No speedometer!?

I need advice to diagnose and repair speedometer. 1981 380SL. No odometer/speedometer. It has an inductive pickup in the transmission. Everything else seems to be working.
Where 'O where must I start?? help please.

[Gilly]

I don't think there is an inductive pickup on the trans, there is a electric solenoid on the rear, I think it's the kickdown solenoid. if you look on the drivers side of the tailshaft, there should be the hole and clamp for the speedo cable. The pickup for the cruise would be either in-line with the speedo cable or part of the speedo head. I am confused why you state "no odometer/speedometer".You mean none of it works, or you don't have one, or what? This is all very specialized work, most moderate sized communities have a speedo repair shop, try calling your nearest MB dealer to get a recommendation for a speedo repair shop.
Gilly

Gilly, I am sorry I wasn't more clear on this. When I reinstalled the transmission the other day, I only had to connect the 4-prong clamp (large, round with locking ring) and the cylindrical plug that clamped by means of one outside, small bolt in the tailshaft. The kick-down cable is more toward the front and locks in place by clip on the inside of the transmission. I drove the car to the mechanic to check on the adjustment of the valves (they did the valve job). When I left, I no longer had a speedometer nor odometer reading. We did not look at anything on the speedometer while checking the valves. I suppose it is just coincidental that the instrument quit functioning. The cable looks like an inductive pickup (no gears or exposed connection other than simply sliding into the socket in the transmission.)It did work on the way to the shop.

[Gilly]

Typically if the speedometer is inop, the odometer won't either, the odometer recieves it's "info" to advance from the speedometer input. The large round electrical connector i believe is for the neutral/safety switch. There should also be the vacuum hose for the modulator. The speedo cable should have the inner cable inside that engages into the square hole in drive gear in the tailshaft. I can't imagine why you didn't see it when you installed the cable, it had to be there on the way to the shop if the speedo worked. I guess I would start by taking the speedo cable off the transmission and examine the inside of the cable end for the inner piece. I don't think the inner part can fall out, I would hope someone would have noticed it on the floor if it did. It's pretty hard to screw this up, worst case scenario is you need a new speedometer cable.
Gilly

Gilly, now that I reflect upon this situation, I did notice some crumbled pieces of plastic while I fiddled with the large connector that has the 4 (?) prongs and the large locking ring. I guess that may have been the inner piece, (possibly). Anyhow, I think maybe I ought to pop off that connector, (lordy, that sucker is hard to get back on) and check the cable. Thanks for the insight, I believe you may have just put me onto the right track.

[Gilly]

No, the piece with the locking ring is the neutral safety switch connector. The speedo cable is black and about as big around as a "normal" pinkie finger. It goes into the hole farther back from the neutral safety switch, the one that has the one bolt that clamps the cable into place. There should be a flexible inner cable with the end squared off, the squared off part rotates, it is turned by the speedo drive gear inside the tailshaft.
Gilly

I am going back to the shop this week to finish up a couple of things. I will get them to point out the cable, fix it and tell me what those other little do-dads are.

I did not take a good digital picture of the transmission connectors upon disassembly and, quite frankly, don't remember what I hooked up. I do remember taking a couple things loose after seating the engine mounts so we could tilt it again to get the darn connector (lock ring) in place. I must have fiddled with that connector for 30 minutes. (I am still not sure what I did to line it up, I think maybe I placed the ring onto the transmission and then aligned it with the cable.)

[tstarr]

I have an 82 300SD and had the speedometer go out last week, as well. It (and the odometer) were completely non-functional until it rained Friday. When it rained, the speedo started working again. As the rain disappeared, so did the speedo. Since I had the actual speedo guage replaced when I bought the car 2 years ago, I'm assuming something similar is going on at the transmission. Make sense?

[Gilly]

Nope, not at all. I would try pulling out the whole instrument cluster and drive it, watch if the inner part of the cable rotates or not..
Gilly

Gilly, on my particular model the speedometer uses a potentiometer to measure the magnetic pickup on the transmission according to M/B dealer and Phil. My question is where to start 'home diagnosis' of the problem piece. I appreciate your help these last few days. I am sure glad you and others are there to help us. I have the car back onto the jackstands. I think I'll loosen the pickup, wiggle it a little (maybe even malletize it some...just kidding )

If you have any other suggestions, I am all ears...uh..eyes.
Thanks.

[Gilly]

Ah crap, you were right all along, it has a pickup in the trans for the speedo. What MB says to do is remove the pickup from the transmission and then take the broad side of a large screwdriver (honest!) and tap the end of the pickup rapidly and see if the speedo needle jumps or not (need a helper to watch obviously). This is simulating to the pickup what the trans is supposed to be doing. If the needle jumps, the pickup is fine, the problem is in the transmission. If it doesn't jump, then pull the cluster out and find the black 4 pin connector on the back of the speedo. With a VOM set to AC volts, 0 to 3 volt AC range, connect to pins 1 and 2 on the 4 pin connector, and repeat the "tapping" part of the test. If the voltage "jumps" when you tap, then the problem is the speedo itself. If it doesn't jump, then it's the pickup at fault. The pickup isn't available seperately (It wasn't when my book was written anyways), you need to replace the transmission electrical harness then.
Gilly
ps Sorry about the confusion, I really was sure this had a speedo cable. Learn something new everyday, if I remember half of it I'm doing OK.

Thanks Gilly, your description of the procedure is just what I need. I guess I wasn't too far off with the 'using the mallet' idea.....hehehe.

Anyhow, I appreciate the tip. I will try it tomorrow and let you know the results.

Gilly, the transducer did not make the speedo needle move when struck with screw driver on the end. I did notice a pretty good magnetic pull from the pickup, however.I did not realize the pickup was 'hot' while sitting at rest in the garage. Next step will be to remove instrument panel.

Thanks.

[Gilly]

Yes, it's magnetized, I believe they key has to be on to do this, if you didn't have it on, try it again. Your not really "hitting" the thing with a screwdriver, you're taking the "flat" part of the end of the screwdriver and moving it towards the tip of the pickup and then back, rather quickly. It's OK if you tap the pickup, but the point is by moving a metal object such as this in front of the pickup, you are simulating the movement of the exciter in the tailshaft. If the speedo needle jumps, you then know the problem is with the piece in the tailshaft. You have to be sure to have someone watch the needle when you are doing this.
Gilly

[ctaylor738]

I am working on a 1985 380SL that sat for three years.

The speedo and odometer work intermittently.

Gilly - are you there? - should I go after the speedometer end or the transmitter end first?

Thanks,