Speedometer, CASIS, Cruise Problem

[CrossT]

Odd, but maybe not, I have a situation where my speedometer drops to 0 and comes back up. Coincident with that the CASIS light will go out. Also, if the cruise is on it will be erratic or cut it/out. When the speedo drops to 0, the trip odometer stops. These are clearly related to something amiss. While this goes on, the car runs without a hitch. Is there a common element here I should look at?



I tried a search on 'speedometer fluctuation' and that resulted in 0 hits, which surprised me.

thanks
todd
89 Targa

[burgermeister]

Check ground points in front luggage compartment - there are at least 3 (one by the battery, 2 by the front of dash, LH & RH side).

My LH front-of-dash ground was pretty corroded and buggered up due to a mouse calling the area home during the car's stay at the PO. Mouse pee is apparently quite corrosive. Also had to replace a couple of wires that had served as mouse food, and the copper wire itself had turned to green crumbly stuff. Not sure if this caused my speedo dropout, but it needed fixin' anyway. And the car is done until better weather now ...

[CrossT]

I know where the ground point is by the battery and on driver's side. Last I looked, they were good. Wasn't aware of one on the passenger side. Where is that?

thanks
todd
89 Targa

[burgermeister]

Beats me - but one of the books I've got says there is one on the RH side.

[CrossT]

I could not see another grounding point on the passenger side. Maybe behind the air duct . . . I'l dig a bit this Sat

todd
89 targa

[rick-l]

All the indicators/systems you have described rely on the speed sensor to function.

How the SENDER works.

If you can get your meter on the green wires on the back of the speedometer and see you get 8 switch closures (<1 ohm to chassis) per revolution of the rear wheel it should answer a lot of questions.

I think there is a connector in the tunnel that gets corroded.

[CrossT]

Interesting. I thought about that as a possibility, but know little about the sensor. The effect is simultaneous. So, having a look at this tomorrow I hope.

thanks
todd
89 Targa

[CrossT]

Maybe a very loose fan belt was the issue. It was quite loose where I could turn the fan/slip the belt by hand easily. Moved a shim outboard and haven't seen the problem yet.

Sometimes the simplest thing . . .

todd
89 targa

[CrossT]

The loose fan belt was not the cause. Will have to investigate the speed sensor as was suggested . . .

thanks for the info and the links.

todd
89 Targa

[Kaliv]

I'm having the same issues. But I have redundancy using a GPS for the speedometer, so I haven't thought about it much. Sometimes it just fluxates down to 0 and back up...and sometimes it will be down for a 30 minutes and then start working again. I'm taking it to a shop for some minor repairs today, so maybe I'll ask some questions about it there.

[CrossT]

That does sound familiar
Please report back if you do ask about it

Looking in the catalog here for a speed sensor results in nothing. Are the speed sensor and reference sensor the same part number?

thanks
todd
89 Targa

[Kaliv]

The guy didn't really look at it, but said that it's connected to the transmission and sometimes the wires go bad, which makes sense seeing how the last time I dropped the engine, that mechanism did look a bit flakey, but I wasn't exactly sure what it was.

[cashman]

I'm reviving this thread because I have a similar problem and I see no resolution above.
The past 2 times I have diven my car, I have noticed the CASIS light and speedometer not operating for the first 5 minutes. After that, they both kick on together and work fine.

I just got done installing a new speedo sender purchased from our host. I went on a 20 minute drive and neither worked at all. The new sender had a brown and green wire which is the same as the one I removed. I attatched Brown-Brown, Green-Green. I have read a couple other post but have never really found the solution.

Any ideas, or better, any solutions?

[cashman]

I have done some additional testing and I have some intriguing findings:

I pulled the new sensor out of the transmission and apply a small magnet. Every time I apply the magnet I get continuity. When I install the new sensor back into the transmission and rotate the tire, I get no continuity at all. (I have verified the sensor is fully seated and installed correctly).

I perform the same magnet continuity test with the old sensor. Every time I apply the magnet I get continuity. When I install the old sensor back into the transmission and rotate the tire, I get (4) contacts per revolution. Everything I have read on this board has mention 8 contacts unless you loose magnets in the disk. I found one report of someone mentioning only receiving 4 contacts but there was really no explanation following the statement as to why. My contact points are evenly spaced across from each other and I would assume I would receive more random spacing If I have lost 4 magnets.

