928 U.K.

hi

replaced my odo gear cog (£20 for part as it was bust) and cleaned up the printed circuit board/replaced the broken bulbs/ cleaned up all the screws/conectors and terminals

my odo now works perfectly/hesitant speedo works perfect/before high reading voltmeter now rides between 12 to 14 as it should and dash now illuminates really bright (6 bulbs shot)

for 3 hours work the dash is better than its ever been

thanks Randy for the links to pod removal and cog replacement

cheers Tim 84 s2 auto

Randy V

Glad I was able to help, Tim.

928 U.K.

Hi

unfortunatly after a few journeys the speedo and odometer stopped working and then started working again

this is a fault i have had for years so it needs further investigation

i thought cleaning up the connections on the back had cured it

the replacement cog does now turn the odometer when the speedo works

as this is an intermittent problem it is very difficult to solve

any suggestiions?

cheers

Tim 84 s2 auto

BrianG

Since the speedo and the Odo are both electrically powered but separate issues, an intermittent but concurrent failure indicates a common flaw. I'd first suspect the pod ground........ it is definitely common to both. The next thing is to look at the schematics for your MY and see if the "power-side" shares a common fuse. If so, clean up that contact also.

Marc Schwager

Tim,

I am fighting exactly the same problem on my 1980. As I write this, the speedo is in a box in the front seat of the volvo going over to Palo Alto Speedometer (a VDO repair depot) tomorrow.

The obvious thing to check is to see if you are getting a clean signal from the sender. I checked mine in the spare tire well, at the fuse box, and at the connector coming into the cluster. All check out fine. You can see the resistance go from a low value to infinity as you roll the car a few feet. The speedo shares a ground with other gauges, and they all work correctly, so I'm reasonably confident about the ground.

I then replaced the printed circuit on the back of the cluster. I got similar results to you - my voltmeter finally reads correctly. However it did not help the speedo.

So in one of my "i can fix anything" moods, I took out the speedo and tore it down into small pieces. My guess was that the connector prongs were not making good contact with the unit. I could see no obvious problems and I checked continuity around the circuit board and wiring in the speedometer. I reassembled it and the speedo still doesn't work, so the speedo is off to Palo Alto for some discipline.

I'll post the results, but if they don't find a problem with it, I'm not sure what the next step is!

_Thomas_

Hi

I also had that speedometer working/not working problem. What fixed it was to solder the copper rivets on the back of the speedometer (solder on both sides of the print).

Cheers Thomas

Randy V

By Jove, I bet Thomas has got it!

_Thomas_

Actually I got the tip from this board ... First I also thought cleaning the contacts alone solved the problem, but it eventually came back.

BTW: Same for the cruise control (I also had an intermittent problem there): remove all the copper rivets (the cable connector pins there are also riveted to the printed circuit) and solder the pins back on and solder ALL the connections on the print (even if nothing looks corroded, solder everything with no exception, e.g. the pins of some of the resistors on top don't meet the printed circuit).

Cheers Thomas

928 U.K.

Thomas

do you mean you soldered the joints inside and on the outside of the speedo or on the main printed circuit board which covers the whole instrument cluster

cheers

Tim 84 s2 auto

Rob M Budd

I had the same problem with a replacement speedo. I installed it & then nothing. When installing it, I made sure the connectors were clean, but didn't check for making contact. doh!
I took it out again, and if memory serves me right there's 4 slots on the back the connectors slide into. I adjusted them to be sure these made a tight fit and ressembled. It has worked fine since.

_Thomas_

Hi Tim

I meant to solder both sides of the rivets, because they have to make contact to the copper stripes on the printed board and to the bits of metal which go to the pins to connect the speedo itself.

To be able to do the soldering on the backside of the printed board of course you will have to undo some of it from the housing of the instrument cluster which involves unplugging/unscrewing bulbs etc.

Cheers Thomas

928 U.K.

Rob

the four pins must need to make contact to drive the speedo and odometer?

cheers

Tim

84 s2 auto

Marc Schwager

I just got my speedo working!!

To answer the last question, those 4 pins drive both the speedometer and odometer.

Back to the story. Palo Alto Speedometer tested my unit and found no problem with it. ($48 thankyouverymuch). I knew I had good sender signal coming to the connector (check the brown and red wire on the drivers side connector far right pin, look for varying resistance between it and ground as you roll the car a couple of feet). The printed circuit board was brand new so I was confused. Out came the continuity checker and I found the problem in about 15 seconds - the new PC card from Porsche was stuffed up!

In the middle of the flex circuit there is a little PC board discussed earlier with the 4 prongs on it. Those prongs are soldered on the front side and the circuits run down to a set of rivets that bring the signal through 4 (5 actually, one is not used) holes to the contacts on the back side of the flex circuit. The 4 rivets were missing! I threw in a couple of tiny nuts and bolts to make the contact and it worked fine.

Net net is that replacing the PC board would have solved my problem if Porsche had manufactured it correctly.

Tim - I hope this gives you a few ideas to work with.

Marc Schwager

Just because I am obsessive I went back to trace the intermittent speedometer problem on the old flex circuit that I replaced. It turned out to be a poor contact problem with the grommet that carries the speedometer sending signal from the flex circuit through to the little solid PC board.

Here is a picture.



The grommet that was making poor contact is the one that the exacto knife is pointing at. I was measuring resistance from the pin that inserts into the speedo and the connector. It should have read just a few ohms, but I was getting readings in the 100Mohm range - also it varied as the circuit was jostled! This would really mess up the speedo which counts on a signal from the sender that varies regularly from a small resistance to a high resistance.

While the new PC board solved the problem, I could have also just given the grommet a wack with a small hammer to reseat it. Seeing as you have to take the board completely off to fix it, it is probably worth spending the $50 to replace it.

928 U.K.

Thanks for all your responses and suggestions on this topic

Tim

84 s2 auto