bronto

I spent last night trying to fix my speedometer, unsuccessfully (Thanks Dwayne for the excellent pod how-to). Here's a synopsis of the issue:

* Speedometer stopped working correctly in July the morning I left for a CA to SLC road trip. I don't think the needle twitched even once on the whole trip. The odometer didn't advance either.

* While in SLC I stopped by Motorsports and had them look at my AC, which also failed on the trip. I had them do a quick diagnosis on the speedometer as well. They found a suspect connection towards the rear of the car that didn't solve the problem, and put a meter on the circuit at the fuse box. Dave determined that (IIRC) a signal was being sent to the instrument panel, but not received back. His diagnosis was that it was likely a failed printed circuit. There was no time to resolve the problem though.

* After the trip, the speedometer started registering speed and mileage intermittently. When it did "work", the speedometer never registered above about 40 mph, although the odometer *seems* to register accurate mileage. Both the speed and odo behavior were in sync; when one registered activity the other did as well; when it didn't, neither did.

* I bought a new PC from 928 Intl at Sharktoberfest along with a full compliment of replacement bulbs.

* Yesterday I finally got the opportunity to do the work. All went pretty smoothly. However when I drive it, the speedometer still doesn't register. The state of the speedometer before I started working was not registering any speed or mileage, and is the same after. I haven't driven it much since then, so I don't know if it will start registering again. In other words, I can't tell if it's worse now than it was before, but it definitely isn't fixed.

Ideas?

WallyP

The sensor circuit is pretty simple, but there are several connections.

The circuit starts at the speedometer, and a trace on the flexible printed circuit board runs from there to Terminal 1L. A brown/red wire runs from 1L to H1. H1 is the lower left connector on the 8th plug from the left end.The circuit runs behind the Central Electric Panel from H1 to T1. Count from Plug A on the left end up to T, skipping I and skipping the two short plugs in the middle. Again, 1 is the lower left terminal. There should be a brown/red wire in the terminal. The circuit runs from there to the sensor, changing color to solid brown at the sensor connector. The sensor is a simple magnetic switch, operated by a ring of magnets mounted on the differential carrier. The circuit goes thru the switch, and returns on a brown/red wire to Terminal T2 on the Central Electric Panel. T2 is next to T1. The circuit runs behind the panel from T2 to H2, and a brown wire runs from H2 to the instrument panel ground on Terminal 11R on the printed circuit board, then to Terminal 7R, and then on a brown wire to ground at MP 3.

When the sender switch is momentarily made (contacts close) by the passing of a magnet on the ring, the circuit from the speedometer is completed to ground, and both the odo and the speedometer receive a pulse.

You can start troubleshooting the speedometer/odo sending unit circuit by checking for voltage at H1 with the ignition switch on. If there is no voltage here, the problem is probably a faulty connection on the flexible printed circuit board. Could be at the speedometer or at the edge connector. If there is voltage here, move to H2 and check for pulsing voltage when the rear wheels turn. If there is voltage at H1, but no pulsing voltage at H2 when the wheels rotate, the problem may be a faulty connector or broken wire. Start by cleaning the connector in the spare tire well.

bronto

Not to argue, but doesn't it seem unlikely that the problem is a bad connection with two different printed circuits, the current one being brand new? I did clean all the connections as I replaced the PC.

The connection in the spare tire well is the one that Motorsports found. But yeah, it's easy enough to get to there's no sense in not starting there anyway.

Dwayne

Hello Bronto,
When I had the speedo problem on California, I verified I had the pulse signal to the instrumentation cluster. This narrowed it down to either the cluster or the pickup sensor at the rear. The best method that worked for me was to test the H2 signal (as Wally suggests) to the instrument cluster using an analog multi-meter. I found it much easier to actually "see" the pulse with the needle moving at each pulse rather than using a digital multi-meter. I drove the car very slowly so I could see the individual pulses. If I was getting pulses but no speedo readout, it pointed to the instrument cluster - perhaps a ground problem or signal problem not getting through. Looking forward to what your tests reveal.

bronto

My next opportunity to deal with this will probably be MLK holiday weekend.

bronto

I am preparing for another stab at this over the MLK holiday this month. I want to have on hand whatever parts may be needed for repair of the issue whatever it may be - within reason. As noted, I have already replaced the PC and will assume it's OK until proven otherwise.

