Club 80-90 Forums

Hi,
I have an aftermarket oil gauge and tachometer for my 1981 2.0 Aircooled.

I am finding that they both act very erratic:
The oil gauge is fine unless the engine is running. It'll sit happily all day long on its 0.0 bar reading. It'll also show the engine pressure dropping off after the ignition is switched off, all nice and smooth. But while the engine is running, the gauge will bounce around and "crash", rebooting and doing its little intro sequence.

The tacho also bounces around up and down, doesn't work in any way smooth like the aftermarket electronic one on my Polo. Obviously, I can't test that while the engine is off.

So far I have tried:
Running a new ground (I was using the one for the radio). The gauges and sensor have their own dedicated ground now.
Different combinations of ground connections.
Connecting power directly from the battery.

No good. Symptoms persist. So, what's likely to be happening here?

I do know that the alternator doesn't have a suppression cap on it. Is that the likely culprit? If so, how can I replace it? I do have some very basic electronics knowledge, but no oscilloscope or anything to test the electrics with.

Have you checked the product specification regards supply voltage and wether they have a bit of latitude, e.g., 10v to 14v? If not specified, you could try running the gauges of a 12v spare battery that is not part of the cars electrical system to see if they perform without issues, most of these gauges are very low regards current consumption so you could even try a 9v domestic battery as an alternative.

If there are no problems with an auxiliary battery you might need to think about a stabilised voltage supply. You could try using the regulated 10v that feeds the temperature and fuel gauges by simply adding a wire with a ring terminal and place it under the nut that secures and connects the temp gauge to the membrane circuit, the nut is the one closest to the voltage regulator. I can post a pic if that helps.

AngeloEvs wrote: ↑15 Jul 2021, 15:42 Have you checked the product specification regards supply voltage and wether they have a bit of latitude, e.g., 10v to 14v? If not specified, you could try running the gauges of a 12v spare battery that is not part of the cars electrical system to see if they perform without issues, most of these gauges are very low regards current consumption so you could even try a 9v domestic battery as an alternative.

If there are no problems with an auxiliary battery you might need to think about a stabilised voltage supply. You could try using the regulated 10v that feeds the temperature and fuel gauges by simply adding a wire with a ring terminal and place it under the nut that secures and connects the temp gauge to the membrane circuit, the nut is the one closest to the voltage regulator. I can post a pic if that helps.

Thanks! I'll give this a go now, I have a spare battery. Just going to tie the grounds together.

I had something similar with my pressure gauge, the needle would hope around and not be stable. Turns out the metal strap that came with my kit on the pressure sender side did have a decent enough ground so I ran another cable to the engine block and all is well now.

Using an external battery hasn't helped.
Even put together a little capacitor/diode setup to try and clean up the current, no good.

I'll have a fiddle with the sensor ground tomorrow.
Will also try using the regulated supply.
Also I have an oscilloscope on the way (long past the time I should have got one I suppose...)

If the problem was still there when connected to an external battery that was isolated from any possible interference from the vans electrical system ( other than for a common earth) then I doubt a regulated supply will work either. If it’s not a supply issue then the problem is in the input signal side or the gauges themselves.