90 S4 speedometer not working (also cruise control)
Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
90 S4 speedometer not working (also cruise control)
Hi!
I have a 1990 Porsche 928 S4. The speedometer is not working. I replaced the reed switch on the differential housing (brand new) but it still doesn't work (waste of money). I jacked up the car, disassembled the tachometer and checked the cables. A multimeter continuity test shows that the reed switch and cables are fine (it beeps when I spin the rear wheel). Everything on the speedometer works except for the km/h (and of course the cruise control doesn't work either).
What could be the problem? I opened the speedometer (it's the newer digital version of the S4) and I don't see any burnt or broken parts.
Any ideas?
I have a 1990 Porsche 928 S4. The speedometer is not working. I replaced the reed switch on the differential housing (brand new) but it still doesn't work (waste of money). I jacked up the car, disassembled the tachometer and checked the cables. A multimeter continuity test shows that the reed switch and cables are fine (it beeps when I spin the rear wheel). Everything on the speedometer works except for the km/h (and of course the cruise control doesn't work either).
What could be the problem? I opened the speedometer (it's the newer digital version of the S4) and I don't see any burnt or broken parts.
Any ideas?
Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
And there is no ABS error.
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Joined: Aug 2005
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From: Riyadh, KSA
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Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
So, I took the dash apart and replaced all the capacitors, but that didn't help. However, the digital part of the dash became more contrasty and brighter. So the capacitors had to be replaced 
The speedometer still doesn't work. Everything on the speedometer works except for the km/h.
Does anyone know what voltage the reed sensor power supply should be? I measured 5V, but I read on several forums that it should be 12V instead of 5V. But these are older versions of the 928. Mine is a 1990 S4 with a digital speedometer, but it still uses the magnetic reed switch in the differential to read the vehicle speed. Maybe the speedometer has a fault and only has 5V instead of 12V. Maybe. But maybe this is a stupid idea because I have no other ideas
By the way, where is the cruise control module in this 89-90 S4? I read several forums and it should be under the center console after the radio, but I can't find it. There aren't any unused wires/connectors there, so it should be located somewhere else. The car has the CC controls and wiring behind the steering wheel, so there should be a CC amplifier.
I checked the wiring diagrams and the CC uses the same wire to read the speed, and it's possible that a CC failure could be causing the speedometer to fail. Another one maybe...
So, does anyone know the pull-up voltage for the differential reed switch sensor on a 90S4 with a digital speedometer, and where is the CC amplifier located in this car?
Thanks in advance.
The speedometer still doesn't work. Everything on the speedometer works except for the km/h.
Does anyone know what voltage the reed sensor power supply should be? I measured 5V, but I read on several forums that it should be 12V instead of 5V. But these are older versions of the 928. Mine is a 1990 S4 with a digital speedometer, but it still uses the magnetic reed switch in the differential to read the vehicle speed. Maybe the speedometer has a fault and only has 5V instead of 12V. Maybe. But maybe this is a stupid idea because I have no other ideas
By the way, where is the cruise control module in this 89-90 S4? I read several forums and it should be under the center console after the radio, but I can't find it. There aren't any unused wires/connectors there, so it should be located somewhere else. The car has the CC controls and wiring behind the steering wheel, so there should be a CC amplifier.
I checked the wiring diagrams and the CC uses the same wire to read the speed, and it's possible that a CC failure could be causing the speedometer to fail. Another one maybe...
So, does anyone know the pull-up voltage for the differential reed switch sensor on a 90S4 with a digital speedometer, and where is the CC amplifier located in this car?
Thanks in advance.
Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
Hi everyone,
The speeod finally fixed and I thought I’d share my findings in case it helps someone in the future.
When I bought the car, both the cruise control and the speedometer were not working. I suspect the previous owner had already tried to diagnose it but didn’t find the cause.
Here’s what I did:
Hope this helps someone in the future. Thanks for all the suggestions earlier!
The speeod finally fixed and I thought I’d share my findings in case it helps someone in the future.
When I bought the car, both the cruise control and the speedometer were not working. I suspect the previous owner had already tried to diagnose it but didn’t find the cause.
Here’s what I did:
- First, I replaced the reed switch in the differential — no change.
- Then I disassembled the speedometer and replaced all the capacitors and the comparator IC connected to the VSS input — still no change.
- I thought maybe the magnets in the differential had fallen off, so the reed sensor couldn’t detect them. While looking for another diff online, I realized that my sensor had been installed upside down (probably by the previous owner). I reinstalled it correctly, checked continuity, everything looked good — but still no speedo.
