Early 944 Temp Sensor Issues

[delgadostarside]

Today i regapped everything as it should be, 0.8mm to the Starter ring gear on the Speed sensor. Checked with an O-scope for good voltage before putting the fuel pump fuse back in and nothing. Same thing only running on 2 cylinders then it died quickly. Then i remember you posting this...

There is a note in Volume 1 which states "It is important that the reference oscillation starts with a positive slope..." while in your oscilloscope video it starts with a negative. This is likely due to probing it opposite, and I can't tell from either the schematic or the manual which way it should be probed. It is of note though that they thought it important enough to mention. Hopefully you did not swap wires around while changing the harness.

Threw my probe on, one on DME pin 25/yellow (or white in my case since i re-wired), ground on DME pin 26/black and saw it was reversed as was in the video. When i saw you write "they thought it important enough to mention" it really bothered me all day until now. I flipped my probe and saw it was what you'd expect. Hm, i wondered. "What if I just reverse the two and see what happens". Well I'll tell you because Holy f***ing s*** this right here was the holy grail. Gave her a crank and IMMEDIATELY fires on all 4 no hesitation. I couldn't believe it.

Now i wanted to rip my hair out thinking i must've reverse my wiring as you say however... That is not the case. I traced it 4 TIMES because i seriously could not believe that i didn't screw this up. I cant figure out how to post an Image on this forum since Imgur wont let me copy a link for images. But i';m telling you with 100% certainty, DME pin 25 is going STRAIGHT to YELLOW on the Ref Sensor and DME pin 26 is going straight to BLACK on the Ref Sensor. If Porsche made it worth noting this, Did something change in the wiring between 83' and the years beyond?!?!? Maybe it was originally this way, and was reversed after this year? Sensors manufactured differently? Besides noting for incorrect sensor wires, why would they mention this?!?

About the noise though, I'm going to rule that one out since she runs great now and try not to think about it too much

I plan on doing a full write up on this in a few days so no one has to go through this nightmare ever again. It's too late to do more testing so tomorrow i want to compare the Ignition Coil and Ref sensor signal with and without reversing DME pins 25 and 26. This needs to have this documented somewhere online. But wow what a relief its over. Had to take a a few shots after this one to celebrate.

[blueline]

Tom's pic posting guide:
https://www.carpokes.com/viewtopic.php?t=410

Post from saved images rather than urls.

Regarding Imgur, save the pic to locally on your PC and then it's easy to post on Carpokes. Or, screenshot the image and save it to your PC (or other device) and post the same way you would a photo.

If still problems, let us know and will try to help figure it out.

[delgadostarside]

Thankyou blueline! That's so much easier to post i don't know how i missed this before!

Heres a pic of my DME harness and the Aftermarket connectors i use for the Sensors. (I know it's not the OEM ones but i couldn't be bothered to buy the BOSCHE connectors when i did this 6 months ago.).

IMG_1294(1).jpeg (154.85 KiB) Viewed 3047 times

Heres the Speed Sensor

IMG_1296.jpeg (1.74 MiB) Viewed 3047 times

Heres the Ref Sensor REVERSED (This is after I got it running on all 4 today)

IMG_1295.jpeg (1.66 MiB) Viewed 3047 times

You'll notice how close the pins are on the ref sensor that's because i ran out of the housings and i just having the pins together for the test i did.

(Ignore the crusty brake level sender cable)

[Tom]

Awesome! Congrats. I saw that comment, but wasn't able to follow the scope enough to comment. A hundred Carpokes points goes to @944er !!! When I tested on the bench, I confirmed the only way for the slope to start down is to reverse the wires. Spinning the flywheel backwards didn't do it. What brand is the sensor? Are both sensors the same brand? Someone went through something kind of similar with brand new FAE sensors that were incorrectly wired and grounded the pulse to 0. Part of the issue is the aftermarket mfg try to cover a lot of cars with one part, and may not realize subtleties like polarity really can matter. Anyway -- awesome find and definitely one for the mental databank!

[delgadostarside]

They're actually Bosch OEM sensors i picked up on pelican (Part#: 944-606-115-00-M14). Definitely OEM with the part number written on the sensors. The backup sensor i bought off of Ebay which was supposed to be a Bosch OEM but most definitely was not, no part number markings and it was missing the different colored plastic washer at the wire outlet. Even worse, these ebay ones showed a lower signal voltage without adjusting sensor clearance and just swapping with the Bosch OEM i got from pelican. Alot of this was on me for missing that part of the Porsche manual but i feel like these parts are the ones that got me here.

[944er]

Happy that I could contribute. I enjoy looking into these vehicle problems as puzzles, and they have taught me so much about the car that has been in my garage for 40 years, even if nothing is currently broken.

As to why this works, it is possible that the schematic is incorrect (wouldn't be the first time) and in the factory engine harness the wires are reversed on the reference sensor.

[Tom]

Here's a possibility maybe... The factory wiring diagrams show the colors of the wires in the factory engine harness. Yellow to 25 and black to 26 as you say. However, the speed and reference sensors are shipped as a sealed unit with no 'spec' for the wire colors inside. They could be purple and pink for all it mattered, as long as they terminate in the right spot on their original connector. Any chance you just assumed the colors matched up? From your report, seems pretty clear they don't. Did the sensors come with a factory style JPT connector and, if so, did you check to see which wires went to 25 and 26?