Rallye dash amp, fuel, oil and temp failed at the same time

[440magnum]

I had forgotten to connect the negative body ground from the battery pole to chassis last time I started the car, after this they all won't work.  Checked all fuses but all are ok. I looked at the challenger-70 electrical schematic and there seems to be a device between these gauges and the 12V incoming. Appreciate any advice on this.

[MEK-Dangerfield]

There is a 5 volt limiter between the 12V and gas/temp/oil pressure gauges. That doesn't explain the amp gauge not working though.

[440magnum]

My mistake, the amp IS working. I will check that there is 5 V to the gauges tomorrow - thanks

[MEK-Dangerfield]

The limiter has a 12 volt feed into it, and it puts out a pulsed 5 volts to those 3 gauges. It might be bad, or it could have fried the 3 gauges. So all 3 gauges worked before??

[Oldschool]

Sounds like a bad ground connection to me..   

[440magnum]

Yes, they all worked before. But Im not sure they all died because I forgot to ground the chassis, but thats the only thing I can think of that has happened since they worked the day before. The voltage to the gauges are fluctuating between 1 and 6 volt, is that normal Incoming voltage to the limiter is ok (stable at close to 12 V)

[JayBee]

The voltage to the gauges are fluctuating between 1 and 6 volt, is that normal

Yes, it's close enough. Some meters can't react fast enough or peg beyond the 5v reading.

The limiter has a 12 volt feed into it, and it puts out a pulsed 5 volts to those 3 gauges.

[1of1]

Yes, they all worked before. But Im not sure they all died because I forgot to ground the chassis, but thats the only thing I can think of that has happened since they worked the day before. The voltage to the gauges are fluctuating between 1 and 6 volt, is that normal Incoming voltage to the limiter is ok (stable at close to 12 V)

They all share a common ground loop, check that.....

[MEK-Dangerfield]

They all share a common ground loop, check that.....

Actually, the daisy chain loop of wires is the feed from the voltage limiter. The gauges get their ground feed by being tied to the dash pod, which is mounted to the dash frame.

[1of1]

Actually, the daisy chain loop of wires is the feed from the voltage limiter. The gauges get their ground feed by being tied to the dash pod, which is mounted to the dash frame.

I learn something every day Thanks!

[440magnum]

My mistake, the amp IS working. I will check that there is 5 V to the gauges tomorrow - thanks

The alternator is not charging either since the f-p, thats why I thought this gauge was fried also. Maybe the rectifiers died, will take alternator apart today and do some pegging...

[Chryco Psycho]

the power is fed from the voltage reg , , it is a pulsing voltage averaging around 5 v , the power feeds out to each guage with the black wire , then passes through the guages & is sent out to the sending units grounding at the senders , the varying resistance through the sender controls the reading on the guage . To clarify there is no gauge ground on the dash , they are isolated from ground .

[dodj]

To clarify there is no gauge ground on the dash , they are isolated from ground .

If they were not isolated from ground, all the gauges would peg full scale.

[MEK-Dangerfield]

Sorry about that guys.   

It's been too long since I had my gauges out and I couldn't remember if both posts of a gauge were insulated from ground or not. Now that I think about it a little, if they were grounded, where would the signal wire go?  DUH!   

[1of1]

Maybe this will help with the solution...