Speedo Often Doesn't Start Working Until Around 30 MPH

[wrxnofx]

Like the title says, that's what my '87 speedo does fairly periodically. Once it starts registering, however, it will work all the way down to 0 mph.

Recently I saw a screenshot someone posted for some other problem. It said something to the effect that the speed sensor may be the problem if the speedo exhibits the same behavior I am having.

Can anyone repost that screenshot or confirm that is what's probably happening? The car is currently tucked away for the winter, but I'm not sure I'd know how to troubleshoot this issue anyway. Any suggestions?

[porwolf]

The speed sensor is basically a micro switch activated by rotating magnets in the transmission. I think it is 4 switch closures per revolution. You can simulate that by attaching 2 wires to the proper terminals on the back of the speedometer and touch them together at varying rates. When the speedometer is powered up those contact closures should move the needle according to the rate at which you touch the wires together. If the meter needle moves at low closing rates the problem probably is the speed sensor. Otherwise it is the speedometer itself.

[RichardNew]

Here's a gauge article I just posted. It's not Porsche specific but you might enjoy it.

Richard Newton

[wrxnofx]

Originally Posted by porwolf

The speed sensor is basically a micro switch activated by rotating magnets in the transmission. I think it is 4 switch closures per revolution. You can simulate that by attaching 2 wires to the proper terminals on the back of the speedometer and touch them together at varying rates. When the speedometer is powered up those contact closures should move the needle according to the rate at which you touch the wires together. If the meter needle moves at low closing rates the problem probably is the speed sensor. Otherwise it is the speedometer itself.

Thanks for the info, porwolf. So, dumb questions follow-up time.

Does the speedo need to be hooked up to a power supply and the key turned to accessory in order for it to register my manual wire "pulses", or can I literally remove the gauge from the dash, bring it to my workbench, hook up 2 wires to the proper connections, and rapidly touch/no-touch them?

[porwolf]

Originally Posted by wrxnofx

Thanks for the info, porwolf. So, dumb questions follow-up time.

Does the speedo need to be hooked up to a power supply and the key turned to accessory in order for it to register my manual wire "pulses", or can I literally remove the gauge from the dash, bring it to my workbench, hook up 2 wires to the proper connections, and rapidly touch/no-touch them?

It should work with the speedometer removed to the work bench. You need a 12V source at the bench. Hook up minus 12V from a battery to the ground terminal, plus 12V to the +12V terminal, and two wires to the sensor terminals (one is the ground terminal). Then touch the sensor wires and see what happens. Or, you could leave the speedometer connected to the car 12V system and do the same with two attached sensor wires. But that is a little cumbersome with the seedometer hanging from the dash board. But it could be done.

[asalcedo]

This is exactly what is happening to my speedometer too. It goes from 0 to 30 directly and sometimes it doesn't go at all and I need to tap the glass to get it going. I also noticed that the odometer seems to be working properly.

Regards

[wrxnofx]

Originally Posted by asalcedo

This is exactly what is happening to my speedometer too. It goes from 0 to 30 directly and sometimes it doesn't go at all and I need to tap the glass to get it going. I also noticed that the odometer seems to be working properly.

Regards

Yeah, now that you mention it, if I tapped on the glass it would start working (haven't driven it since November, unfortunately). I had totally forgotten that part. Either way, I need to get out there and test it per porwolf's information. A trip to NH Speedo may be in my speedometer's future...

[asalcedo]

Originally Posted by wrxnofx

Yeah, now that you mention it, if I tapped on the glass it would start working (haven't driven it since November, unfortunately). I had totally forgotten that part. Either way, I need to get out there and test it per porwolf's information. A trip to NH Speedo may be in my speedometer's future...

I would be very interested in knowing how it went when you finally tested it. I do not have the expertise nor the tools to test it so your case will be a good benchmark for me.

[agfours]

I've recently been working through a speedo issue, and although my issue was a non-working speedo altogether, I learned about the mechanics of this quite well through researching many posts.

I would doubt your case that the sensor is the issue if tapping on the glass causes it to work.

Before going through the hassle of removing the sensor or the speedo, you can easily test whether the sensor of the speedo is at fault. I believe to bench test the speedo sweep at different speed, you need an oscilloscope to dial up the signal.

The midpoint between the two is where you will do your testing...Now, open up the access panel in the trans tunnel on the back seat floor. There will be a clear plastic connector there with a brown wire and a green wire. To test the speedo, turn the ignition on to the accessory position (do not start the car). Tap the connectors on the brown and green wires together...the speedo should jump a little. If not, its a speedo issue. If it does, then proceed to testing the sensor.

Testing the sensor: - lift the car, and remove driver wheel and go straight through the opening to access the sensor. Remove the screw/clip from the sensor (the sensor is attached to the top section of the transaxle cover plate.) Pull the sensor out of the trans housing. Hook up the rearward green and brown wires (in the access tunnel coming from the sensor) to an ohm meter. You can trail the ohm meter out the driver door so that you can see the reading. Use a magnet to test the sensor by waiving it closely over the sensor. You should see the reed sensor close and 'ohm out'.

Also, you can do what was advised by others to clean off the magnets that are attached to the differential disc carrier inside the transaxle. I got a clean terry rag and put it on the end of a long screwdriver which I inserted into the hold in the side plate. I then spun the axle a few times. (note that the magnets are on the outer circumference of the disk - they pick up metallic sediment over time and can reduce the effectiveness of the signal pickup in the sensor).

However, as mentioned, my guess is that the speedo needle itself is sticking in the 0mph position (mechanically) and you may need to have it rebuilt. There's a chance that some of the diodes/caps may be leaking and could cause the issue (not sure on this point, but common in old electronics, and when I had mine rebuilt, while it still worked, there were some leaking capacitors) Make sure before spending $$ to get it rebuilt that you ask for a bench test.

Overseas Speedometer here in Austin is great. Met the guy in person and he was very helpful and easy to work with. I've also had tach work done at North Hollywood Speedo and can recommend them as well.

Good luck!

[wrxnofx]

Thanks, Paul! I'll give this a shot and report back.

[pfroede]

Hi I have the same problem on my speedometer and was wondering if you found out what it was. I checked the resistance in the magnet sensor and it gives me readings right from the start. So I'm pretty sure it comes from the speedometer itself. Does anyone have experience rebuilding the motor? Thanks

[jrwilson]

Old electronics often suffer from what's called "cold solder joints" where the soldered connections inside on the circuit boards have cracked and only partially or intermittently connect. Open up the unit and re-flow the soldering points with a soldering iron and that might fix the issue.

[airkewldfan]

Is the odometer working? Mine started doing this when the odometer gear broke. Once I replaced the odometer gear it works perfectly from zero again.

[agfours]

Originally Posted by pfroede

Hi I have the same problem on my speedometer and was wondering if you found out what it was. I checked the resistance in the magnet sensor and it gives me readings right from the start. So I'm pretty sure it comes from the speedometer itself. Does anyone have experience rebuilding the motor? Thanks

A speedo rebuild was the fix. Mostly capacitors , etc, which degrade over time.