Rob Frantz

Hi Everyone!

I'm going through the process of restoring a 1989 928 auto and I've got some gremlins popping up that are really stumping me. I've been trolling the forum for a long time gleaning all the info I can about these awesome cars but I've finally hit a point where I need to ask for help. This is my first time posting, so I want to thank everyone who're posted in the past because it's been a HUGE help.

Now for my gremlins:

1. The car runs a little rough and is hesitant to engage the transmission when I press on the gas (both in drive and reverse) from a stopped position. I already adjusted all 4 wires on the throttle quadrant, but it may need more. The attachment ball of the Bowden Cable is almost to the end of the threading, so I don't know how much more I can tighten it. I have already replaced the spark plugs, plug wires, distributors, and rotors. Once the car gets moving, it seems to engage the transmission normally.

2. The Odometer, Speedometer, Tachometer, and oil pressure gauge are not working correctly. The odometer is not increasing (assume it's because of the bad speedometer). Both the analog and digital speedometers are not working and no miles to empty given on digital display. The ABS warning light is NOT on.

When I start the car, the tach jumps to about 2900 RPM and stays there. If I rev the engine pretty high the tach drops, going to zero if rpm increases further.

The oil pressure gauge seems overly touchy. When idling it sits at about the 2. As soon as I touch the gas pedal, it pegs to 5. If I let off the gas it goes back to ~2.

I've cleaned off the speedometer connections in the back and replaced fuse 24, 25, and some others that looked questionable, but I haven't checked the sensor itself. I've also pulled out the instrument pod, cleaned off all the pins in it, and cleaned off all the ground connections in the spare tire well and the steering column.

The car starts right up each time, so I have no idea whether these problems are all related or just coincidental. The car was sitting unused for about 3 years before I got to it, although the PO did recently change the fuel pump and filter. Does this sound like one of the computers could be bad? Or possibly a short somewhere?

I plan on replacing all the relays and fuses, and cleaning all the grounds to see of that does it, but any help from you experts would be appreciated.

Thanks.

Bilal928S4

Did you make sure that the 4 connectors in the back of the cluster are fully seated and locked. If they are not seated properly then you can get erratic instrumentation behavior.

Rob Frantz

Yeah, I double-checked the connections back there to make sure they were well seated without any play. The instrument behavior was the same as before I took the pod out.

Bilal928S4

Had anybody been in there before?
Are you sure that all the connectors are connected to the correct terminal on the cluster board?

Rob Frantz

The main wiring panel looked like it had never been touched before, but the PO did put in an aftermarket radio and the A/C control panel appears to have no power going to it (fan selection **** does nothing, but defrost blower works). I'll check the back to see if maybe there's something tapped back there causing a problem.

auzivision

Hi Rob, welcome to the tank.

One thing about the autos is they are very quirky if not completely full of fluid. Not just "in the window", but at the "top of the window". The procedure to check isn't too bad... filling is a little more complicated. In both cases the vehicle needs to be level and running. Quirks are typically related to 'slippage'. However, you stated "hesitant"... as in perhaps slow to engage when shift in from neutral... that would be normal. If in gear already and just at a stop and you hit the go pedal and it hesitates then bang... takes off, that could be low fluid.

Can't help much with your pod (hopefully Alan or others will chime in), but speculate that it still could be due to a faulty ground, or a fuse... or maybe even a relay (928's virtually have a relay for everything, not sure about the pod, guessing not since it's mainly electronic as opposed to electrical). The fact that so many systems aren't working makes me think it's not all the sensors. There is a good article on cleaning all the ground point in the "Must Read section" for newbies... found here:

https://rennlist.com/forums/928-foru...questions.html

specifically this one:
https://rennlist.com/forums/2220328-post33.html

That might be a good place to start,

Rob Frantz

Hi Kurt, I'm glad to be here!

The symptom is exactly as you stated: Car is stopped, already in gear, hit the go peddle and nothing for a second, then Bang - off to the races. I had forgotten that the PO stated all fluids, including the AT fluid, had been changed recently, so I'm guessing the AT fluid was not topped off properly. I definitely never would have thought of that being an issue. Thanks!

auzivision

Yup, that is classic low ATF behavior.

You can check it easy enough with a mirror... Rough location to position mirror:
https://rennlist.com/forums/attachme...f-location.jpg

What to look for in mirror:
https://rennlist.com/forums/attachme...-reservior.jpg

Note how my fluid is all they way to the top of window. Mine starts to slip when it's about .25 inch down going around corners and all the time at about .5 inch. Hasn't slipped in years now that I keep it topped off. Also, I run a little tran-x based on advice from Bill Ball... stopped an annoying drip/leak at torque converter.

Best to clean up and around the reservoir bottle before opening (with engine running or you'll be sorry... fluid will come out when not running) and pick up a cheap gear oil pump something like this:




Lots of discussion on what fluid to use, not much on how to check/fill. I've had good luck with Mobil 1 Synthetic:

https://rennlist.com/forums/928-foru...o-use-now.html
https://rennlist.com/forums/928-foru...ion-fluid.html

Rob Frantz

Kurt, you're awesome. I had a chance finally to top off the ATF and that worked like a charm. Thanks!

Now for the rest...

soontobered84

Check Fuse 24 for your instrumentation. Diagnostic port fuse.

Rob Frantz

I've replaced fuse 24 (had some charting in the fuse connection on the board). I did some wire tracing and testing for the tach problem and found some interesting stuff, assuming I tested correctly. Without taking the pod out again, I checked for voltage at diagnostic pin 14, CE panel wire M11, and wire W11 (tach signal from LH) all have a constant 5.8 volts, no matter what I do with the peddle. I assume this is bad, but I have no reference to compare. Anyone have ideas on what might be causing this? Am I measuring incorrectly? Could there be a bad wire on the CE panel?

I also removed the kick down relay and that didn't make a difference. Any ideas would be helfpul.

Thanks.

Gary Knox

Be sure fuse 5 is good. It controls speedometer, brake lights, and some other stuff.

The Forgotten On

The A/C not working correctly is a relay pack located in the back of the main A/C system housing right behind a rubber connector that connects the blower motor to the A/C system. It is relatively common to have happen on these cars and is well documented in the forums. Part number is 92868610100. Good luck.

Rob Frantz

Thanks, I'll take a look. From what I've read it seems like that resistor pack's kind of a pain to get to.