W124: no speedometer, but odometer works
 

Hi,

I've been fighting with a strange problem for the past week.
I have a W124 1992 300D with om603 and 722.3.
I've been using the original speedo cable that is a few centimeters too short for this combo, so it got kinked and had some low speed wobble.

Decided it's time to replace it, so installed a proper 148cm cable from low miles car, pulled the cluster and repainted the needles.

Now what's strange: The speedometer needle wouldn't budge. Even if the cable is not attached to tranny pan.
I know that my tranny tail end plastic socket for the cable is worn and aftermarket cables wobble like crazy, so I go only with genuine.

But the strangest thing is that odometer turns, so the cable must be turning. When I push the lower left or right corner of my instrument cluster, the speedo needle immediately jumps back to life.

Since this is W124, the odo is mechanical, but speedo needle is driven by rotating magnet in the speedo head. I replaced the whole speedometer, same issue. I replaced the speedo head with a low miles one, same issue.

I thought it is something electrical in the cluster, because my low washer fluid light went off and the oil pressure needle jumped up only after 10 seconds or so. I opened the cluster again, resoldered the pins, changed out the bulbs. It fixed the light, oil pressure gauge and speedo needle upon installation, but when I buttoned everything up, the speedo problem came back.

So when I start driving, odo turns, but needle doesn't. Push on cluster, both work. I stop at the lights for more than 20 seconds, the needle is dead again, I have to press it again.

Do you have any suggestions what's the problem?
Broken speedo cable?
Bad ground in cluster?
Bad harness? (I checked wires and everything is shiny and nice)

Cruise control works and everything else. Don't tell me it's the needle paint ;)
I'm planning on borrowing a whole cluster and testing. I got the tail end socket ordered and will be getting a new cable probably.
Thanks

Could the speedometer needle just be spinning on the axis? IE not tight?

Nope, because it works outside the car with a drill, or hand.
Besides, I tested another speedo which was never taken apart - same story.

I just tested a 300E cluster, that worked all the time, although I couldn't push it all the way into the dash.
This leads me to believe it's my cluster left side gauge fault, that distributes power and grounds across the cluster. But everything checks out fine inside :confused:

Edit: it seems that removing the big round 15-pole connector from cluster fixes the speedo issue. So the culprit would be left gauge, somehow. I'll let the car sit overnight and test again.
What's weird, it just takes pressing the cluster to make it work, or go over a speed bump fast.
I also took a spare speedo and connected it directly to speedo cable, without cluster. Works great.

Edit 2: so this morning I drove the car to work and.....speedo dead :( no turn signals either, but with the big connector off, that may be expected. Pushing the cluster fixed it temporarily.

Does anyone know what voltage reading should show on the circular plug for VSS with green wire, behind the cluster? Mine shows 7.7V with key in position 2, seems weird. I thought it's speed output.

Cruise control module plug shows 10V DC at pin 11 (speed input).

All wiring around looks good.

I replaced the speedo cable again, nothing helped.

So:
- checked fuses,
- checked cluster PCB
- checked with another speedo cable
- checked with round plug disconnected
- checked with round plug connected, but everything else disconnected.
- replaced speedo with another good unit,
- replaced cluster enclosure,
= all of this causes the same issue, no speedo, but odo works. Since odo is mechanical, that means it is receiving the cable input.

However, speedo works in 3 circumstances:
- when tested in cluster outside the car,
- when placed in car without the cluster
- when cluster is gently pushed on the left side. When you stop the car for more than 10 seconds, it stops working.
- when you go over speed bump fast, it starts working, alternatively slap the dash above speedo.
I also connected a 300E cluster, but did not push it in. Speedo worked every time.

Since odometer is working, the transmission end must be getting good grip. Waiting on used gauge set, will post results. Probably the problem lies in there.

Issue resolved. New gauges did not fix it.

I replaced the instrument cluster enclosure and everything worked.
When I put back my renewed enclosure (which had the front glass knocked out and glued back), the issue came back. However, I wiggled it a bit in place and everything works now. It seems that the curvature of the enclosure affects pressure on ground points or something. It is weird, but it worked. It's not a mechanical issue related to the flexible shaft.

Just in case someone encounters similar issue.