- Next, I measured the sensor’s voltage feed — it was barely 5 V, which is too low. Checking the wiring, I found that the VSS feed is shared between the speedometer and the cruise control. Since the speedo has its own pull-up and capacitors, the signal should be around 10 V. I suspected the cruise control module was dragging the voltage down.
- I unplugged the cruise control connector — and the speedometer immediately started working perfectly.
- The VSS sensor was installed upside down.
- The speedometer capacitors were dried out.
- The cruise control module was faulty and pulling down the signal voltage.
Hope this helps someone in the future. Thanks for all the suggestions earlier!
Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
Everything is working :)
Hi,
Another update. My last post was about the speedometer finally working after I fixed the sensor position and replaced its capacitors. But when I reconnected the cruise control module, the speedometer stopped showing any speed again. So I suspected the root cause was actually a faulty CC module.
I measured the VSS sensor feed voltage: with the CC module disconnected it was around 10V, but once I plugged the CC module back in, it dropped below 5V, which is not enough.
I replaced the capacitors in the CC module and tried to trace the faulty component, but I ran out of luck.
I visited a local Porsche wrecker (they have a huge amount of transaxle Porsche parts), and they were very kind — they gave me two CC modules for testing for free. This morning I installed one of them, and everything is working as expected. I measured the feed voltage again and it's still around 10V, and during the test drive both the speedometer and the cruise control worked perfectly.
It was a long journey, but luckily I finally found the issue.
Another update. My last post was about the speedometer finally working after I fixed the sensor position and replaced its capacitors. But when I reconnected the cruise control module, the speedometer stopped showing any speed again. So I suspected the root cause was actually a faulty CC module.
I measured the VSS sensor feed voltage: with the CC module disconnected it was around 10V, but once I plugged the CC module back in, it dropped below 5V, which is not enough.
I replaced the capacitors in the CC module and tried to trace the faulty component, but I ran out of luck.
I visited a local Porsche wrecker (they have a huge amount of transaxle Porsche parts), and they were very kind — they gave me two CC modules for testing for free. This morning I installed one of them, and everything is working as expected. I measured the feed voltage again and it's still around 10V, and during the test drive both the speedometer and the cruise control worked perfectly.
It was a long journey, but luckily I finally found the issue.
Team Owner
Joined: May 2004
Posts: 31,335
Likes: 4,290
From: Philly PA
Im glad you found a resolution.
FWIW if your car has been started with a low battery,
or a booster charger has been used to start the engine.
Then its very possible that this event could have damaged the CC module,
other modules that can be damaged are the LH,
the cooling fans controller, the warning lamp system computer,
the ABS relay on the CE panel , the alternator diodes and regulator.
Bottom line keep the battery charged,
charge it without the rest of the charging system attached.
I use a 6 / 2 amp charging device.
It would be better to disconnect the battery ground strap,
if the car will not be used for period of a week or longer,
as opposed to using the battery tender.
FWIW if your car has been started with a low battery,
or a booster charger has been used to start the engine.
Then its very possible that this event could have damaged the CC module,
other modules that can be damaged are the LH,
the cooling fans controller, the warning lamp system computer,
the ABS relay on the CE panel , the alternator diodes and regulator.
Bottom line keep the battery charged,
charge it without the rest of the charging system attached.
I use a 6 / 2 amp charging device.
It would be better to disconnect the battery ground strap,
if the car will not be used for period of a week or longer,
as opposed to using the battery tender.
Thread Starter
Track Day
Joined: Sep 2025
Posts: 16
Likes: 10
Im glad you found a resolution.
FWIW if your car has been started with a low battery,
or a booster charger has been used to start the engine.
Then its very possible that this event could have damaged the CC module,
other modules that can be damaged are the LH,
the cooling fans controller, the warning lamp system computer,
the ABS relay on the CE panel , the alternator diodes and regulator.
Bottom line keep the battery charged,
charge it without the rest of the charging system attached.
I use a 6 / 2 amp charging device.
It would be better to disconnect the battery ground strap,
if the car will not be used for period of a week or longer,
as opposed to using the battery tender.
FWIW if your car has been started with a low battery,
or a booster charger has been used to start the engine.
Then its very possible that this event could have damaged the CC module,
other modules that can be damaged are the LH,
the cooling fans controller, the warning lamp system computer,
the ABS relay on the CE panel , the alternator diodes and regulator.
Bottom line keep the battery charged,
charge it without the rest of the charging system attached.
I use a 6 / 2 amp charging device.
It would be better to disconnect the battery ground strap,
if the car will not be used for period of a week or longer,
as opposed to using the battery tender.