OJ GTS

Hi all,

I'm just finishing up on a major refit of the interior of my '95 GTS (UK) and I'm having a few issues. Basically, the digi dash seems to be going a bit wild with the speedo (and digital readout) bouncing around, the PSD light coming on and off, and the odd spurious pad wear alert, stop light failure or 'differential control off' warning. This is all occuring with the car running and sitting on my driveway, I'm still waiting for my new seats to be delivered so I haven't run it round the block to test what it's like in motion yet.

I've carried out a fair bit of work on the car including:

- Removal of a badly fitted and then even worse partially removed 90's car phone install
- Removal of a poorly fitted 90's head unit to be replaced with a Porsche classic head unit with new speakers and a compact amp located in the original position and picking up the original speaker wiring
- LED upgrade to the instrument cluster including one of Ed Scherer's PWM dimmer units
- Deactivation of the airbag ECU using a Porsche bypass plug to allow the fitment of a sports wheel without airbag warnings
- Complete carpet set replacement

The only pattern I can gauge is that it happens more consistently when the car is started with more electrical load (headlights, blower fan, stereo turned up), and that revving up the engine seems to trigger it. If the car is left running for a while before turning any of these on then it doesn't happen at all. The car has sat around while I've undertaken the work in my limited spare time over the last year or so and the battery did go flat a while back. I recharged it while disconnected from the car on a CTEK, and has since been fine, although perhaps a bit lazy to crank. Worth saying all of the work has been undertaken with the battery disconnected too.

I was hoping this might be down to the car being sensitive to low voltage or perhaps slightly iffy earth or connection to the digidash and that I might only have to go through the misery of pulling and refitting the pod, but worryingly the PSD sounds like it's actually actuating when the speedo jumps and the PSD light is triggered. Having reviewed the wiring diagrams and other info it makes it sound like a problem around the ABS unit rather than the digidash, although apart from replacing the carpet around it and maybe shifting the loom around a bit I haven't touched or unplugged any part of it...

Has anyone seen an issue like this before, or do you have any suggestions on where I might want to start looking?

Bigfoot928

sounds like low voltage. Check your battery, as well as voltage from Alternator and jump point in the engine bay. Also check voltage with the car running at the starter as it is a contact that goes from the alternator to the battery.

Ed Scherer

Check to make sure things are properly grounded at MP IV; you probably messed with this grounding point when you installed the AILD-1. Two wires from the cluster (plug 2, terminals 14 and 15) are grounded at MP IV. Plug 2 also carries all the power (both ignition-switched and unswitched) to the cluster. I suppose it's possible that plug 2 might not be properly seated, but that seems unlikely if you inserted the plug all the way and the "wings" are in the locked position.

As Sterling already pointed out, the instruments go crazy with low voltage. Two fairly easy check points:

• at the fuse for unswitched power ("30" bus) to the cluster; I think it's fuse #25 on your '95
• at the fuse for switched power ("15" bus) to the cluster; I think it's fuse #39 on your '95

On my '90 (I didn't check to see if it's the same on the '95), there are also three RE/BK wires (two of which go to the cluster, plug 2, terminals 16 and 17) and a RE wire tied together with (and fed by) one fatter RE wire. I don't know where that happens. Probably not far from the cluster. Regardless, that could be another failure point.

OJ GTS

Thanks for the replies guys, relieved that it sounds like it could just be low voltage

i was careful to make sure the MP-IV ground point was secure and that all four connectors were seated properly and locked, although when I was testing the cluster I did have a couple of similar problems even though they were all correct so perhaps I should have made sure the connectors were all clean and contacting well.

That said I fired up the car again last night and it all seemed fine... helpfully I left my multimeter on and the battery was dead so I couldn’t check anything!!

I’ll have a check round with a better unit borrowed from work at the weekend

daveo90s4

Ensure that all of the red wires that attach to the 'T' on the positive on the battery, and the T itself are all properly secure.

Cheers

Dave
Aus.

The Forgotten On

Check fuse 24 as well. It supplies power to a good portion of the cluster and can cause all kinds of weird things when it blows or goes bad.

OJ GTS

Thread from the dead just in case anyone has the same issue…

Car was dry stored in my parents garage over COVID and I finally got back around to trying to sort it a few weeks ago

After pulling the pod and half the interior back out, checking and cleaning all the earths on the car, then finally deciphering the hideously complicated wiring diagrams and meticulously multimetering/oscilloscoping all the wiring between the wheel speed sensors, the ABS unit, the dash, and any earths and power feeds associated, I found absolutely bugger all wrong with anything!

Eventually I found a post on here describing how the speedo input is produced from the ABS unit, and on later, post 93? cars like mine it comes from the left rear wheel speed sensor.

Unplugged it, no bouncing.

In the absence of any other explanation I ordered an eye wateringly expensive new sensor and after wrestling the old one out of its badly electrolytically corroded hole I now seem to have no more speedo weirdness.

Science!

I’m guessing the sensor had degraded and any sort of minor voltage fluctuation was causing it to pulse and fool the ABS into thinking the left rear wheel was spinning with everything else stationary, hence the Speedo bouncing and the PSD going crazy

Now it’s off to a specialist to have the rest of it checked over and recommissioned.

Oldewoodupes

Glad you figured it out. Your problem has a lot of similarities to one I had that was cured with a new ignition switch. If your problem returns, hopefully not, something to look into. Take care