Tach Sensor Location??? - PeachParts Mercedes-Benz Forum

Gregory · #1 02-09-2014, 09:56 PM

Where is the tach sensor located on the 1983 300D or 300SD 617 engine?

mach4 · #2 02-09-2014, 10:02 PM

There is a pin on the front pulley that is read by a variable reluctor sensor. This signal goes to a tach amp on the inner fender well that feeds the tach gauge.

Gregory · #3 02-10-2014, 12:45 PM

I looked for the sensor location on the ring between the eng/trans. Nothing but smooth metal there.

I like it when it start getting better.

The PO pulled the engine from my 84 300SD and installed what is certainly a 1983 engine. The pin is on the front pulley and the tach sensor is right there. That wire goes to the "tach amp" on the fender well.

However, there is no tach amp inside the little trash can thingy. There is a pin base for the test plug, and another wire comes out the bottom, that should go to the tach in the cluster.

So it looks like I need the entire tach amp & holder, because what I have, the test point, is too short to hold a tach amp board inside of it.

What years/cars will have the same base holder for the tach amp? Will the 300D swap with the 300SD?

It looks like 83/84 was the break point for this part on the 300SD.

Which one will work with what I have? 1983 pickup over the pulley, and a 1984 tach in the dash.

JamesDean · #4 02-10-2014, 12:52 PM

This is what the tach amp looks like normally:

http://www.benzworld.org/forums/attachments/w126-s-se-sec-sel-sd/324090d1281385077-1983-300sd-tachometer-not-working-tach-amp.jpg

Gregory · #5 02-10-2014, 01:05 PM

Quote:

Originally Posted by JamesDean

This is what the tach amp looks like normally:

http://www.benzworld.org/forums/attachments/w126-s-se-sec-sel-sd/324090d1281385077-1983-300sd-tachometer-not-working-tach-amp.jpg

Mine is much shorter. Also, one female pin is missing. Mine has contacts at 2, 6, 7, 8, & 9.

This article: http://mbelectronicupgrades.com/Tachometer%20Amp%20User%20Guide%20Rev%20A3.pdf says a pin is also required at #3, or the tach will not work.

So I will need a need base for the tach amp, can anyone confirm that a 1983 part will work for my situation?

JamesDean · #6 02-10-2014, 01:17 PM

I seem to recall in 84 they moved the tach stuff over to a control box I think.

I think your best path would be to either convert to the 83 system or the 84. Not a hybrid.

I would either acquire a 84 sensor and hook it up in the normal way the 84 is.. or get an 83 base for the amp and and 83 tach/clock pod for the instrument cluster.

I dont know much about the stuff they put on in 84/85.

It might be far easier to just get the 81-83 base and tach/clock pod.

I'm not sure what kind of signal the 84's tach pod is expecting. I know that MB changed it in 84, presumably with the rest of the tach stuff.

Gregory · #7 02-10-2014, 01:23 PM

Making it work as an 84 requires removing the transmission and replacing the 83 eng/trans adaptor ring with the 1984 ring, so the 84 tach sensor can be reinstalled. I have those parts, but its a big job to R&R the trans.

Changing the 84 test plug to an 83 tach amp would be real easy to do, if that config will work with the existing 84 RPM gauge in the cluster. Does anyone know if that will work?

JamesDean · #8 02-10-2014, 01:38 PM

Yeah I would just adapt it back to the 83 setup. Way too much work to pull all of that stuff unless theres a really good reason.

Its hard to say if it would work.

I can tell you what kind of signal the 81-83 tach amp will output (I have oscilloscope shots of it) but I don't know what kind of signals the 84 tach gauge expects. They might be totally incompatible.

Moreover I think that if it is electrically compatible the signals would be different, the pickup on the balancer might have less pulses per revolution than the pickup at the flywheel.

Gregory · #9 02-10-2014, 02:19 PM

Kris - Here is a thread with good info on how the 1983 tach sensor and amp work: How the Tach Amp works

Can you compare that info to what you know about the signal needed for the 1984 RPM gauge?

toomany MBZ · #10 02-11-2014, 08:19 AM

MB changed the set up in 1984.

You must use the proper year tach gage with the proper wiring harness.

Gregory · #11 02-11-2014, 08:43 AM

Quote:

Originally Posted by toomany MBZ

MB changed the set up in 1984.

You must use the proper year tach gage with the proper wiring harness.

Will the 83 tach gauge fit into the 84 cluster spot?

Gregory · #12 02-11-2014, 08:56 AM

What range of years will work with the 1983 tach sensor at the front pulley? Which cars, 300SD, 300D, 300CD, 300TD?

I need to buy:
Tach amp with its base
Tachometer gauge for instrument cluster
The wire between those parts.

toomany MBZ · #13 02-11-2014, 09:02 AM

The tach will fit, but the wiring harness plug is different.

Any tach set up from 1983 and earlier will work, from any 617.

JamesDean · #14 02-11-2014, 12:57 PM

Quote:

Originally Posted by Gregory

Kris - Here is a thread with good info on how the 1983 tach sensor and amp work: How the Tach Amp works

Can you compare that info to what you know about the signal needed for the 1984 RPM gauge?

I'm afraid I dont know anything about the needs of the 84. I know the what the 83 tach amp will put out and the 83 head will expect. I know nothing of 84 system.

Probably the cheapest/best route would be to get a 83 tach head and wiring harness. Its only two wires that run from that tach amp socket to the cluster.

Gregory · #15 02-11-2014, 01:00 PM

Here is good and critical info:

MBZ 300SD part numbers:
81-83 Tachometer p/n = A 002 542 21 16
84 Tachometer p/n = A 002 542 73 16