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Old 04-10-2008, 07:25 PM
Jeremy5848's Avatar
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Join Date: Mar 2006
Location: Sonoma Wine Country
Posts: 8,394
Originally Posted by DeliveryValve View Post
Regarding the trimming plug Jeremy. I understand what you are saying about the rack position sensor but in this post, M-Shop member Bstef says otherwise on post #11. Maybe controlling the emissions output also has some effect on the idle. (Pic #1 of the trimming plug is from Bstef)
I suppose that is possible. The FSM (1985 "Introduction Into Service") doesn't discuss adjustments to the trimming plug, just prints the electrical diagram. I don't have a late edition FSM to check for detailed instructions. Later cars, like my 1987 300D have a similar trimming plug in the engine compartment and it does indeed change idle speed.

"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
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Old 04-11-2008, 09:56 PM
Join Date: Apr 2008
Posts: 7
OK, tach is still not working, despite the following:

- rpm sensor appears to test OK (I only get readings when I touch the probes to the male posts, not to the female sockets)
- a/c and cruise control both working fine, so OVPR and Klima relay probably working fine
- green oxidation cleaned from the contacts in the control unit
- trimming plug also cleaned, though it can't actually be rotated- there are only two pins and sockets in it, and small fins prevent it from being rotated anyway (probably not relevant anyway because of the next item)
- EGR and AVR vacuum hoses are blocked off
- all ALDA lines and banjo bolts nice and clean, ALDA screw turned out to max
- clock in the tach works fine

As Jeremy recommended, I briefly touched my multimeter's probes (in ohm mode) to the tach's contacts (there were three contacts, so I tried all combinations), but the tach needle never moved... so perhaps it's simply a bad meter in the tach after all, even though it worked for a few weeks several months ago.

I'll have to watch for a tach on eBay and such...

At the risk of sending this thread off topic, one of the reasons I want my tach working is because I recently learned about the bowden cable, so have been dialing in the shifts to my liking.

Of course, without a tach I can't really check the rpm's to what others recommend, and with no other experience in a 300TD, I wouldn't know what the rpm's should sound or feel (other than what sounds and feels reasonable). Thus, I'm erring on the side of caution, though would like better 0-60 times.

I'm only getting 0-40 in 12 seconds, and 0-60 in 24 seconds, when I should be able to get to 60 in something like 16 seconds. Granted, the stretch of freeway I'm using is not perfectly level and straight (I live in LA), but I wouldn't think it's curved or sloped enough to account for such slowness.

I recently found a few posts about the kick-down switch, and I realized I've never experienced what that switch is supposed to do. It turns out my pedal doesn't actually come in contact with the switch anyway, so I added a short piece of pvc around the linkage behind the pedal so it would press the switch when floored- but still no difference in shifting, so I've ordered from Fastlane (of course) a new kick-down switch to see if that'll help with any of this.

Then again, I'm also talking my wife into driving along side me and telling me her speedo readings as a comparison of my speedo.

Thanks to everyone here for contributing to a thread that will hopefully prove useful to others.
Federal 1985 Mercedes 300TD, 220k
Blue exterior
Palomino interior
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Old 04-18-2008, 07:18 PM
Join Date: Apr 2008
Posts: 7
Here's an update on my still non-functioning tach.

I re-tested the rpm sensor leads with my somewhat cheap, battery-operated multimeter, and after reading it more carefully, it appears my rpm sensor was not reading within spec.

I only got readings by testing the male ends on the rpm sensor, and with the engine off I saw a reading of 2.08 k ohms, which is greater than the 1.9 k ohms max. value in the rpm sensor test that DeliveryValve posted earlier in this thread. With the engine idling, I also saw only 2.8 VAC, which was less than the 4 VAC in the test.

So, today I replaced the rpm sensor with a new one- aaaand the tach still does not work.

The new sensor gives a reading of about .5 k ohms with the engine off, which is within spec. However, with the engine idling I see only 3 VAC- more than what I saw with the old rpm sensor, but still less than the 4 VAC it should be.

I also used speaker wire to connect the rpm sensor contacts directly to the contacts in my old tach and a used one I bought last week, but neither tach worked. Since that bypasses the control unit, I think the control unit is not the problem... so I'm thinking the problem resides in where the new rpm sensor picks up its signal.

I'll probably be away from Internet access for the next 2 weeks, so perhaps there will be more discussion in this thread when I return.
Federal 1985 Mercedes 300TD, 220k
Blue exterior
Palomino interior
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Old 06-24-2008, 06:28 PM
Join Date: Apr 2008
Posts: 7
After stumbling across this thread

1985 300 D Tach Problem

I got a used EGR control unit and replaced the slightly corroded one in my wagon (also apparently a replacement from the previous owner), and the tachometer is now working. It sometimes stops working, but I suspect that's because of a loose connection somewhere.

But at least the tach now usually works. Thanks again to DeliveryValve and others who contributed to this thread and the other thread linked above.
Federal 1985 Mercedes 300TD, 220k
Blue exterior
Palomino interior
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Old 06-25-2008, 02:30 AM
JimmyL's Avatar
Rogue T Intolerant!!!
Join Date: May 2004
Location: Sunnyvale, Texas (DFW)
Posts: 9,673
Originally Posted by corchard View Post
It sometimes stops working, but I suspect that's because of a loose connection somewhere.
There are quite a few wires inside that harness that plugs into the EGR computer, and if the old unit was crusty then some of that moisture probably got to the wiring inside the connector. About 3 wires on mine popped off after I got the plug apart. Just barely touched them and the wire broke from the crimped portion of the pin.
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
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'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John
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