fltechpilot

Looking for any pointers on a failed speedo that still doesn't work after replacing the printed circuit board with a shiny new one.

The car is a 1988 S4. I started having a feeling the printed circuit board wasn't going to fix the problem when I tested the continuity between the old printed board and the speedo pins and it all checked out. It's always nice to find something broken that you expected to be broken on a project. Unfortunately that wasn't the case. Also checked the speedo itself for continuity and all seemed fine.

My next step is to jumper the speedo pins that go to the gauge cluster and check the continuity on the plug in the trunk. If I have continuity there then that should mean that there is continuity throughout the whole circuit, correct?

Anyway, if anyone has any pointers or pics of the connectors in the trunk that would be great. My concept of electricity is pretty lacking so any help would be great!

Mrmerlin

Ok when was the last time the speedo was working?
What was the last thing you fixed?.

fltechpilot

It's actually my parents' car and I am working on it a bit while visiting them on vacation. The speedometer didn't work when they bought the car last summer. So I don't have much info to go off of there.

Mrmerlin

OK how many miles on the speedo?
was the speedo gear replaced?
was the transmission replaced?
Was any work done in the spare tire well?
was the pod worked on?

fltechpilot

To the best of my knowledge the car has roughly 82,000 miles on it. When I did the work, I was planning on replacing the odometer gear, but saw that someone had already put in a replacement rennbay gear. This made me a little nervous because I knew that someone had been in there and have no idea of the caliber of work that they performed.
Original tranny to the best of my knowledge.
There was an aftermarket sound system installed by one of the PO's that was professionally installed (have the receipts) so they were working around that spare tire well.

I went back out to the car and checked out the connector in the spare tire well and there seems to be continuity between the wires and the plugs. Next step is to jumper the pins on the wiring harness up at the gauge cluster and check for continuity at the spare tire well. This should establish whether or not there is continuity through the whole circuit. By any chance. Do you know which pins I should jumper?

If I get continuity through the whole circuit I guess I'm looking at a fault with the gauge itself or the impulse sender at the diff.

Wisconsin Joe

I think you can put an ohm meter across the plug (trans side) and see if the impulse sender is working.
The sender closes the circuit each time it 'registers', so the ohm meter needle (or readout) should twitch from open to closed as it sends a signal.

I'm working off of memory of posts on here. I did a fair amount of research on speedo repair for mine, but it turned out to be bad gear and bad connection at the rivets, so I never went into the sender or wiring in back.

If I'm wrong, Alan or one of the other experts will correct me, I'm sure.

TrishB

Fltechpilot 's Mom here. It's my car that the question is about. I bought the car in July 2014. The initial drive home was a little over an hour and the speedo did bump up off the needle a couple of times (at highway speeds). The PO did alert me to the fact that the speedo/odometer did have issues. Since having the car, we did the timing/accessory belts. I was able to get a couple of small rides in before this endless winter started. The speedo did move a couple of times since the belts were changed, always enough to get off the needle but never up to the actual speed of the car.
I lurk here a lot, I have so much to learn, but thank you all for your input.

Mrmerlin

Hi Trish thanks for being a caretaker of a fine machine,
I was asking questions to give you my best guess on where to begin looking.

So ,since your getting some jumps,
that would indicate that the trans sensor is working,
and its signal is being sent to the pod,
and the pod connection is good .

Or any one of these connections could have a loose connection.

I would start at the spare tire well and deoxit all of the connections as well as the grounds,

find the plug for the speedo feed on the bottom of the CE panel and do these as well.

Usually the loss of signal is at the push pins on the back of the pod or the slide on connectors,
so make sure to deoxit these as well, Do the slide on connectors too.

Take the pod out and take the speedo apart,
inspect the motor for the odometer,
inspect the row of number gear wheels where the new gear was replaced.

NOTE if there is play within the gears and their usually is,
then a thin washer should be added to this stack to remove the play,

NOTE the looseness can cause the gears to bind and stop working.

Also inspect the new gear teeth a magnifying glass is essential for this job.

NOTE due to improperly made replacement gears sometimes molding flash is included on one or more of the teeth,
this flash is enough to cause the odo gear to bind.

NOTE if its possible get a spare pod and plug it into your car and test it,
this should quickly tell you where to look

Please report your findings

TrishB

Thank you for sharing your thoughts. We are working out a game plan now. Will definitely let you know the results.

TrishB

Sorry for the delay in checking back in but it's been far too cold to work on the car until yesterday.
After putting a meter on to check continuity, we were able to narrow down the issue to the sending unit.
One of the connections had become completely detached. Removed the sending unit and re-soldered the connection. Once we put everything back together, we tested it and I finally have a working speedometer! Thank you for your advice and a HUGE thank you to my own private Pit Crew.