Late 944 Bouncing Intermittent Speedometer

AKA_951 · 02-07-2013, 10:54 AM

I'm hoping this is a one-stop-shop as far as fixes go since they COULD be related.

SYMPTOMS:

Tachometer does not move, even when I tap the dash.

Temperature gauge reads no higher than 1/4. I drove it home yesterday. The low-speed fans came on at about 1/4 when idling in the garage. Then the highs came on until I turned it off 30 seconds later. The temperature gauge was barely above the 1/4 mark. It should be 1/2 then 3/4 repsectively.

Last week I noticed the speedometer wasn't working after driving for five minutes. I gave the dash a tap and it worked again. Same thing happened today when I left home for work.

A respected Porsche repair man told me that the tach is most likely bad. This was before the second two symptoms occurred. I cleaned the ground contacts that are underneathe the dash (what a back-breaker). I also used a potentiometer and matched resistances to temperature readings using Clark's recommendations. They checked out fine. I also used a laser temperature reader to check various temperature throughout the engine bay and radiator. They are also in an acceptable range. I put a new used double A/C relay in. I tested it and it's "ok" despite my A/C not working at all due to low pressure switch acting.

I'm thinking that cleaning the back of the cluster will solve my issues. I will double check my oil pressure (typically reads around 5) and voltage readings today when I leave work.

Thanks in advance. Porsche problems :/

Gawernator · Gawernator's Garage

My first electrical project

If this doesn't work it probably is bad but shouldn't be too bad to get a replacement. By the way I have a volt/oil gauge in good shape if you need one of those. Hope my tutorial fixes it

AKA_951 · 02-07-2013, 03:21 PM

Oil pressure is pegged on 5, and I'm getting ~12.8 constant voltage. Just pulled the cluster for cleaning. I'm also going to replace bulbs and reflector material. I'll let you know if it works. In the mean time I'll check my bellhousing grounds. What's that hose that's in the way? 1" 90 on a turbo car. Sorry I usually post here for anything unrelated to the turbo.

Gawernator · Gawernator's Garage

Sending unit bad? Get the genuine one from Sonnen or Sunset Porsche. Can't help on 951 specific

AKA_951 · 02-08-2013, 06:48 AM

It's the heater control valve it turns out. It's in the way of the bellhousing grounds. Why wouldn't I have to drain the coolant AGAIN. I'll order a new sender. I did attach 3" jumpers to the original wiring harness next to the senders because the female harness spaids were bad. It also made it easier to plug in with everything in the way. Because it's an electro-mechanical sensor and not a thermocouple, I believe the added resistance from the jumpers is fundamentally negligible to the accuracy of the temperature circuit. Correct me if I'm wrong. I have also installed all new fan switch and thermostat previously. Cars are FUN

AKA_951 · 02-08-2013, 07:18 AM

actually, first I will clean cluster and reinsert. If that doesn't work, I will replace sensor and access bellhousing grounds. As for all other gauges, I'm hoping the cluster is to blame.

Gawernator · Gawernator's Garage

Clean that corrosion!

AKA_951 · 03-07-2013, 01:12 PM

Cleaning the cluster did NOT work. Now I am delving into the bellhousing grounds. Otherwise known as bloody knuckles with a Porsche. I will also clean the grounds under the dash. They look to already be clean. We will see if either thing helps... I usually like electrical problems, because the job is fairly clean, but Porsche designed this car as if nothing would EVER go wrong, putting everything in unobtainable, obscure places

Gawernator · Gawernator's Garage

There's more than one bell housing ground? Whoops lol

porschetub · porschetub's Garage

Replacing my bonnet on the 85,PO dropped it on tools cause he was to mean to replace one strut

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Anyone have the paint code for an August 85 series one in black?

AKA_951 · 03-07-2013, 07:22 PM

not sure what ^^ he's talking about but,

I cleaned all of the engine bay grounds I think... Cluster is still acting weird. I tapped the speedometer and it started working again... not sure how long that will last... Do the 3 connectors on the back of the cluster ever start messing up? I'm curious as to why my stuff happens when I tap the dashboard. I am going to reclean the grounds under the dashboard next week. They have some dielectric grease on them. I think my temperature gauge is a bit more accurate now as the fans came on when the gauge got to 87C, so I must be doing something right... My tach might need to be replaced. Is that hard?

AKA_951 · 03-07-2013, 07:29 PM

Well I didn't get all of the grounds,

Still need

Battery area main ground
Hatch interior ground
One under the headlight motor
And Re-clean under the dash

Hopefully that will fix it. After that, I'll buy a tach. Cars are hard...

AKA_951 · 03-11-2013, 08:35 AM

Does anybody know if I can start the car without the cluster in? I was planning on removing the cluster, removing the tach, and hard wiring the tach to the harness to check for cold solder joints by hand. I could also use a multimeter on the tachometer to see if I have infinite resistance across critical joints. Is there a possibility I can ruin the tach this way? Clark's makes a huge deal about ruining your cluster because it's ESD sensative.