Speedo reading too slow
 

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.

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.

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

Bumpin'

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 $.

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 :

http://forums.pelicanparts.com/uploa...1074523596.jpg

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

Rob,

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

Thanks, Rick

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

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

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.

http://forums.pelicanparts.com/uploa...1074526346.jpg

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.

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.

http://forums.pelicanparts.com/porsche-911-technical-forum/76869-speedometer-adjustment-tire-size.html?highlight=speedo

http://forums.pelicanparts.com/porsche-911-technical-forum/97658-speedometer-error-need-some-help.html?highlight=speedo

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.

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

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!!!

Hi Warren,

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

Thanks!

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.

Ron,

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

Rick

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.....

I have a GPS/Radar Detector unit I use in the car. One of the functions is speed. It updates about once per second. All cars I throw this thing into seem to show they are running under speed by about 0.5-2%. I checked the GPS unit against several measured miles and the GPS unit is accurate (Cruise control holding the GPS at 60 mph on the measured mile and timing).

I'd be careful about trusting the speedo of another car. I'd find me one of those speedo traffic signs or find a friendly police officer willing to help you out. Or get a GPS unit that reads speed...

This is how I tested my cars speedo out and found the speedo to be 2-5mph over.

The GPS solution is the easiest since its actually pretty cheap. $100-$200 for a Garmin handheld device.

My GPS/Radar Detector was $225 (UNIDEN)

It's funny, I did get the speedo working this weekend, and it is pretty way off. I'm not worrying about adjusting it. It needs to be sent out to be turned into a "real" speedo, since it's the 85mph job.

James

GPS is how I measured mine and I'm fast by about 7-8 mph @ 70.

Thanks Ron

ack. mistype.

I used my GPS as well just for fun, it was pretty close. Then had the GPS in the boat that weekend and my buddy waas scratching his head trying to figure out how the boat was doing 85... That's a lotta wind baby.. I never told him ;)

Does anyone know where I can get one of these? Or at least know what it's called so I can refer to it properly when inquiring?

Thanks!

You could use somethng like this...

http://www.jaycar.com.au/productView.asp?ID=KC5380&CATID=&keywords=speedo&S PECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keywo rd2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

This is an Aussie website, but I'm sure there'd be something equivalent over there.

I just noticed my speedometer reading about 30 mph too low at high speeds (80mph shows ~50). Started happening today. I'm worried about the dropped magnets in the gearbox. My question is, would draining the transmission fluid likely expel this magnet, or would it likely stick to the housing or something else. I would imagine if it stuck to a non-moving part, it should be OK, right?

I wouldn't be too hopeful of getting the magnet out while draining the fluid. However, you could do the simple test to see if that is actually your issue.

Yes, I will be doing that test. However, if the answer comes back as a missing magnet, I'm trying to decide if at that point it would be necessary to take the transmission apart. My thinking is a dropped magnet would bounce around until it either jammed a gear, got crushed, or stuck to something non-moving. Since I haven't jammed a gear yet, I'm thinking it is either crushed or stuck to something non-moving so probably harmless at this stage.

Ok, I tried the test above and got no contact closure readings. I'm obviously doing something wrong. I jacked up the back so both wheels were off the ground. I pulled out the two leads that attach to the side of the trans, and stuck my ohm meter leads into the holes of the connector on the trans. I then rotated one of the wheels slowly, several times in neutral, observing the resistance reading. It read open the whole time, no flickers. I then tried in first gear and got same results. What could I be doing wrong?

You need to have one of the wheels blocked if you do not have a LSD.

Yes, but you must block the correct wheel, the one to which the magnet disk is NOT attached!

- Manfred

Which one would that be? The one opposite side of the connector? Also, I assume then the car has to be in gear?

Make sure you are reading across the two wires that lead to the Hall effect pickup coil (puck) mounted to the side of the transmission. On a 930 the leads go into the tunnel and the connector is next to the shift coupler. I believe a 915 transmission would be the same.

Yes, that is what I'm reading across. My question is which wheel do I turn, the one on same side or opposite this connector, and do I put the car in gear or not.

I had started another thread and posted this same thing there as well:
I was able to do the test today and got only four readings. I raised the right side rear wheel and connected my ohm meter to the brown/red wires that connect to the speedometer. Since I don't believe that I lost four magnets at once, I'm trying to figure out what is going on. It seemed each reading lasted a good 20-30 degrees wheel rotation, then no reading for 20-30 degrees and so on. The readings were consistent and at the same angles as I did the test multiple times. Based on the size of the magnets, I suspected I would get just brief blips (8 times) as I slowly rotated the wheel. What could be going on?

This thread is timely for me, as I just had my speedo fixed after having it not working since last fall. Put the speedo in last night and it was reading very slow, I was thinking maybe the tech put a MPH gear in instead of KPH, he says he didn't and suggested perhaps a magnet fell out, so I guess i'll be trying to do this test.