Quick Tach Amp Question 85 300TD - PeachParts Mercedes-Benz Forum

Fisherman · #1 03-10-2003, 10:13 AM

In my study here I see reference to a "male" pin set in the tach amp cap but when I unscrewed the one that was being worked on it does not have anything in the cap.
The female pin connections are then exposed and some appear to be a little loose. Is there a male connector that bridges the female plug?
If so, does anyone know the pin out on these that it could be bridged with wire if that is in fact what is needed?

RSH · #2 03-10-2003, 11:44 PM

I don't know if this will help, useing my 83 300SD as reference, yes there is a male connector that corresponds with the female connector. I did a continuity check across the male pins (six pins) numbered 2,3,6,7,8,9. Pins 7 and 8 have continuity, and pin 2 has continuity to both 7 and 8. The other pins did not have continuity. My tach is fully functional, I recently picked up two tach amps at a local pick a part junk yard for spares, they only charged me $5.00 I did a quick swap in and out of the two spares, one worked, the other did not, Iam going to remove the potting material (silicon rubber) that holds the male pins in place from the non working amp to check how the pins are connected and see if I can repair it.

mercedesmatt · #3 03-11-2003, 12:10 AM

i have repaired several of these tack amps. in short, take off the plastic case, dig out all the silicoln you can, not an easy job, remove the round circuit board, being careful with the wires.
now for the tricky part, you have to get all the silicoln off the board, what i have used is a very fine wire wheel (stainless bristles) on my bench grinder, then ever so gently and lovingly buffed the silicoln off the board. next i gently clamp it in my "panavise" with soft jaws. and resolder every connection on the board. use only a 25 watt, pencil type, soldering iron. with a very fine tip. be very careful not to "bridge" any solder connections. and im sure this will let you wide open for disappointment to say the least!!! goodluck guys!!!

mbdoc · #4 03-11-2003, 08:23 AM

The 1984 & 1985 cars don't use the amplifier! The 1984 & 85 cars use a FLYWHEEL ring gear sensor that produces A/C current. The 1984 tach operated directly off of that signal.
The 1985 car has a computer that processes that signal & then sends it to the tach & a/c compressor relay. Many times if the tach is inop the overvoltage relay fuse is blown form jump start OR battery replacement. The OVPR is behind the rt outside dash vent.

#5 03-11-2003, 08:33 AM

MB Doc is right.. the fuse is on top of the relay on the passenger side to the right of the glove compartment.... in my case it was a bad connection in side the relay.. just took it apart and frixed it..

Ken

Fisherman · #6 03-11-2003, 09:39 AM

I'll go fishing on his car this aftenoon.

Fisherman · #7 03-11-2003, 11:50 AM

This car has the pickup mounted to the front balancer... The grey wire then comes up to the drivers side wheel well where it connects into the round, capped block (this one had no cap and his other car has no male pin assembly in the cap). when you look at the round device (amplifier I am assuming) you cna see all of the female pins with no connection between them. Then there is a black wire that comes off of the bottom of the round assembly, next to the grey wire, and heads through the firewall I believe.
I need to trace this further but does this mean it is using the "old" style pickup with amplifier?
If so then I assume that he needs to get a new cap with the male pins/circuit board still inside in order to even begin to troubleshoot the system correct?

#8 03-11-2003, 04:16 PM

The 85 cars used the old style pickup on the front to the connector so you could read RPM while plugged in to a tester.. However the pickup for the tac is on the injection pump and goes to the computer on the right side kick panel area.. So yes it does have a pickup on the front.. but does not use it.. The one going to the computer of sorts takes care of the idle ect.. as they no longer have the twist idle knob.. the tac runs from that..

Ken

Fisherman · #9 03-11-2003, 04:19 PM

Phew, am I glad you guys are here...

#10 03-11-2003, 04:20 PM

Sorry I should have also said.. a cap over the test connection is a good Idea.. However there is nothing plugged into it like the older cars.. so do not even try to plug any thing except a tester into the plug..

Ken

Fisherman · #11 03-11-2003, 05:53 PM

hey Ken, sent you a pm.

marketwize2003 · #12 03-23-2003, 07:59 AM

can you tell me where the tach amp is located...83 300sd, thanks

shear2001 · #13 03-23-2003, 08:30 AM

Where exactly is that fuse for the tach on the 85 model. Is it under the dash or in the engine compartment?? Thanks

#14 03-23-2003, 09:03 AM

Not sure whitch 85 cars you mean..

On the 85 300 TD, CD, and D the fuse is on the passenger side.. remove the lower cover under the glove compartment and its on top of a relay mounted about where the lower dash board bolt is..

I have no idea where it is on the other 85's

Ken

shear2001 · #15 03-23-2003, 07:55 PM

thanks I'll give it a try when I feel ambitious!!