gauge anomoly, again

[David Anderson]

I wrote about this before and kind of put it on the back burner, but now am chasing the problem again.   Someone else posted about the same thing not long ago, also.

My water temp gauge and oil press gauge read one set of numbers when the lights are on and a different set when the lights are off.  The readings are more accurate when the lights are on.   I've tried all different grounding configurations to no avail.

I checked voltage and power to the gauge with lights off is 13.8.  With lights on is 13.4.  I was wondering if I put some type of regulating resistor on the line to force the voltage to stay at 13.4 +/- volts regardless of a higher voltage input.  Is there something at Radio Shack I could by and install on the 12v+ side of these two gauges?   I don't have any ideas as to what else to try to get this annoyance stopped.

Thanks,

David

[Hartley]

Your ground is flakey that grounds the gauge cluster to the chassis.

Run a new ground wire from somewhere solid away from the guage cluster to the ground lines
on the gauges.

What happens is the dash lights coming on is causing the weak ground on the gauge cluster to gain
some voltage and not be grounded.

Makes the readings fluctuate up or down with the lights on or off.

My MC9 needed the extra ground and now everything is reading correctly.

[David Anderson]

You were absolutely correct.  I pulled the grounds from both gauges, ran a dedicated ground to the frame and the fluctuations stopped.  Yipeee.  3 years of annoying gauges gone.  I'm glad I finally took the time to ferret the problem out. 

   Thank you for your help.

David

[FloridaCliff]

Way to go Dr Dave and David,

That is the cardinal rule of DC problems....Always verify the ground and test....

Dr Dave has another successful remote diagnosis

And David has one less headache

What a great board!

Cliff

[belfert]

I seem to have grounding problems with my dash as well.  Would a #6 or so THHN wire back to the frame work?

Brian Elfert

[FloridaCliff]

Brian,

I would run a temp ground from the gauge to the drivers control panel grd point.

If that solves it I would  trace the gauge Grd and check both ends from the gauge and at the ground point.

Many times its just easier to replace it then to tear everything apart,

#6 THHN is probably overkill to the gauge, but you could use it to provide Grd to other gauges if needed.

Cliff

[David Anderson]

Brian,

I used #16 gauge wire.  That is what all my gauges use.  They draw very low amperage.  I only rewired the two suspect gauges.  The rest of the cluster was not affected.

David

[belfert]

Someone recently had talked about using a #6 ground run all the way from the engine up to the dash.  I suppose #6 is overkill since everything else is much smaller.

Brian Elfert

[mdainsd]

I designed TV news gathering trucks for years. They have incrediably complex electrical systems, as one can imagine.
Multiple gensets, AC, DC, A/C's and the requirement for very clean power.
DC low voltage grounds were the cause of most of the problems encountered.
As a minimum: in addition to the large battery to engine ground cable (#2-0 or larger), a large (we used  #1-0) ground cable from the battery bank to the frame. There should be a fairly large conductor also from the battery bank ground to the body of the vehicle. All of these cables should be as short as possible. In larger vehicles we would then attach another ground cable (again #1-0) at the battery ground to frame point and run forward through the vehilde making a stop at a frame location near any panels, generators, transmitters, dash etc. Each point had this cable grounded to the frame, and then an additional ground cable from that point to the equipment in question.
With this design, the grounding problems were eliminated.

Note: faulty grounds like the ones that show up in weird gauge operation arent the only ones, more serious are over-charge (high voltage) or under charge conditions of the battery banks because the regulator systems were getting flaky grounds.