checkmate1996

I've been chasing down a speedo issue with a 1985 Euro S on and off for 3 years now. (See details from another thread here) Here's the consolidated backstory as it may help others and current state in shortened form:

**Situation: PO installed an LSD from an 87+ into the 85 transmission Box. Speedo no longer works. PO installed magnets around CV joint and re-positioned the reed pickup over the magnets as a partial fix. Speedo still registered off by about ~10mph**

• Problem: The Reed magnet no longer picks up the magnet carrier. No Pulse signals from the magnet were being sent to the speedo.
• Solution: Obtained a later model differential plate that matched up with the magnet carrier. (Trust me, this took a lot longer to solve than it appears!!) Pulse signals now can be picked up by DMM. (Wait...more to come on this one)

• Problem: Speedo still registers 50% of the current GPS speed.
• Experiment #1: Try another Speedo. No Difference.
• Experiment #2 : Replace original flexible circuit board with new one. Looks pretty. No difference.
• Experiment #3: Position the magnet into different positions.Made it worse. Only picked up 2 tones.

Back to the drawing board. Start Troubleshooting again:

• Confirmed: Magnet Carried has 8 magnets.
• Confirmed: Speedo has a new Odometer gear. Odometer gear matches up with partner gear to make '70' total teeth. 49+21. (Odo gear)

Back to the drawing board...

• Troubleshooting #1: On my other 1985 euro Auto, it has a working speedo. Measured 4 LONG test tones with DMM.
  See video here
  .
• Troubleshooting #2: On the euro that is not working correctly, it measured 4 SHORT test tones or chirps with DMM .
  See video here
  .

Question: What does the different tones mean? Is this contributing to the speedo issue? Do I have a magnet or carrier problem?


I appreciate your comments!

NewToPig

Do you need to invert the signal? One car is long connection short break and the other is the opposite.

checkmate1996

I performed the test the same way on both cars plus we are just measuring open and closing resistance. Unless you are thinking something else. best!

NewToPig

The signals are inverted so if the car is sensing the break as the signal (instead of the connection similar to how a coil only fires on the break) then the one that isn't right might be dwelling too short to trigger the read (hence the low reading). You could flip it with a transistor. I have no idea if there is a circuit difference between years. That's just my observation with the videos.

checkmate1996

interesting. So you are pointing to swapping out the impulse sender as a potential culprit?

copperstew

I've never opened one of these diffs, but just by hearing your magnet closing the reed switch makes me suspect that the "carrier disk" with the magnets is simply constructed differently.

Looking up the parts for the transmission, I find two different rings.

Carrier Disc With Magnets (UP TO YEAR 85) 92831802402
Carrier Disc With Magnets (FOR YEAR 1986 & Up) 92831802405

If these disks are different enough to require separate part numbers, do they have a common reed switch?

Early reed switch is 92860621000
Later reed switch is 92860621001

Could it be that the later reed switch is required for the later carrier disk?

Eplebnista

I would suggest getting a cheapo scope instead of listening to beeps: . I bought a similar one to diagnose tachometer issues and it worked fine.

This is a hack work-around so I expect criticism for suggesting it but you can fix your problem with one of these: Dakota Digital Speedo and Tach Interface.

Otherwise you either have the wrong number of magnets or the pickup is not working. I also hope that Porsche used a hall sensor and not a reed switch for the pickup but who knows what the technology was in 1978. Reed switches suck.

checkmate1996

Thanks for suggestions above as they gave me a few ideas to tackle. I did reach out to Tim at Dakota to see what they thought. Here was his response

This weekend I plan on:
- Changing out the impulse sender with the other car that is working
- Verifying the Speedo works via a Oscillisope
- Putting an Oscilloscope on the non-working car to determine the differences and measure. I'll post the results later.

Shirah

Lots of cars have 'improved' additions as mods and very few are 'purists' at this point in time. A mod is a mod.
I know an 89 S4 running with this Autometer GPS and it was a 3 wire connection. Wham! Bam! and done.
All 3 connections in the truck (Battery, Ground and purple wire to the OE Speedo's original lead).
Just sayin!

checkmate1996

Some quick updates-

the pulse sender in the good car had most current part as listed above but it also had an addition number with 3.85.

the non working car had most recent part number as listed above but also had 4.86.

one thing I noticed is the the newer differential cover that I installed is actually cross referenced to a 1991 model year ONLY. (928 301 306 06)0It is not the rear housing cover for 87-91 928 301 306 00(of which the lsd is from) It appears there is a 4mm difference between the two housing covers. I’m now wondering if that 4mm difference is enough to throw the pulse sender off and read just the outer edge of the magnet carrier. Thoughts??

=======EDIT 5/9/24======== I thought I had a 06 because there was a Porsche sticker on the cover but it turned out it was an 00. So I had the correct one all along.

Kevin in Atlanta

1991 was a year with PSD. In those years 90 through 95 speed was pulled off the ABS if memory serves.

So don't you need the correct differential cover?

FredR

Further to what Kevin posted PSD was introduced in MY1990 and even then the rear cover is different to the MY1991 and later covers in that the level tell tale is higher and thus fills with 3 litres of diff oil instead of the later model cover that fills at 1.9l of diff oil. When fitted to even earlier models I have no knowledge of whether that lower level inventory would have an impact on lubrication inventory but note that the PSD rotating assembly has a greater volume and thus will displace more oil when rotating than an earlier open diff unit.

As to what/why someone fittted a later diff cover that is not configured to carry a speed sensor signal is baffling and I dare say might well explain why you have an erroneous reading especially if some well intended DIY mechanic fitted the cover with mods to accept the earlier sensor given the castings are present but not kitted out for such.

checkmate1996

Pulse sender is 83-91 iirc. However, there are three rear housing covers listed for those years. The 83-85 didn’t work at all. When I got the cover from mark af 92& we thought it was for 87 but when I checked it again today it’s for a 91.

Kevin in Atlanta

Umm, no. According to AutoAtlanta the -90 (92830130600) 91- (92830130606)

checkmate1996

Fred, let me recap. The original PO put the LSD from an 87 and KEPT the original rear cover as IS. He didn't realize at the time pulse sender location was off due to the NEW location of the magnet carrier. Based off Bill Ball's situation back in 2012, I asked Mark @928intl for an 87 differential housing cover. I got one; not realizing until today that it's part number is for a 91 ONLY. (PS. This has nothing to do with fluid or volume. just sender location).

@Kevin in Atlanta We are in agreement. (see my edited posted above) about the rear housing covers. The PULSE sender is from 85-90. (928-606-210-01). So my new thought is I need to go back and get the 87-90 cover. 92830130600. Do you have a spare laying around to sell?

======== Reference ONLY Pulled from Porsche Classic PET =========

1. 78-82 928 301 035 04 Housing cover
2. 83-86 928 301 305 09 Housing cover
3. 87-91 928 301 306 00 Housing cover -90
4. 87-91 928 301 306 06 Housing cover 91-
5. 92-95 928 301 306 04 Housing cover