When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Recently I have been tracking down a no start issue with my 1984 Porsche 944. I believe I have tracked down the issue to be that the ECU/DME isnt getting any signal from the speed sensor.
Using the guide from Clarks Garage, I traced down the cables coming from the sensors to test their resistances using this image:
The reference sensor pin 25-26 reading was completely normal, about 950 Ohm, but the speed sensor reading pin 8-27 was far too high, measured in M-ohms type high.
The strange part however was that I was not able to get a reading on neither 25-78 or 8-23. My multimeter simply wouldnt detect anything, as though there wasnt even a complete circuit. I am already planning to replace the speed sensor due to its strange readings and my tachometer not moving, but would this be cause for concern enough to also replace the reference sensor?
Recently I have been tracking down a no start issue with my 1984 Porsche 944. I believe I have tracked down the issue to be that the ECU/DME isnt getting any signal from the speed sensor.
Using the guide from Clarks Garage, I traced down the cables coming from the sensors to test their resistances using this image:
The reference sensor pin 25-26 reading was completely normal, about 950 Ohm, but the speed sensor reading pin 8-27 was far too high, measured in M-ohms type high.
The strange part however was that I was not able to get a reading on neither 25-78 or 8-23. My multimeter simply wouldnt detect anything, as though there wasnt even a complete circuit. I am already planning to replace the speed sensor due to its strange readings and my tachometer not moving, but would this be cause for concern enough to also replace the reference sensor?
Thanks in advance!
When you say you could not get a reading for 25-78 or 8-23, your multimeter should read either of the two: 0 ohms or infinite ohms. Which one are you getting? 0 ohms would mean a closed circuit (wires touching each other or something broken in the sensor), infinite (or 999M it depends on the multimeter model) would mean an open circuit (that's what I understand from your posting, just wanted to confirm). It might just be you multimeter that considers anything higher than 1M ohms as an open circuit. As per Clark's spec, this should be ok.
Martin
Last edited by choupette.944; 04-05-2024 at 11:03 AM.
Hello Martin, thank you very much for your help. I thought the display showed 0, but it appears it showed an O, indicating that the reading was too high, ergo infinity. Thats why you read your manuals I suppose.
Then I will be replacing the speed sensor only to begin with, and hopefully Ill be out on the roads soon again.
Passing the tests given in Clarks Garage are no guarantee that the sensor is working properly.
The only test that guarantees that the sensor is working is to use an oscilloscope as documented in the factory workshop manual, and compare the waveforms to the waveforms in the manual.
Passing the tests given in Clarks Garage are no guarantee that the sensor is working properly.
The only test that guarantees that the sensor is working is to use an oscilloscope as documented in the factory workshop manual, and compare the waveforms to the waveforms in the manual.
You are right, however but getting bad ohms values, might indicate that the sensor is bad. But as you said, getting the right ohms values doesn't mean they work correctly.
Thank you for your help everyone. On friday I installed the new sensor, and the car now runs like a charm. The tachometer seems somewhat blocked, since it initially locked at about 2.3k RPM, and now after about an hour of driving at 2.6k RPM. I am just assuming that this is a physical block, and one that I honestly cannot be bothered to fix since the car still feels properly timed above those RPM values.
04-05-2024, 08:04 AM
944 speed and reference sensors - help!
