I was going to suggest checking the running voltage at the cluster for fluctuations due to the ivr and the charging system itself because the whole electrical system might be doing the same thing but not noticed.
 
Case in point 73 Valiant ( no oil gauge). After engine was at running temp, turn engine off, 10 minutes to go in the store whatever, then turn switch back to on, temp needle would race to 'H' then fall back to normal reading while fuel gauge never did anything out of the ordinary. So the only time I saw a wild needle swing was with hot water. Faulty mechanical limiter was the problem. Solid state regulator cured it.
 
Just curious, have you disconnected the ground to the wiper switch?
 
Tim C said:
Just curious, have you disconnected the ground to the wiper switch?
Click to expand...

I have not gotten a chance yet... a major issue with this situation is that the car is 3.5 hrs away and I really only have about a weekend a month to get over there and get work done on this and 3 other vehicles.

Not ideal, but its what I have to deal with. Which is why I would like to get a list of things to try in hopes that one may work out and would be able to get this car back together so I can put more miles on the thing.
 
That is very wise. Take that ground off first, I had a very similar issue, removed the ground, no issue. As they say try simple first!
 
So I will be heading to back to where the car is in the next week or so and I will try that ground issue, try that resistor tool (already made one with those resistances, thank you), and see if that voltage regulator change would do anything.

Just brain storming, but if there was a ground issue with the wiper switch or the voltage regulator needed to be changed why would those issues only affect the oil and temp gauges and not the fuel gauges? I am just trying to understand the problem...
 
While your checking things, make sure you have a good ground from Negative battery terminal to body and a ground on right rear head to cowl. I've seen gauges go wacky without them. I had a truck that seemed haunted until I realized truck engine wasn't grounded. Just a thought .
 
Okay, so an update:

taking the ground off at the wiper switch didn’t change anything.

I did find out that if I ground the wire ends the gauges will go to peg, but then oil and temp gauges will go back to their bouncing thing.

I did take the negative post of the temp gauge and grounded it. The gauge went to peg but then started to bounce again. So that tells me that what ever the issue is before the gauge...

Any thoughts?
 
TrailBeast said:
Dang Del, great explanation.
I do diags a little different and I was going to suggest checking the running voltage at the cluster for fluctuations due to the ivr and the charging system itself because the whole electrical system might be doing the same thing but not noticed.
Click to expand...


So how would I go about this?
 
So i was able to check the voltage on the cluster in the car, but in the process I blew the fuel gauge by touching too many things at once... So I removed the cluster out of the car and started doing bench tests. From what I could tell I believe the issue was with the connections to the voltage regulator itself. I was able to get it to replicate the bouncing outside of the car, but after a couple changes out of the voltage regulators it stopped. So I have to get a new fuel gauge from my stash, and I ordered the solid state voltage regulator. I will hopefully no more after this coming weekend.
 
okay so I got this situation figured out. I had a spare known working cluster to work with. Ran the same tests with my make-shift Miller gauge tool and kept swapping parts between the two clusters to find out what combination worked and what didnt. It boils down to the gauges are messed up:

charger gauges.jpg


The two gauges on the right are the ones that were in the car and were giving me issues and the two on the left were in the spare cluster. So the problematic ones are some type of aftermarket gauges and I guess they just decided to give up when I did the recent work on the cluster a couple months ago. So the gauges on the left are disassembled and painted waiting for new gauge faces to be applied so I can test a couple more times before the cluster goes in, for good!!

thanks everyone for your help!
 
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