Speedo reading too slow

[v9ff]

Okay... my speedometer is all whacked out. When I'm actually going about 45 mph (second gear at 4000 rpm), it only reads about 23 mph. When I get to freeway speeds and am going around 85 mph (forth gear at 4000 rpm), it only reads about 60 mph.

Here is my frustration, I've already replaced the speedometer and speedometer sender with new ones and still have the problem.

Can the problem actually be in my transmission?

Thanks in advance.

[yelcab1]

Yes, it can be. There are magnets inside the speedo plate inside the transmission. These magnets give rise to the square wave that the speedo sensor is reading. If one of the magnet falls off, you will get a slower wave form, and a slower speed reading. It is ... rebuilt time if that is the case.

[v9ff]

That sounds like my problem... what sort of rebuild are we talking about here? What would it entail? Approximately, how much will this hurt?

[v9ff]

Bumpin'

[Wavey]

You need to do a search - this can be a serious issue. There was a post on this BB during the last year from a guy who lost a magnet, and it ended up lodging in his final drive, between the ring and pinion gears, which caused the rear wheels to suddenly lock while he was in a turn, which caused him to wreck the car. The guy was flabergasted about why the rear wheels locked until they dug into the tranny.

I don't think you necessarily have to rebuild the whole tranny, but you do need to get the loose magnet out of there and replace the plate. Have no idea regarding $.

[1980SC]

You can remove the differential with the drivetrain still in the car. You'll need to replace the ring of magnets which is pressed onto the differential. Here is a picture of the magnet ring with it's eight magnets :



I lost 3 of the 8 magnets before I pulled my gearbox to rebuild it. My problem was that the pinion gear was disintegrating and chunks of it were sticking to the little magnets. Then as the differential spun, there wasn't enough clearance for the chunks of pinion gear, so it would rip the magnet out. These magnets were ground up within the gearbox with no damage to anything else.

If you don't feel like fixing it just yet, you can simply adjust your speedometer to compensate for the missing magnets. I was able to adjust my speedo so it was perfectly accurate at 60mph, when I had only 5 of the 8 magnets left.

Now something to make you worry :

A few years ago someone totaled their 911 and felt is was because of one of those magnets. Evidently a wheel locked up and sent the car off into the woods, destroying it. After disassembling the gearbox, they found one of those magnets stuck to the ring gear, so when it tried to mesh with the pinion, it locked the differential. This story is what made me go ahead and disassemble my gearbox.

My experience was that the 3 magnets I lost were harmlessly ground up within the gearbox.

EDIT - Wavey beat me to the story above...

-Rob
1980SC

[Silveresrty911S]

Rob,

How did you adjust the speedometer? Was it the small screw in the speedo itself?

Thanks, Rick

[red-beard]

I was looking for an adjustment on my speed yesterday and I found no adjustment. Is the pot under the cover?

[1980SC]

RoninLB has given some great directions on where to drill the speedo case so that the tiny adjustment screw could be reached with the speedometer assembled. He posted pictures too. I did this, but on my speedo case there is bulb holder in the way, and I never could get a screwdriver through the case. I had to partially disassemble the speedo for each adjustment. Tedious, but it works. There is plenty of adjustment available.

-Rob
1980sc

[1980SC]

This is not a very good picture, but it's the best my camera will do. The speedo's needle is pointing towards the trimpot.

[masraum]

Just a note, there was a Rennlist member who had a low reading speedo. He was tooling along one day when the car spun for no apparent reason (public roads). It felt like the rearend had locked up. When he got the car and started pulling the tranny apart he found one of the magnets from the speedo embedded in one of the gears in either the tranny or diff, I don't remember. But basically that little magnet had been floating around and eventually stuck to a gear and then brought the drivetrain to a screeching halt.

[red-beard]

OK, I see the trim pot. But there is no access to it from the outside, right?

[1980SC]

Originally posted by red-beard
OK, I see the trim pot. But there is no access to it from the outside, right?

No access. You'd have to drill a hole.

[RoninLB]

Speedometer Adjustment for tire size?

Speedometer Error. Need some help

[CHILI]

Steve,

I heard a story about a guy who's wheels locked up and..... ohh, I guess it's been told 3 times in this post already. LOL!

Sorry to hear the bad news. It might be worth asking Rob on more details about how to remove the diff while the motor's still in the car.

All of the sudden my mechanical speedo doesn't sound so bad, huh? Muahahahha

Again, sorry to hear about this.

[Early_S_Man]

There is no need to get all worried about missing or 'thrown' magnets before you find out if that is the problem!

There is a very easy test to do with a multimeter -- either analog or digital will work fine!

1. Put the back end up on jack stands.

2. Disconnect the speed sensor from the vehicle wiring harness. No need to mark the wires ... polarity doesn't matter!

3. Connect the multimeter leads to the sensor wire connectors with the meter on the lowest resistance scale or continuity function.

4. Rotate one of the back wheels EXACTLY one revolution ... and look for eight contact closures from the magnetic reed switch sensor. If you observe eight contact closures and openings during the test ... don't worry, there aren't any missing magnets!!!

[v9ff]

Originally posted by Early_S_Man
2. Disconnect the speed sensor from the vehicle wiring harness.

Hi Warren,

Which wiring harness you referring to? At the transmission, under the shift coupler cover plate or at the guage?

Thanks!

[Early_S_Man]

Steve,

At the side of the transaxle was what I was referring to, though at the speedo would work too, if you already have it out. You would need a long set of test leads ... unless an assistant is available to count the contact closures.

[Silveresrty911S]

Ron,

Would this work on an 84 electronic speedo? The drill and adjust method I mean.

Rick

[RoninLB]

Originally posted by Silveresrty911S
Ron,

Would this work on an 84 electronic speedo? The drill and adjust method I mean.

I have the speed sensor on trans style electronic speedo.. So I assume the internal pot is in the same place. The adjusting screw on the pot is very small.....