bouncing needles on new gauges

[mach1one]

The coolant temp. guage never worked in the 73 mach, so I decided to update it with new guages (along with the oil pressure/volt. guage) while troubleshooting the problem

With a new sending unit the guage doesn't read, it just bounces below 100; even with the car warm. To test the wiring I ran a new line directly to the sending unit, didn't fix it.

The oil pressure guage is now acting the same way. It bounces around way too much to work right. Before the guage swap it too was working.

The alternator guage is working right and has always worked.


Is this the symptoms of the IVR being bad? I only have a schematic out of a haynes manual and I'm having problems understanding it. Wiring is not something I understand very well.

 

[BullittStangV8]

Do they have a good ground or share the same ground?

 

[Rusty67]

The wire that goes to the sending unit is actually the ground wire. The sending units provide a certain amount of resistence which keeps the needle from pegging one side or the other. Make sure you have your wires setup correctly. Try running a wire directly from a regular 12v source to the gauge and then another to the sending unit. Actually, don't try that unless the guage can take 12v continuous. I'm not sure if your aftermarket gauges need an IVR. Check the instructions for the gauges and see if they need one, otherwise by pass it.

 

[wicked93gs]

bouncing needles(or pegged) on the autometers I used in my neon always had bad grounds...my oil pressure didnt bounce since the Isolator acted as the ground through a stainess steel line....however if it isnt getting a good ground through the engine, you can always run a ground line to the frame as well(for the isolator...I had it setup like that once) anyway for the other gauges doublecheck all wiring and grounds...I assume you have all your splices soldered right?

 

[Edster]

If the gauges are after market and the IVR is hooked up that could be your problem. try it w/o the IVR and see what happens.

 

[mach1one]

The IVR wasn't hooked up; originally I wasn't sure if it was suppose to be.

So here's an interesting conclusion. I went to check the engine/chasis ground and there was none! So I bought one, installed it, problem solved.

The previous owners had also wired the guages such that they were on. So we swapped that over to a switchable power source too. Now if I can just get the gauges mounted I'll be all set

 

[wicked93gs]

wow...with no grounds I'm kinda surprised your engine even ran, I've always had to have 2 or more grounds with most of my engines(even my old 67 had two grounds if I remember correctly)

 

[mach1one]

Yeah, what's even crazier is that its been running fine. No charging or electrical problems aside from the one non-working guage.

 

[Rusty67]

After hearing that, I'd recomend you rewire the entire car just to be safe. Sounds like these guys were the type of people that make gremlins.

 

[mach1one]

It looks like the engine will need to be rebuilt anyway so I'll have time to take that on. That is probably a good idea since I wasn't very fond of some of the things the previous owners did.