New benz guy- where is the tach sensor and "amp" located - PeachParts Mercedes-Benz Forum

surfblau · #1 04-23-2002, 04:14 PM

I have read all the posts on this board to try and revive a dead tach on an 1984 TD, but I am having problems locating the sensor and the elusive "amp".

Since it is an 84, the cigarette butt trick won't work, because the plactic cylinder with the tethered black cap is only used for diagnostics. I checked, there is nothing in there except female connections (no circuit board).

In laymans terms, does anyone know how to locate the tach sensor (from above or below the engine, is it attached to something, what's a bell housing) and where to locate the "tach amp" if such a thing exists (one post said it was in with the EGR control electronics behind the passenger leg space- if that is true, I probably will not be fixing my tach...).

The previous owner claimed that there was a burning noise and smell when the old tach ceased working, but when I took it apart, I could see nothing wrong with the guage. It wouldn't have been the first time he was wrong.

thanks to any patient readers...

mbdoc · #2 04-23-2002, 05:42 PM

The 1984 & 85 engines use an inductive pick-up on the FLYWHEEL ring gear instead of the tach amplifier & pick-up on the front of the engine. ON the 1984 cars the A/C current reading from the flywheel goes directly to the tachometer. Should put out 5-6V a/c at idle.

surfblau · #3 04-24-2002, 01:05 PM

M.B.DOC ,

That was perfect. I will check it out this weekend when I have time to get to the tach leads on the instrument cluster. If I don't get the voltage, I will be looking for the sensor on the Flywheel ring gear and see if it fell off or is broke.

I am new to all this car repair and trouble shooting, but where is the flywheel ring gear and sensor? I looked on the shop CD and saw a detail of the flywheel ring, but I don't know exactly where it is on the block. It must be on the back of the engine block immeadiately in front of where the transmission is, surrounding the crankshaft (sorry for the pedestrian description). If thats the case, it sounds like the only way to see the sensor would be from underneath (exhaust manifolds, etc).

Do you have any idea where the pick up or sensor is on the engine? Driverside, etc. Since it is ac, does that mean only one wire coming off it?

thanks a lot

The Warden · #4 04-24-2002, 03:26 PM

The flywheel is bolted to the back of the crankshaft, so yes, it's at the very back of the engine. The torque converter (part of the transmission) is attached to the flywheel.

BTW, the flywheel teeth mentioned are also what the starter turns to get the engine started.

And the bellhousing is the metal case surrounding the flywheel and the torque converter (if you had a manual tranny, this is where the clutch would be).

Hope that helps somewhat...good luck!

#5 04-25-2002, 12:33 PM

To fix the tach on my 83 wagon I removed the plastic cylinder cover and sprayed motorcycle chain lube in the female connections. You also try wd40 or liquid wrench. This fix first came from an old time MB mechanic local to me. I tried it about 3 weeks ago and it still performs flawlessly.

give it a shot, you have nothing to lose!

thanks,
Andrew

stephenson · #6 06-06-2002, 12:23 PM

My 1991 350 SDL has a tach problem that started intermittently, then generally permanent with only occasional operations.

When it stops, the air conditioner compressor also stops engaging (everything else is OK).

Does this sound like any of the symptoms anyone else is having?

Any advice on where to start? Did the standard fuse thing ... unrelated at that point, I think.

Could be a matter of connections being corroded or loose?

surfblau · #7 06-06-2002, 12:28 PM

Sounds nothing like any of the problems that I had, but have you checked the grounding wire for the tach? I have noticed a bunch of brown wires coming together under the dash and mounting to the chasis. Maybe that would be a place to look.

Sorry I can't help more....

stephenson · #8 06-06-2002, 01:23 PM

Appreciate the feedback, surfblau,

Unless there is a feedback loop to air conditioner at the tach, or it shares a ground, may not help, but I will look at it just to make sure.

Thanks, again.

amptique · #9 06-07-2002, 11:30 PM

M.B. Doc's suggestion fixed my ac/tach problem.

Bill

surfblau · #10 06-08-2002, 10:25 AM

The only problem that I have now is an intermittent, faint, screaching noise coming from either the tach or the sensor, usually under load and around 3000 rpm. If anyone has ideas on that, let me know.

Otherwise thanks for all the help.