Grounding vs. the alternator?

[johnmh]

Greetings to all. Here goes my first post.

My car runs well, but has some chronic gauge issues.

The coolant temp needle and oil pressure needed wiggle significantly while driving, even with steady throttle. The coolant temp can swing from 80c to 100c once a second, while the oil pressure gauge can go up and down by 1 bar at the same rate, even if the engine rpm is steady. Both needles move together at the same rate.

At idle both gauges stop their oscillation and go to normal numbers. If I then rev the car, the oscillation starts and becomes more pronounced. Other gauges do not wiggle at all.

My voltmeter is pretty constant, it does not swing with steady throttle. When the car is cold, the gauge shows a healthy 13.5 volts at idle. However, when it is warm after a bit of driving, it shows only 12.5v at idle.

Does this suggest one or more bad grounds somewhere? More resistance when the car gets hot? Maybe resistance from the alternator itself, or from the battery to the frame? Something basic? The ground straps all look tight and clean.

I am considering sending the alternator to be tested, but if it had spikes in voltage it would show up on the voltmeter, or the alternator light would come on.

Car is a 1992 Lamborghini Diablo. The alternator lives near the catalytic converters, but has a heat reflective shield between it and the exhaust system.

Thanks to all for any insight.

 

[JHZR2]

I’ve had a jumping fuel gauge due to poor grounds. It’s not inconceivable that this is a grounding issue.

How hard is it to pull gauges/cluster? Can you chase those grounds? Sometimes they go back to a common point.

 

[johnmh]

I can pull the cluster from the dash pretty easily and have a wiring diagram. I am just not skilled enough to read it to understand where the ground(s) for the two gauges are located.

 

[RooflessVW]

I'm willing to bet it's the main grounds, not the grounds to the gauges themselves.

 

[rijndael]

When you're looking at the alternator, I'd also verify the AC ripple measurements (at the alternator and not at the battery).

https://studylib.net/doc/18144475/too-much-ac-in-the-dc-charging-system-ripplevoltage

http://tmoss.efidynotuning.com/Automotive Fluke Testing.pdf

 

[willbur]

cool engine. HP?

 

[Jetronic]

I can pull the cluster from the dash pretty easily and have a wiring diagram. I am just not skilled enough to read it to understand where the ground(s) for the two gauges are located.

fix a lead to the ground on the battery, then measure voltage between that and the ground on the gauge, while the fluctuations are happening. If you see anything but 0V you have a ground issue.

 

[RedSpider]

This might help:

Diablo Gauge Fix

My friend has two Lamborghinis. Absolutely incredible cars.......going 0-100 with a V10 roaring behind you is quite a rush.

 

[1444buicking]

I would look at the Diablo Gauge solution suggested by @RedSpider first.

If you suspect a general power issue here is what we do in our backyard.

1. How old is battery? Get it load tested at a battery store. I don't think I would have a battery store load test the battery in the car (or the alternator in the car either).

2. Make sure the battery terminals and clamps are clean and tight. Inspect the crimps on the wires.

3. One clue to a serious voltage drop is hot battery terminals after a good drive. The terminals should be cool to the touch, generally.

4. Do some more serious voltage drop testing, particularly on the ground with the engine running and accessories on. You can test from the battery to different points on the car (e.g. body grounds near battery, engine block, alternator, etc. Do some research here so that you don't create problems. This basically just requires a multi-meter and takes a few minutes.

5. Check the connections at the alternator for cleanliness, corrosion, etc (with the battery disconnected). Pay special attention to the condition of the fat wires at the connections (if the wires are corroded under the crimp, at the start of the insulation, that can cause a serious voltage drop.

6. You can put a scope on the alternator to get an indication of condition . I suppose brush wear or diodes failing (bridge rectifier that converts AC to DC) could be typical failure modes that can generally be rebuilt. There are shops that specialize in testing and rebuilding alternators.

Wear some goggles and PPE messing around with the battery. They rarely explode but it happens.

 

[johnmh]

Thanks to all that responded, especially Redspider. I can pull the cluster and find a better 12v source. That seems a possibility. I thought the heat near the alternator might be the issue.

A Diablo stores its battery in the driver’s side rear wheel arch in front of the rear wheel. No corrosion or loose connectors from what I can tell. Cranking a 5.7 V12 with a loose terminal wire would not happen and the car starts right up.

If the dash fix does not work, next stop is the alternator, battery and main wiring harness.

Diablo is a 2wd version, about 500hp and drives as you would expect for a bright orange car named after the devil that is endowed with a huge v12. Not too many places to rev it out, even in lower gears. Good, but scary, fun.

 

[f355spider]

Thanks to all that responded, especially Redspider. I can pull the cluster and find a better 12v source. That seems a possibility. I thought the heat near the alternator might be the issue.

A Diablo stores its battery in the driver’s side rear wheel arch in front of the rear wheel. No corrosion or loose connectors from what I can tell. Cranking a 5.7 V12 with a loose terminal wire would not happen and the car starts right up.

If the dash fix does not work, next stop is the alternator, battery and main wiring harness.

Diablo is a 2wd version, about 500hp and drives as you would expect for a bright orange car named after the devil that is endowed with a huge v12. Not too many places to rev it out, even in lower gears. Good, but scary, fun.

You should really be a member of the various Lamborghini forums. The information sharing is invaluble as you see from that post above. I used to own a couple older Ferraris and it was amazing whenever I ran into what I thought was "my" unique problem...there would be three or four people chime in that had experienced it before and had the solution, including parts sources. Often saved me trips to a mechanic and I could fix it myself.

 

[johnmh]

I am a member of those forums, but I suspected that people on this site may have more technical insight that the average Lamborghini forum visitor. Fchat is pretty good however, I have a few older Ferraris I work on myself and have gotten a lot of help from that site. Not a lot of Diablo experts out there, as there are not a lot of Diablos.

 

[johnmh]

So, I suspect Redspider’s advice is spot on.

At the battery, engine off, the battery develops 12.7 volts. Resistance from the battery negative terminal to the engine block is 0.6 Ohms.

At the water temp gauge, the ground wire at the back of the gauge to the frame has a resistance of 7.2 ohms.

With the key on but engine not started, the positive feed (not the signal wire) to the same gauge delivers only 9.9 volts.

Just not enough power getting to the upper gauges. I imagine resistance goes up when the wiring gets warm.

 

[meep]

Good troubleshooting. You’re on it. I came here to suggest grounds to the cluster… nice car!

 

[berniedd]

A thought came to me just now. Many high end cars now come with plenty of carbon fiber panels. Do these panels conduct electricity just as good as grounding wires and therefore don't need them?