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gauge silliness

Posted By: sharpie

gauge silliness - 04/09/10 08:18 PM

Okay, interesting problem in the 71 Demon. I have five Auto Meter gauges in my panel - voltage, oil press, fuel level, speedo, water temp. No problems with water temp, speedo, fuel level. But some interesting silliness with the oil pressure and voltage.

You see, everything works out beautifully when the car starts and runs. All gauges show correct. But when I pull the light switch to either parking lamps or headlights, my oil pressure and voltage cease to work. Oil pressure buries itself past 100 and my voltage zeros out. No instrument lamps go on in these gauges either. Additionally, the dimmer switch on the light switch can be used to change the values of these gauges. At this point you're probably saying, "haha, this problem's easy! You have it on the instrument lamp circuit!"

But that's where I can't figure this out. You see, my instrument lamps and my +12V leads to the gauges are on completely different circuits. Further, and here's the kicker, all the instrument lamps are in parallel with each other, as well as the +12V leads on a separate wire(see photo 1).



So you'd assume that if two were having problems, the other three would be, too, because they get their power from the same place.

So I decided to trace it - maybe I wired it wrong somewhere? I traced it back, and all of my +12V are going to the Dark Blue with tracer wire (DB* on the chart in photo 2).



So not sure what's going on. Maybe a bad ground? Maybe something else. Any ideas as to how to test this?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:19 PM

Sounds like a ground issue.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:23 PM

How are the headlights grounded?

Headlight switch?

The engine block has a good ground to the firewall?

How about the instruments?
Posted By: sharpie

Re: gauge silliness - 04/09/10 08:27 PM

Quote:

How are the headlights grounded?



Dunno

Quote:

Headlight switch?



Dunno

Quote:

The engine block has a ground to the firewall?



The block has a ground to the battery (-)

Quote:

How about the instruments?



The instruments ground through an extra opening in the bulkhead connector and attach to a wiper motor nut/stud. My bulkhead connector may be bad, as could be the grounding point on the wiper motor, I suppose.

But why wouldn't they all have issues then?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:34 PM

Quote:

My bulkhead connector may be bad, as could be the grounding point on the wiper motor, I suppose.

But why wouldn't they all have issues then?



As soon as you pull the headlights on if it has a bad ground, it will look for any ground it can get. this includes the instruments.

Remember your oil guage and your voltage guage uses a ground. I'm thinking your headlights is stealing this to complete it's circuit.

Assuming it is doing this I would think your headlights are not the brightest either when it is on.
Posted By: sharpie

Re: gauge silliness - 04/09/10 08:35 PM

So you think the headlights have a bad ground, as opposed to the instruments?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:37 PM

Quote:


Quote:

The engine block has a ground to the firewall?



The block has a ground to the battery (-)






Very bad, put another ground from the block to the chasis and another from the battery to the chasis
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:38 PM

Quote:

The instruments ground through an extra opening in the bulkhead connector and attach to a wiper motor nut/stud. My bulkhead connector may be bad, as could be the grounding point on the wiper motor, I suppose.



Is this your only ground for the wiring system? How does it get back to the [-] battery?

You should also have another ground from the wire harness to the core support, but you still need a wire from the battery [-] to all of this...


I mean how does your system get ground if there is nothing going back directly to the battery? You have power going to everything but you don't have ground going to everything.
Posted By: sharpie

Re: gauge silliness - 04/09/10 08:45 PM

Quote:

I mean how does your system get ground if there is nothing going back directly to the battery? You have power going to everything but you don't have ground going to everything.



Good question. I am not sure. I guess it grounds to chassis then chassis to engine then engine to battery?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:48 PM

Quote:

Quote:

I mean how does your system get ground if there is nothing going back directly to the battery? You have power going to everything but you don't have ground going to everything.



Good question. I am not sure. I guess it grounds to chassis



Yes

Quote:

then chassis to engine



How?

Quote:

then engine to battery?



Yes, yes, and yes.

You can help it by installing a ground from the chassis to the battery [-]

For the gauges to work properly you need a good ground from the chassis to the engine.

This would be a good start.
Posted By: stumpy

Re: gauge silliness - 04/09/10 08:52 PM

How are you grounding the gauge lights?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 08:56 PM

Quote:

How are you grounding the gauge lights?



