When I dove into my pool my speedo went down.... But seriously I could`nt resist that one. When I accelerate my speedometer goes down instead of up untill I let off the gas then it bounces back up to normal. The previous owner admitted to having run the diff dry, and it howls slightly when I am on the gas and quiets down when coasting. Could the speedo problem be related to the ring gear being loose and moving the sensor pick-up out of alignment? Any theories about the second problem would be appreciated, the first promlem was a broken draw string.

Thanks

1800es/780/744t/780/744/240

OOPS!! I apologize. I thought it was your tach needle that was fluctuating. I read it the second time and noticed you said it was your speedo. Anyway, just in case you get this problem you'll know what to do.

I had a similiar problem a few weeks before my 86 765T failed to start one morning. It turned out to be the Ignition Controle Module located on the driver side front fender close to the battery. Cost me $90 to replace after I replaced everything else. I would replace it soon before it leaves you stranded.

Check the speed pick-up in the rear diff. for any metal on it . This would cause the speedometer to be erratic.

volvonut is right: I had exactly the same problem as what you describe and my diff ran dry too. (The lid was rusted through and I lost all the oil in about one hour of high speed highway driving). After I replaced the lid the speedometer was fluctuating, just as you describe. It turned out to be that toothed wheel hitting against the hall sender for the speedometer, which is mounted to the lid of the differential. The reason was that some of the bearings inside the final drive unit were so worn, that the whole assembly slightly moved around. At that point there were shavings in the oil already but I could not hear anything. A thick washer between the hall sensor and the lid fixed the problem (the problem with the speedometer that is) temporarily. I used the rear axle for another 30000 km before it became replaced, so there was lots of time to look around for good used rear axle. I hope I did not spoil your day and maybe your diff is still OK. I would definitely check for shavings in the oil and try to shim the speedometer sender with a washer. Although, it may be difficult or even impossible to remove the sender without destroying it and I remember that thing being ridiculously expensive.
I hope this was somewhat helpful.
Toby (740 TIC, 740 GLE)