Diagnosing digital dash issues

[bcar240]

I have an '86 z31 where both the instrument cluster and and climate control are doing very wonky things. The only part of the entire instrument panel/HVAC that consistently works properly is the tachometer and odometer displays. Here is a mostly comprehensive list of issues, I am sure many of them are related, but trying to figure out if there could be one root cause:

-The speedo occasionally switches on its own to km/h (both reading and units) and then quickly back to mph, also just recently the speedo started going to 0 for a few minutes then coming back (odos/mpg/accel don't work during this period)
-The three engine gauges flip out occasionally, the voltage reads top of scale, oil pressure reads 4 to 5 bars low, temp reads bottom of scale, even though everything is normal. They will eventually return to normal.
-Fuel gauge sometimes reads a random amount between the actual level and empty. The digit readout sometimes gets screwed up as well, but doesn't necessarily correspond to the bar graph. Ex. I have a 3/4 tank of fuel and the graph will show 1/4 tank and the digit says 1 gallon, or something like that.
-Average speed display doesn't work at all.
-The climate control functions correct as far as I can tell when it comes on (other than having no freon charge), but sometimes the control unit is just completely dead and unresponsive (no backlights, temp readouts blank, pressing buttons does nothing), the manual defrost DOES work but that's it. Then all of a sudden the whole thing will start working again.

All this stuff happens sporadically and comes and goes. It can happen together or as individual problems. Sometimes everything works 100%. It also seems to be affected by temperature. For example, the speedo acts up in the 80°s, engine instruments usually work fine above 80° or so, HVAC tends to act up more when cold, etc... I have done a lot of research and found that there are a lot of problems with the power supply unit for the dash, and there is a guy out there that refurbs them. Does this power both the dash and HVAC? I am not really sure where to start since nothing seems to be working consistently. When I bought the car about a year ago it did not have these issues. They started slowly until now they happen pretty much on every drive. Any help appreciated!

[bcar240]

I'm really trying to get this fixed now after ignoring it all summer as the speedometer has decided to not work about 60% of the time and this is rather inconvenient for me. My main questions right now in case anyone has some insight:
Does the digital HVAC share the power supply or is connected to the digital dash at all, can their be a single failure point that causes issues in both of them?
Are the speed sensors a common issue with these cars or is the dash likely to be the issue for the 0 speed reading?

[evildky]

I think they share a power sully but AZ Zbum I'm sure will know. The speed sensor on the 86 is actually under hood attached to the end of a spedometer cable, could be a bad cable? might be worth at least checking both ends are secure.

and your buttomes has me wondering it theey might not need to be taken apart and cleaned?

[bcar240]

Hmm, so the speed sensor is merely connected to the end of an old school mechanical cable, not a pinon or something in the transmission. That is very interesting. I would think mine is electronic in nature due to the way it comes and goes based on heat and after I take corners or hit bumps. Seems like a balky connection somewhere.

On the buttons, I know some of them on the dash are suspect, but when the climate control is acting up, it is totally dead. I get nothing from it in terms of displays or response except for emerg defrost.

[AZ-ZBum]

The digital climate controls and instruments don't share the power supply.

The wonky fuel level reading is probably the fuel level sending unit in the gas tank.

The speed issue is most likely the pinion gear seals at the transmission leaking and contaminating the sending unit by allowing transmission fluid into the speed sensor.

The three readouts flipping out is most likely the power supply. It's found above where your right knee is. Just remove the lower dash cover.

[evildky]

the 87-89 had the speed sensor bolted to the trans, I believe the early cars (84-86) with digital dashes have the remote speed senson at the end of the cable.

[bcar240]

Lot of good info and points to check here, thanks! The only thing that I'm still pretty worried about is that if there is nothing connecting the dash and the hvac, I wonder what the heck is the issue with the climate control. Maybe just a wonky hvac unit? Are there any known issues with those to check like the soldering on the power supply?

[evildky]

http://www.xenonz31.com/DigitalDashRepair.html

it wouldn't hurt to pull the dash and check all the electrical connectors and grounds

[vh45300zx]

Also helped myself, so thanks.
Bob

[AZ-ZBum]

the 87-89 had the speed sensor bolted to the trans, I believe the early cars (84-86) with digital dashes have the remote speed senson at the end of the cable.

Only for the analog cluster models. ALL US Z31 digital cluster models have the speed sensor on the right side strut tower.