So I have a couple questions:
1 - Can someone explain why the new sensor will react to a small magnet but not work while installed?

2 - Can someone verify there are only 4 magnets on a G50 1989 transmission?

3 - If I get my sensor reading accurate and I have verified I have 12V to the speedometer, can someone offer any other suggestions to my random working speedometer?

Mark

[rick-l]

It is just a reed switch. You should see 8 pulses per revolution.

[cashman]

I would like to provide an update and some conclusions and ask for some assitance on this ongoing issue.

I have performed some magnet sensitivity test with a weak magnet and found the following:

The original sensor would activiate if the magnet was anyehere within 1/2" - 2" from the sensor tip. During one of my installtion trials, I noticed the reed swith would remain closed during tire rotation. This would explain why I had intermmittent working issues. I would get 4 closures per tire rotation when it was acting correctly. One every 1/4 turn. Speedometer read 10% high which is normal for the VDO.

I performed the same magnet test on the new cable and found it would only activate if the magnet was withing 3/16" - 3/8" of in inch. When installed, this sensor would not receive any closures. Therefore, no readings and no working speedometer. I have determined this reed swith to be too stiff and not providing any readings when insalled.

I contacted our host and I ordered another replacement cable. The magnet test would activite between 3/8" - 1" from the sensor. When installed, I would receive 2 contacts extremly close to each other every quarter turn of the tire rotation. I was now happy to be receiving contact while installed and I was getting a total of 8 hits per rotation. Now my car reads 50% fast and the both odometers are reading as If I am travelling 50% fast.

I have come to the following conclusion:
1 - old sensor was was reading acurate 4 hits per rotation. Was a very sensitive reed swith that would occational stay closed. I did find a small piece of bare wire which may have been the issue as well.

2 - New sensor was too stiff, Reed swith would not activate while installed in transmission producing no hits and no speedometer reading.

3 - Second replacement sensor is between the two. I should only be receiving 4 hits per rotation but I am actually receiving a quick cluster of 2 hits every 1/4 turn. This is causimg my speedometer and odemeter to read falsly high. The CASIS light will not operate due to the whacked out reading it is receiving of 80MPH at 2500RPM.

I don't want to keep replacing cables but I am convinced this is my second bad cable.

Can anyone offer assistance, advice, or explanation? Can someone recommend a person to contact at Pelican to review this situation? I have noticed that both new replacement VDO sensors are .010" smaller in diameter where it fits into the trasmission. I don't think this is the issue, it fits in the hole easier. It is installed straight.
Mark

[cashman]

I wanted to provide some closure to my portion of this thread in hopes it may help someone else some day. I hate searching threads for hours that never have a conclusion or ending.

I ended up ordering a third cable from my local Porsche Dealer. The third cable has been installed and I receive 4 contact closures every tire rotation. One every quarter turn.

My speedometer now reads its accurate 10% fast and the CASIS light is working as before. For what its worth. The OEM cable I just received was the same size as the original which is .010" larger in diameter than the after market VDO cable? Go figure

[rick-l]

I'm trying to understand the 4 pulses per revolution thing.

sensors for speedo in trans

If one wheel was stationary in a non lsd car would the plate rotate at 1/2 the wheel rpm?

[cashman]

I searched all the relevant post I could find before I started this project including the one you reference above. I did find one other post that someone mentioned only getting 4 contacts but there really wasn't any further comments or resolution in the post. I can not explain it either.

Old cable - 4 hits per revolution every quarter turn that last about 2" of travel.

first replacement - No returns and speedometer would not work.

2nd replacement - Acted like a double hit every quarter turn and speedometer read way fast. Almost like it "hit" once on the magnet and once off?

Last cable - 4 hits per revolution every quarter turn that last about 2" of travel and speedometer now works?

[Cattler]

I don't know about the revs / hits issue, but can say that the speedometer sensor cable is very important to CASIS function. Before I replaced my cable, the CASIS system would only light after a few minutes of operation and then normally only for shifts from 3-4 and 4-5. It now works perfectly as soon as I leave my garage and through all shifts. In my case, I think its because I wasn't really accurately measuring my speed below ~40 mph with the old malfunctioning sensor.

David