Technical SW gauge wackiness

I replaced all the gauges in my car with a set of older SW gauges. Normal Volts, Water temp, Oil pressure, Fuel level, all electric. They are mounted in the stock gauge pod, share a common hot wire, and are grounded to the pod and the dash by the pod mounting studs. They have worked fine until recently, which they still read out, but when you press the brake pedal, they rise a couple or more needle widths, and when I turn on a turn signal, they all jump upward with the impulse of the flasher. If I have a half tank of gas, when I touch the brake the gas gauge needle rises to 3/4 tank. Weird! Kind of like a back feed surge of current, but only when the brake lights are on or the signals are flashing. Does this sound like a ground issue with the dash, or do I need to look elsewhere?

By the way, it's a Speedway 27 circuit wiring harness, wired up according to their directions. The gauges I wired myself, again by their directions. Car has been wired and operating 8 years without problems, so this is something that just popped up recently.

 

Yes, a ground problem would be my first guess.

 

I'd have to go with ground, If it is a swing pedal that is attached to the dash on one end you may be moving the dash enough to cause a contact to either loose contact or improve contact. If they all move pretty even I'd be looking at the other end of the ground wire that they are all tied into.

 

Does the gauge power have its own fuse?

 

Connect a voltmeter between the gauge panel and the battery negative post (will probably require a jumper wire). With everything turned on, hit the brakes and see what the voltmeter reads. If the ground for the panel is good it should read less than 0.1 volts. I'll bet you will see something above 0.5 volts which tells you the panel is not properly grounded.

 

Thanks guys. I'll take a look at the ground tomorrow.

 

Haven't gotten around to checking the ground yet, too many other irons in fire. This part time work has gotten out of snyc and screwing up my play time, lol.

Edit: Had a few minutes to look at it this morning before I leave to go to work. Looks like the pod is loose, losing ground. Screwed around with it, got the gauges stable, but lost my gas gauge in the process. Looks like I will have to pull the pod and see if a wire came off or out of the connector. Will probably have to pull the speedometer too, just not enough room to get my hands up in there and you can't see much either. The pod is held in position with a couple of br*** 3/8" wrench sized nuts, hard as hell to get my hand in there and get one started on one of the studs to pull it up tight. Must have gotten distracted when I put it in the last time and left one of the nuts off.

Maybe I can get to it this long weekend coming up.

 

Finally getting around to updating this. Cleaned the studs with a wire brush as well as the gauge pod hold down holes. I managed to get the other nut on the gauge pod and pulled it up tight. Found one terminal on the gas gauge was snug, but not tight, so I tightened it up. Results--three gauges now act normally, gas gauge is reading again, but still rises when you press the brake pedal. Still unsure what is causing this, but I can live with it as long as it's reading some gas in the tank.

 

This is going to sound crazy.I had a similar problem.The temperature gauge would go up when the headlights went on.I read somewhere to run a ground wire to all the gauge ground terminals and ground that wire again.2 grounds 1 wire. Could not believe that would work,but it did.

 

Not crazy at all Hoop. I had a 68 Impala that the gas gauge would drop when you turned on the lights. Electrical stuff can do some strange stuff sometimes.
I put a couple of gallons of gas in the car to check if the needle moved, it did, I'll fill it up next time I have it out and see how it acts then.

 

Is it possibly the gas sloshing around in the tank?

Be sure and check the light grounds because that can cause issue in unexpected places. I had a bad ground on a turn signal cause dash issues.

 

Agreed,
When a circuit has a bad ground it will attempt to find ground any way it can, in this case through the gauges which handily show you the extra added current.

 

No sloshing, sitting still in shop. I would expect some sloshing movement on the road.

Light grounds sounds like a good place to look! Had a bit of trouble with one of the tail lights not grounding, I'll give the other one a try.

 

Had a Corvette once. when you hit the brakes the gas gauge would go wacky. Too small ground to the rear I guess-gl*** body-ran another ground a fixed the problem

 

It could be like the gauge in my T-Bird. Accelerate and the gauge goes down, brake and it comes up. I had a 58 Impala that did the same thing. The float pivot on the sending unit was lined up with the center of the car.

 

Mine has never moved while driving. I think my float runs sideways.

 

Another update.
Been driving the car a bit more since the weather has gotten better. Gauges have been operating correctly except the fuel gauge would still move. Just put a replacement alternator on, and it stopped moving! Old one was a 30 something amp, put a 120 or 140 amp on it, can’t remember which, seems like everything electrical is working better. Flashers are faster, lights are a bit brighter. The old alternator was still charging, but I’m thinking it wasn’t keeping up with the current draw when something was on. And I may just be full of ****, but it’s working better, so I’m happy.

 

Just a note about grounds, on gauges the ground to the face plate, then to the dash (or really anything the grounds through something else) I like to run a ground wire. EVERYTHING has to be clean for the ground path to work right and with old cars that's not always the case, so ground wires to lights, dash, body and frame.

 

With your new LARGE output alternator, I hope you have large enough wires in the charging circuit to handle to increased amperage output.
Just a thought, unrelated to the gauge problem.

 

Don’t remember the size, but yes, not stock, larger than most old factory stuff.
They way I understand it, it only puts out what you need, so it’s actually overkill in capacity. Probably could have gotten by with a 65 amp, but it was cheaper than a new reman and it’s polished, so why not?
I figure the old one was probably only putting out 10-12 amps. Just enough to keep the battery up if nothing extra was drawing current. Probably fine for my old 8n tractor that only has ignition to run, but not enough for HEI, power windows and AC.