so bob, techguy512, pm'd me and sent me a BRAND NEW tach amp! very cool!!! i sent him mine in return so he can do a little research.

i replaced it a couple of days ago and it has fixed all the issues besides the pegging of the tach with the horn blowing. i highly recommend getting the amp off of bob. it's easy to replace and fixes the issues. plus, as evidenced by this thread, the man is honest and easy to work with, and obviously guarantees his work. thanks!!!

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1985 300D - 1984 Euro 280E AMG Clone (SOLD) - 1978 280CE (SOLD) - 1983 300D (SOLD) - 1981 300D (SOLD)

Hi Squabble - thanks for the plug, and I'm glad you're up and running again.

I received your unit back on Wednesday. I gave it a very thorough visual inspection and put it through all the bench tests that new units get. No issues. I then put it in my car, and I've been driving with it for the last two days. It's performing quite well, and has handled the entire RPM range I've thrown at it, which is essentially idle up to about 4200 RPM thus far.

I'm ready to declare this one 'NFF' (no fault found).

I suspect what happened was that over the past 13 months (your unit was originally delivered to your PO on or about June 1, 2008) thermal cycles and engine compartment vibrations contributed to a degradation of the electrical contact between the amp pins and socket walls. I suggest that if this ever happens again that you pull the amp out of the socket, wave it over your head three times while saying an incantation of your choice, and then reseat the amp. The mechanical wiping action of the pins against sockets as reseating takes place will break up any corrosion that may have taken place, establish new contact patches and you should be good to go. If that fails, by all means let me know.

Regarding pegging with the horn, this seems to be a very common problem - I have it in my car as well. I suspect it is at least partially a result of grounding issues, as some people have it, some people don't and I've even heard of it happening with the OEM VDO tach amp. I think the electromechanical horns put a ton of electrical noise into the electrical system when they activate, which just happens to be at a frequency that confuses the amp. Someday I'll dig into this more.

So......for anyone interested it looks like I have a very nice, gently used, FACTORY RECERTIFIED tach amp for sale CHEAP. First $20 takes it - FREE SHIPPING! Don't miss this special offer! PM TODAY!

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Bob
'82 300D Petrol B-G Metallic

Can someone give more clues on what to look for? Under the screw-on cap is there a bunch of empty pins?! Is that what we're talking about? My tech has same exact symptoms described in OP.

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

The tach amp is inside the cap, you'll notice there are male pins to fit into the female sockets on the lower part.

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83 SD

84 CD

Unless you're talking about your '85, in which case your cap will be empty.

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Bob
'82 300D Petrol B-G Metallic

Yes, it's on my '85 - I'm starting to track this down - help, plz.

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

MB went with a different set up in 84, the cap is just that, so that is simply a diagnostic port. The signal comes from the transmission. I understand there is an OVP relay with a fuse, at the red top, located on the passenger side.

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83 SD

84 CD