Yes I should not assume he did that either.
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:02 PM

there's actually two separate grounds for the gauge panel. One is for all of the lights including the signals and hi-beam lights, the other is for all of the gauges. Each runs through the bulkhead and out to the chassis ground inside the engine bay on the wiper motor.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:05 PM

Quote:

there's actually two separate grounds for the gauge panel. One is for all of the lights including the signals and hi-beam lights, the other is for all of the gauges. Each runs through the bulkhead and out to the chassis ground inside the engine bay on the wiper motor.



So there is a ground from the battery to the chassis? Or how is the ground getting back to the battery [-]?

Hopefully not thru the motor mounts.
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:08 PM

Quote:

Quote:

there's actually two separate grounds for the gauge panel. One is for all of the lights including the signals and hi-beam lights, the other is for all of the gauges. Each runs through the bulkhead and out to the chassis ground inside the engine bay on the wiper motor.



So there is a ground from the battery to the chassis? Or how is the ground getting back to the battery [-]?



Through the engine itself, I would think. Let me doublecheck my photos though.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:11 PM

Quote:



Through the engine itself, I would think. Let me doublecheck my photos though.



Hopefully not thru the motor mounts but thru a engine ground from the engine to the chassis.
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:12 PM

Quote:

Quote:



Through the engine itself, I would think. Let me doublecheck my photos though.



Hopefully not thru the motor mounts.



why is that bad? the bolt touches each side, no?
Posted By: stumpy

Re: gauge silliness - 04/09/10 09:17 PM

Motor mounts are rubber and don't ground to the frame. You diffenately need a ground from the block to the body and to the frame. I have seen more than one throttle cable or shift cable burnt through because they became the only grounds to the engine and couldn't carry the amps from the starter draw etc.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:23 PM

Quote:

Quote:

Quote:



Through the engine itself, I would think. Let me doublecheck my photos though.



Hopefully not thru the motor mounts.



why is that bad? the bolt touches each side, no?



Not nearly enough. make sure you have both

a engine ground from the engine to the chassis


and


a battery ground from the battery [-] to the chassis.


You also have to have the engine ground but you already mentioned you have a battery [-] to the engine. Which is not enough by itself.
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:33 PM

Quote:

Motor mounts are rubber and don't ground to the frame. You diffenately need a ground from the block to the body and to the frame.



What I don't understand here is the car runs fine, charges fine, and even has voltage to spare. It's just the gauges.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:40 PM

Quote:

Quote:

Motor mounts are rubber and don't ground to the frame. You diffenately need a ground from the block to the body and to the frame.



What I don't understand here is the car runs fine, charges fine, and even has voltage to spare. It's just the gauges.



Actually your headlights are stealing the ground from the gauge grounds when they are on. Look at it this way how does the headlight get it's ground?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:46 PM

Here is some of the grounds to your headlights. Notice there are several of them. Now, how do these grounds make it back to the [-] side of the battery?

Thru the chassis, then the motor mount bolt, then thru the [-] cable? Or even worst thru the voltmeter negative and/or the temp guage wire and then thru the motor mount bolt? I hope not.

Hopefully you have a cable directly from your [-] side of the battery to the chassis.

Attached picture 5916515-headlightground.JPG
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:47 PM

Do you have a good ground going from the chassis to the [-] battery?

If you do, then you still have to check these and well the other grounds. If you don't you know where your problem is.
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:47 PM

so the headlights are pretty dim, but still, I'm not sure why the dimmer switch works on the gauge needles. The dimmer switch works for the instrument lamp circuit, not the headlights?
Posted By: sharpie

Re: gauge silliness - 04/09/10 09:48 PM

Quote:

Do you have a good ground going from the chassis to the [-] battery?



probably not.

So what all do you suggest I do?
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:51 PM

If you look at your wiring system you are providing power to everything. But just as important is the ground [-]. Are you providing a good ground to everything? The headlights require more power than just about everything else except for the starter. If you have a ground issue the headlights will tax it.

It is a good thing you only have a two headlight system, with a 4 headlight system if you were to put on the high beam you would either burn something out or the headlights would be even dimmer.

Edit: The other problem with dim headlights is voltage loss going thru the bulkhead, light switch, foot switch, bulkhead again, and finally the headlights. Voltage checks will verify where your losses are.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:57 PM

Quote:

Quote:

Do you have a good ground going from the chassis to the [-] battery?



probably not.

So what all do you suggest I do?