Seems to me that possibilities remaining for the cause would be:

* A bad connection or ground somewhere: no special parts needed for repair
* something wrong within the speedometer itself: what would a likely culprit/repair be? Should I get a used replacement speedometer?
* Is there a relay involved?
* Any other switches/resistors/capacitors involved?

I'm also getting a replacement odometer gear to replace WIAIT.

TIA

lrpman

My 1982 does the SAME THING. I did all the cleaning and testing and like when you go to the doctor the test where all fine. Then it worked for about 4 miles. Turned off the car and restarted and NOT working.
I have sprayed contact cleaner in the connections in the spare tire area. I guess my next step is to look at the reader at the magnets for trash, oil etc that could block the magnetic field from giving the pulse all the time.
AS a new guy to working on these is it POSSIBLE to be a problem with the ignition swicth or is this another area altogether.
Wally, thanks for the quick shipping on my parts, MORE to come for sure.

j.kenzie@sbcglobal.net

I agree with Wally. (As usual). I connected my circuit tester light to 12V and then to the central electric terminals that Wally mentioned(H1 to H2 with ignition on). I then pushed my car about a foot or two along the ground and the light went on. Two feet more and the light went off, etc. Thant told me my sensor was OK and the connections in the rear were OK. In my case it was the connections at the rivets where the speedo connects to the PC board. Good luck.

RCinXS

After installing a newly rebuilt trans and posi diff in my 84, I have no pulse when the sensor(brand new) is installed correctly. However, if I rotate the sender 90 degress, I get pulses, speedometer reads twice as fast as I am actually going. All connections are perfect. Printed circuit film verified perfect. Speedometer worked fine before the trans / posi rebuild. I've tried a different known to be good speedometer with the same results.

Car did not have posi before.

Seems like a magnet alignment problem but I am told that that is impossible by 928 International.

Anybody ever heard of this kind of problem before?

Thanks,

RCinXS

bronto

Since this resurrects my thread I'll update on it.

Turns out the speedo needed rebuilding. I finally took it to my wrench in late August and confirmed this was the case, he took it out and put another "spare" in there while it was being rebuilt. The spare worked fine, except it was about 10% fast and got stuck at 80mph once.

The rebuild came back a few weeks ago, I took it in to swap back to the rebuild and the rebuild still did not work. They sent the rebuild back again, and I am still waiting for it to come back. Hopefully by Friday, since I wanted to take it to the SB PCA Concours on Sunday.

bronto

UPDATE:

Here's a weird one, any ideas?

After getting back the 2nd repair attempt, it still doesn't work. My wrench spent some time with the repair company on the phone and they insist that the speedometer tests OK on their bench. But when it gets into the car it doesn't. As I said above, last time it was out for repair I used the car with a "spare" speedometer he had on hand, and it worked fine except for it reading about 10% fast and it sticking at 80 mph once. Wrench says that the spare speedo was from a 944. I am using a brand spankin' new printed circuit I bought at Sharktoberfest last year.

Any ideas?

Brad W

Bronto, Any update? I am in he same place.

bronto

I started a new thread based on new status.
https://rennlist.com/forums/928-foru...naccuracy.html

I still haven't had a chance to drive long enough distance with satnav turned on to get readings at lower speeds.

malcoll

Resurrecting this ancient thread....so I have less than 1 volt at H1 and T1. I've had my speedometer out and found someone has been in there before...
where do I obtain a PCB for a 1979 ROW 5 spd? I fixed a couple of ground issues with the PCB, but it's cobbled together in a couple spots now and probably just better to replace it....but could the speedometer head be bad too...I took it out and OT looks clean, no obviously burned components...the Odo gear is even intact! Where best to send the speedometer for testing and repair?

I know, lots of questions...thank you all in advance!

malcoll

@bronto so did you ever learn what was wrong with your speedo? And did you get it set to accurate?