The tach on my 1985 MB300TD is starting to act up - after only 193,000 miles. I read about the Tach Amp on 300s, putting the cig butt under the top, but the 85s are different - I think?

It will work fine for a few days (currrently working), but then suddenly 0 RPM for several min, hours, or days, then all of a sudden it works again. The AC is not working - has not worked since I got the car in Feb 06. I sprayed some cleaner on the tach pickup on the crank - that did not help/hurt.

Any suggestions?

The amplifier on the inner fender....follow the TDC wire from the crank balancer up to the black round socket/plug.

Failure rate = Medium to rarely.

My 1985 has one ....it's an inductance sensor that pick up the flying pin in the crank balancer.....lot's of late model vehicles have short lived tdc sensors.


.



.

__________________
[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Doesn't an 85 have the sensor at the flywheel end of the engine? Or is that only California 85s? If the sensor is at the flywheel end, maybe you have a bad overvoltage protection relay. Intermittent failure suggests loose solder joints rather than a bad fuse or complete meltdown.

Sixto
93 300SD

The sensors rarely fail, unless you tug on the wire and it breaks, this usually occurs only when an engine is removed from the car and the mechanic doesn't take care to protect the cable from damage. Or cuts it off like I have seen

The '85 300TD does not have a module on the L fender as in other years, its inside the car, under the kick plate on the passenger side footwell and is a large rectangular "box" with a connector on one side. It also processes the RPM for the EGR and is in fact called an EGR module firstly, it just drives the tach secondly.
Therefore the cigarette butt trick is not applicable in the '85's at least all of them I have ever seen have the rectangular module.

Fortunately the "EGR" modules do not fail regularly like the earlier trash can shaped VDO tach modules.

The '85 modules are much more scarce. They have one at the shop where I worked and troubleshooting would consist of swapping a used one in temporarily to see if it fixed the problem, before ordering a new one (they are generally non returnable and very expensive!)

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Ditto on that. The overload protection relay was the problem on my '85. They sell them here at Fastlane. That would be the first thing I would check. You'll see it right under the vent on the passenger side. It's about two inches tall and 1 inch around on each side and has a square fuse in the top.

Scott

__________________
Scott
1982 Mercedes 240D, 4 speed, 275,000
1988 Porsche 944 Turbo S (70,000)
1987 Porsche 911 Coupe 109,000 (sold)
1998 Mercedes E300 TurboDiesel 147,000 (sold)
1985 Mercedes 300D 227,000 (totaled by inattentive driver with no insurance!)
1997 Mercedes E300 Diesel 236,000 (sold)
1995 Ducati 900SS (sold)
1987 VW Jetta GLI 157,000 (sold)
1986 Camaro 125,000 (sold - P.O.S.)
1977 Corvette L82 125,000 (sold)
1965 Pontiac GTO 15,000 restored (sold)

You're right...and then again, your wrong !
Let me explain.

Last couple of months I've been tripping over the engine on my shop floor out of a 1985 300td.
Last night I said the tach' and was on the inner fender....well, sort of.

What I took to be the old style inner fender tach' plug is in fact the 9 pin diagnostic plug.
BUT......I've been looking at this engine on and off for a while now and saw the old style front crank damper mounted TDc sensor.....hmmmmmmmm ?

So this morning I thought I'd better check this out after reading Sixto and DDH's posts.
Vehicle is a 12/1984 300TD.
Fuel pump wears an electronic idle control i/p.
Front TDC sensor, as I said...BUT also looking at the flywheel adaptor plate in the back of the vehicle this morning ( Engine dismantled...came that way ! ) I see a flywheel sensor !

So....I thought I'd see what the difference was. The TDC sensor wire on the crank damper is terminated in a 3 pin female socket half oval plug.
Don't see the pin socket on the vehicle for it though.

The flywheel sensor wire has a gouped 3 pin socket with connector on fire wall.

OVPR absent !
There's no ovpr in the engine bay....

So, what I have here is a Frankencar

So...question...where's the front tdc sensor wire loom plug and the OVPR located ?

Any anyone ?


.

__________________
[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

OK, you have bits and pieces, but not the whole image. And, I'm not the best authority, since my tach still doesn't work......

The round socket on inner fender does connect to sensor up front, but is for diagnostic equipment only, not for car tach function.
The Overload Protection Relay is accessed through glove compartment, under farthest right vent. It in the engine compartment on 126's (I've seen it on some, not sure if all)
Flywheel sensor is the one used in tach function.
AC will not work if tach not working. Must bypass Klima relay pins (pictures available if you search "Klima"

Wish mine worked. I need to find somebody one day with another '85 W123 with working tach, and swap things until it works. Best way to trouble shoot this I'm afraid.....
Jimmy

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

We have a winner ! Thanx Jmmy, I bought this basket case a few months ago...

Never seen a 1984/5 'diesel' wagon before. All the one's I'd seen in UK had gas motors in them.
Learn something every day



.

__________________
[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Thanks everyone for the feedback. I think I may have fixed the problem simply by posting the question - since then it has worked fine. Only if all MBs problems could be fixed with this method

Scott98 - you suggested checking the overprotection relay under the vent on pass side with the sq fuse - under the hood? correct? I just want to know so if the TACH dies again, I will start there.

JimG