Speedo reading too slow

[red-beard]

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...

[moazam]

Originally posted by red-beard
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.

[red-beard]

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

[Silveresrty911S]

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

Thanks Ron

[moazam]

ack. mistype.

[anh911]

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

[v9ff]

Originally posted by 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 :



-Rob
1980SC

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!

[Fishcop]

You could use somethng like this...

http://www.jaycar.com.au/productView.asp?ID=KC5380&CATID=&keywords=speedo&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

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

[jwakil]

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?

[kjchristopher]

Originally Posted by 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!!!

Originally Posted by jwakil

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.

[jwakil]

Originally Posted by kjchristopher

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.

[jwakil]

Originally Posted by 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!!!

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?

[red-beard]

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

[ManniB]

Originally Posted by red-beard

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

[jwakil]

Originally Posted by ManniB

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?

[dap930]

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.

[jwakil]

Originally Posted by dap930

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.

[jwakil]

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?

[puddy]

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.