Need guidance on troubleshooting speedometer problem...

[agfours]

I bought an 87 coupe about a month ago and the speedometer has not worked since I've owned it. Here's where I am in the troubleshooting and where I'm stuck....

Situation:
No speedo, no odometer, no tripometer function.

Lights to speedo work, and I understand this means I have good ground. (however there are additional brown wires going to the speedo which do not seem to have ground...have not been able to figure out what these wires go to...cruise?)

I replaced the sensor twice already (went back to this as a possible cause due to a bad part as I understand is not uncommon)

Based on a tip in another post, i tapped the contacts together under the access panel and the needle jumps, and assumed understanding was that this indicated that the sensor was at fault, and all was good between the access panel wires to the speedo. Also ran continuity checks on the speedo wires to the access panel and those checked out ok.

So, i must be missing something or there is a fault in my troubleshooting logic. I.e. new sensor(s), rebuilt speedo verified with bench test and needle jumping test in situ, continuity on wiring, ground gives me lights thus should provide for the speedo function,....

Other information and looking for comments:
1. Auto heat not working...I understand that this is tied into the speedo for some reason....possible cause? Is there a way to eliminate this from the speedo circuit for testing?
2. Cruise is inop due to broken cable, and could also potentially be not working electrically
3. There are two wires in a dedicated sheath, one brown, one black with brown stripe that do not appear to be connected to anything (other end dangles in the

I will post a shot of the wiring on the speedo for any sharp eyes out there...

[agfours]

Here's a shot of the wiring in the back of the speedo... S+ stands for signal input.

[porwolf]

For comparison: Here is a picture of my '79SC speedometer electrical connections. I I took the picture when I had the odometer worm gear replaced.

[agfours]

Problem solved!!!!! Just wanted to provide a little closure for future generations that might encounter this same issue....

Did a lot of deep research tonight on the forum after reconfirming my earlier findings regarding the electrics and path of logic...took me deeper into examining the posts related to the actual magnetic carrier ring that is attached to the diff that sends magnetic pulses to the sender on the side of the tranny. I found a deep post that someone had found that the replacement senders from VDO are a little different in spec than one that you might get from Porsche. Evidently the newer ones sit deeper into the recess or somehow don't make good pickup from the disc on the diff. The poster had used a piece of heater hose between the diff and the sending unit and got good results, albeit a little less Germanic than intended from the factory.

Well, I decided to try the same idea. I found a little nylon bushing that perfectly fit the diameter of the sending unit where the unit goes into the hole in the tranny. I slipped it over the o-ring to act as a washer about a quarter of an inch thick between the sending unit and the body of the tranny. Worked like a charm. Hooked up the volt meter to the spades in the access tunnel inside the car and started to get a signal when I spun the wheel hub!!! Road test confirmed proper operation, and speed registering correctly per GPS app on my phone.

Let me tell you, again, just a testament to the power of this board and the depth of resource it is! Who knows how much that would have cost in diagnosis at a shop...

Interesting footnote here is that my expert wrench here in town said "probably the sending unit...those are the case 90% of the time...". Well, yes and no. I went back and tested the original sending unit, the first replacement, and the 2nd replacement now in the car, and all worked fine using a volt meter and a hand held magnet waved over the sending unit...just use ohm setting and the circuit will close (ohm out) across the two wires when the magnet passes over the sensor....shoulda researched that to find that test first time around I guess...20/20...

Guess I'll see if I can return one of these sending units....

Thanks to Porwolf for the reply as well!

[agfours]

Never mind me...just having a little chat with myself on Pelican. Update: the nylon bushing I used did not seal adequately and so i have replaced that with a rubber ring with square shoulders and about 1/8" thick. Inner diameter more like a 3/8" hose so was a tight fit over the sensor and took some fanagleing to get it over the o-ring on the shoulder so that the o-ring would not gather...but, should create a seal. Also used a couple of washers between the case and the clip to distance the clip by the same thickness as the new o-ring (which is acting as a spacer between the case and the sensor). ....carry on....

[porwolf]

Thank you for posting the solution to the problem. That is a major help and great contibution to the Pelican forum.

[rinverso]

kudo's to you. thanks for the solution. i ofter talk to myself on this forum also. i guess sometimes my problems are not interesting enough sometimes for the readers. but it is always nice to get a response. side note my son lives in Austin, we love visiting. hoping to go to next years gran prix.

[88911coupe]

I think it was me...I figured out by accident that it only worked when partially inserted. I used a piece of heater hose but I like your idea better, sounds more stable. I also found out that my '88, along with a VERY small number of others, only has FOUR magnets...not 8 like everyone else. Works fine now though.

[agfours]

Rinverso, if you make it to Austin for GP or F1, you should come with our PCA chapter (Hill Country PCA)!

88911coupe, yes! It was your ingenuity that solved this for us all and inspired my solution! I cant say enough thank yous!

Cheers!

Sent from my SCH-I535 using Tapatalk 2

[88911coupe]

I nearly signed up for the PCA event but was not sure the people I was with were interested. Will make it a point to push harder for that next year. Had a blast though at the race and the weekend. Definitely a "must do"!