Take a jumper wire from your wiper ground to the [-] side of the battery and you will most likely notice a night and day difference.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 09:59 PM

But the proper fix is to put a ground from the battery [-] to the chassis, and a wire from the chassis to the engine block. Make sure the chassis ground for the wiper is good too as well as checking the other grounds.

Notice how most battery grounds have a extra wire? this extra goes to the chassis.




The Ground issue may not be your only problem but it is a good start. Especailly if you know you do not have a engine and chassis ground.
Posted By: sharpie

Re: gauge silliness - 04/09/10 10:38 PM

Oh wait, in that case, I have a chassis ground (I think it's grounding on the core support). I have a double-wired negative battery cable.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 10:42 PM

Quote:

Oh wait, in that case, I have a chassis ground (I think it's grounding on the core support). I have a double-wired negative battery cable.



I would check those grounds then (plus add the engine to chassis ground) and make sure they are well grounded.
Posted By: HealthServices

Re: gauge silliness - 04/09/10 10:51 PM

Ok check all grounds first, this includes...

  • the headlight grounds in the circuit pictured.
  • engine to chassis ground
  • chassis to battery ground
  • Battery to chassis ground
  • Check the ground you have at the wiper.
  • check path of power to oil pressure guage and to temp guage.
Posted By: 68HemiB

Re: gauge silliness - 04/09/10 10:57 PM

Brian,

I am late to this dance, and Allen & Stumpy are doing a fine job.

I just want to remind you about the fresh paint in your engine compartment. Fresh paint is pretty, but the enemy of good ground contacts. Make sure the to-do list includes undoing the body ground (the one from the negative post of the battery) where it attaches to the core support. Scrape/sand all that pretty paint away to reveal BARE METAL, then reconnect the (also cleaned) wire eye to that spot of bare metal. Find where the headlight wire harness grounds are (probably similar wire eyes) and do the same clean-pretty-paint-to-bare-metal fix.
Posted By: Fury Fan

Re: gauge silliness - 04/09/10 11:23 PM

Quote:

If you look at your wiring system you are providing power to everything. But just as important is the ground [-].


In an odd, way, grounds are more important than + voltage. No voltage, no output. If you have voltage but bad grounds, though, that voltage will find a different path to ground, and that's where some wierd stuff can happen.

And FWIW, that blue/black ignition feed shown in an earlier post is an adversary to us all. It connects to almost every ign-hot item, it reaches from the alternator to way up under the dash, yet it has no fusing on it whatsoever. At least my 70 300 didn't. DAMHIK.

Anyone doing extra wiring, wiring repairs, or just loves their car should review that circuit in a FSM and put an inline fuse in it somewhere!
Posted By: sharpie

Re: gauge silliness - 04/13/10 06:50 PM

I think I have an idea of what's going on now. I mapped my gauges, and if you look at the gauges that aren't responding, you can clearly see there is one circuit that they share with only each other - the grounding circuit.



I'm going to doublecheck the grounds today by making new grounds for both the oil and volts and grounding them close (i.e. not through the bulkhead). I'll update this afternoon.
Posted By: sharpie

Re: gauge silliness - 04/14/10 03:43 PM

Well, you were all correct. I did a simple test yesterday that yielded the best result possible. I took a lead from my multi-meter out of the contraption and attached it to a good body ground. Then, I turned the ignition to "accessories" and put the other end of the lead onto the ground terminal of the oil gauge with the old grounds still hooked up. Both gauges immediately began functioning. So it was obviously ground.

Now, the only place in this circuit that wasn't soldered and shrink-wrapped was the bulkhead connector. In fact, I had fashioned that connector in one of the extra slots in the bulkhead connector, so it probably wasn't even as good as stock. So I cut the two grounds before the connector and put them to a good dash ground. The gauges function perfectly now.

Thanks for your help, everyone!
Posted By: Pat_Whalen

Re: gauge silliness - 04/14/10 03:46 PM

Sharpie:

What software did you use to recreate your gauge's wiring?

I'm getting tired of a ruler and paper.
Posted By: sharpie

Re: gauge silliness - 04/14/10 04:07 PM

Microsoft Office Visio 2007. I really like it. But there are free ones online, too. Like Gliffy.
Posted By: HealthServices

Re: gauge silliness - 04/14/10 05:36 PM

Make sure your other grounds are good. If turning on the headlights affected the guages then the grounds for the headlight circuit should be checked. Use some star washers at the grounds make sure there is no paint in the way. Checking the other grounds would not be a bad idea either.

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