Greetings to all, and an electrical fault question

[Johnmh]

Hello, I am a new member, with a few older cars which need some help on occasion. Where I live there are no skilled specialists and I usually enjoy the challenge.

One of my cars has a strange gauge issue and I am not fantastic with electrical issues.

In short (no pun) the coolant temp, oil pressure and fuel level gauges work and are stable at idle.

When the car is up to speed, say 3-4000 rpm, these three gauges wiggle, the coolant temp gauge most of all. It may oscillate from just above 80 to 90c and back every second or so. The other gauges follow the same pattern but slightly less movement.

I thought that there may be some variation in voltage causing this, perhaps an alternator / voltage regulator issue, but the weird thing is that the voltmeter is rock solid - yes, it climbs slightly when I rev the car, but the alternator seems to be happy sending out13-13.5 volts.

Before I take out my dashboard and start chasing wiring and bad grounds, I thought to ask if anyone has ever seen this.

As I may be asked, the car itself may be outside of the usual fare for this site, a 1992 Diablo (my newest car!) I am still learning my way around.

[tyang]

Welcome to the site @johnmh.

Yeah, a 1992 Diablo may be too new and different for a forum for Vintage Ferraris, but you never know the knowledge base on this site!

Tom

[Johnmh]

I promise that is my newest car, most everything else I have has carbs and points. I won’t make a habit of it, it’s just that the older cars basically work ok.

Serves me right for buying a ‘new’ car. I expect that the laws of electricity are probably the same. More wires though.

[linen grey ghost]

Perhaps fuses were replaced at some time and according to the linked bbs thread you are experiencing the quoted issue:
". . . Sometimes even the diagrams are wrong, also be very, very careful when pushing in new fuses, on the older models like yours they are prone to push out the wire in the back of the box when you insert a new fuse and you get zero connection. Ask me how long that took to diagnose on a friends car LOL."

link > > > https://www.lambopower.com/forum/index. ... l-diagram/

[Johnmh]

An interesting idea. I have removed the footwell plate and looked at the relays in the past; perhaps something is off there.

I do know there is nothing to push against when you try to install a new relay. Slightly odd design.

Will check it out tomorrow. Thanks.

The car is ex-Japan, lots of additional wiring for extra lights, stereo stuff we carefully removed. No idea why the stereo was necessary, as there is no way to hear it over that engine.

[Jimmyr]

Yes the gauge grounds and all other grounds have an effect on these gauges. It seems like they have a problem when you rev the engine which could point to the charging system. Your charging voltage should be about 14.4 V or so, yours seems low. The gauges could be more sensitive to voltage problems than the gauge you are using to measure battery voltage. Check and observe the voltage right at the gauge feed point ad see what voltage they are being powered at. If they are stable with the ignition on and the engine not running, and react with the engine running; take a closer look at the charging system.

[tim]

Greetings... Since you have a "new" Ferrari I suggest looking for a voltage stabilizer. Most newer cars have them so the instruments have a steady supply . Tim

[Johnmh]

Just to follow up, I finally managed to solve the issue.

While each gauge gets its own signal wire, the power supply to the Diablo’s gauges are fed in series by a common + wire and go to a common ground. After 30+ years, all these contacts generate resistance. After the circuit goes through so many connectors, it ends up at the water temp gauge. Despite a nice, healthy 13.5 volts at the battery with the car running, there was only 9.9v at the + wire which fed the temp gauge, when the - end of the voltmeter is grounded to the frame. The same spot on the frame read normal voltage when read off the battery, so the ground was ok.

I cleaned the connectors, then discovered that NOS instrument cluster wiring harnesses are still available at reasonable cost. Installation was pretty easy.

The issue is now resolved. Thanks to all that replied.