VERY Incorrect Speedometer - 1982 SC
 

Hey guys,

Odd problem. My Tach works great, but my Speedometer is way off. Not your typical 1 or 2 mph, but more like 30% off through the entire range. It does seem to be a percentage of difference vs. always being off by the same value. When I'm doing 50 I'm actually doing 70, haha!

Any ideas? I don't really know where to start since when searching you only find the usual "wrong size wheel" threads.

tire size? Car year?

Lots of folks on here can address this, I seem to recall when swapping out my 85mph for a proper speedo that there has to be the matching sensor for the speedo unit to avoid what you describe. Any sparts changes on your car that would coordinate with the readings being off? Someone on this forum can be much more specific.........

DS, my new to me 82 SC with 183k miles has a very inaccurate speedo also, grossly understating true roadspeed....doing what must be 80, it reads about 60mph, therefore quite useless. It is the car's original unit and tire sizes are stock, so I figure that a) the signal from the thingy in the transaxle (probably called a transducer?) is either present or not, and if it is, then b) the guage needs to be re-calibrated. So I will send mine off to North Hollywood Speedometer, imagine they'll be able to correct it. Best of luck, John in CT.

Great, then my '83's speedo is pristine! It's only 6 mph off. I guess that was the factory spec.

can you get to the rear differential and count the number of (reed) switch closures per revolution of the wheel at the connector?

You probably lost a magnet or two that drive s the sensor in the trans. The plate inside the transmission has 8 magnets. If 2 or 3 fall out you are off between 25 to 37 %.
If this is the problem, it can be removed in the car as its attached to the ring unit in the trans. Has to be pressed off and on.
Bruce

All very interesting... It's an 82 as seen in the thread title. Tires are 255/45/17 in the rear although it was the same on stock tires and wheels.

The car is a Euro so had some of the gauges swapped. Is there a different year that would not be compatible? I haven't checked the date codes on the speedo yet.

Look at the part number for the speedo up under the inside rim, shoot a picture of the face and report back.

JR

Bruce, thanks for that explanation...i figured the signal from the sensor was either good or bad. If some of the magnets fall off, do they fall to the bottom of the transaxle and stick to the drain plug, such that one can retreive them by draining the oil, and re-use them? Can they do any harm to the tranny's innards? Thanks, John

You buy the whole new disc to replace the one in the trans. Magnets are small, they'll be in the bottom. Wait until you pull the ring assembly to replace the disc so you don't keep filling and refilling.
Bruce

I had a feeling your car was a RoW. My 85 RoW had a broken speedo when I got it, so it was sent to Palo Alto Speedometer for repair. Upon inspection, it was reading way off like your car and PAS recalibrated it and it worked perfectly when reinstalled.

Interesting, WPOZZZ. Lets see what happens tomorrow when I pull the gauge. Hopefully its that and not the scary-sounding thing in the transmission.

How much did the repair cost?

I have no idea what it cost as PAS also redid all of my gauges. My guess is it was around $150. I think they change one of the gears to get things back up to spec, but must choose the correct ratio to be MPH instead of KPH.