OM617A Tach Amp
 

After struggling with my tachometer for over a year, and trying all the tricks including contact cleaning
and cigarette butts, I decided that if I wanted a functioning tach I would need to take matters into
my own hands and tackle the project of designing a cost effective replacement Tach Amp.

The results of the project are shown below in photos.

1) Tach Amp installed in the connector socket on the left front fender
2) Cover (minus the old electronics module) being replaced on the connector socket

This tach amp should be compatible with the following vehicles equipped with the OM617A
engine built between 1978 and 1984:

W116 Chassis 300SD 1978-1980
W126 Chassis 300SD 1981-1983/1984 (it's unclear exactly when the tach amp was phased out)
W123 Chassis 300D/CD 1982-1984
W123 Chassis 300TD 1981-1984

My minimum order quantity on PC boards was 40pcs, so I have about 35 left, and I want to offer
them to anyone that is tired of having no tach. Cost is $42 with $4 S&H.

I have attached the User Guide (with photos stripped out to reduce file size) for anyone interested in
learning more about the operation of the Tach Amp.

During this project I learned a ton about the operation of the tach in these vehicles, as well as how
to troubleshoot tach problems. If anyone has any questions, just fire away....

Not sure I understand which cars used a tach amp and which ones did not. If you unscrew that cap and find nothing but a plug, then where did that model take its tach signal from?

I'll have to look at mine. However the tach does work in mine now. Is there an advantage to messing with one that works? Like perhaps a better signal and more accurate signal or just preventive medicine?

Looks neat and tidy thats for sure.

Never mess with anything that works.

The '85 model year gets its tach signal from the EGR computer or some such thing. Your '83 should have an electronics module inside the screw-off cap. When you look up inside the cap, you should see six pins hanging down.

I have diagnosed my issue down to the pickup at the crank pulley.....I have a known good tach amp (its new, and worked in our other two cars which both have working tachs.....and I have tried three different gauge pods, all of them are known working. I have voltage (13.9) at the plug going into the gauge pod....as well as at the diagnostic plug/tach amp port under the hood....so the only last thing it could be is the sensor/pickup itself. It is aligned properly....and the magnet on the balancer is there......there's just no signal.

Others have said its not possible for the pickup to fail.....but if that were so....why do they sell a replacement for it here on fastlane? :D

Should I go ahead at this point and replace the pickup/cable? I am 99% sure it will cure the problem.

Also, I might order one of your units anyways, because of the self-test feature....thats just cool. :D It'd also not be bad to have a spare....

It is completely possible for the pickup to fail. Or the cable to fail. Or the solder joints at the sockets to fail. I talked to one person who had the pin at the pulley crash into the pickup and take part of the face off. New pickup fixed it.

The other issue that you might check into is bad wiring between the amp and the gauge. This new amp has the self test feature to help eliminate that as a cause of problems.

Here are the pin assignments to the pickup. The numbers are shown on the socket connector on the fender (after you unscrew the cap).

pin 9 is input from sensor
pin 8 is sensor return (gnd)
pin 7 is sensor cable shield

Get a DVM, and put in resistance (ohms) mode. You should get the following readings:

pin 8-9 80 ohms (+/- 20 ohms)
pin 7-9 infinity (open)
pin 7-8 infinity (open)

I have an early model of this PCB. It works great. Very steady needle and nice diag sweep at the beginning. My tach did not work at first either. I fixed it by turning the crank by hand with a socket on the crank nut until the pin in the crank is aligned with the sensor. Then bend the metal bracket with a pry bar to get it within a 1/16" of the sensor face. Works like a charm. BTW. on my first attempt I bent the bracket too far and ruined the pickup when the crank pin came around and ripped the front of the sensor. sneak up on it, changing the crank sensor is a real PIA.

Fantastic work techguy,
You should get this stickied or something, or an announcement at the top.

THIS is why I hang on this forum (I sold my Benz Diesel last year...)


-John

That is very impressive Techguy! Your forum name says it all I guess. :D
Alas, I have a non-working '85. I hope you buy one just like it soon........;)
Good job Bob!

IIUC, the '84's and '85's did not have an amp.
My CD does not have one, my SD does.
I'll agree with techguy512, if it ain't broke...
When I purchased the SD, the tach did not work, replaced the amp, has been working since. Not sure where the '84 gets it's signal.

I was trying to read ohms on the socket today and couldn't get anything from it (engine was running).....I did every combination of pins (including between 8 and 9) and it just said "0.00"....so I guess I can assume the pickup is dead....where the wire goes into it is kinsa loose and icky looking....I'm betting its damaged (or the cable shielding is) I'll order a new one and try that.....and I might still order one of your amp units soon too. :D If I can get this thing working I will be a happy man! :D

I am on the garage pc... :D ......I just did an extra ohm-test on pins 8 and 9, and there's nothing, nada....a few times I got a brief "181.3" reading (for less than a second), but then nothing. So I am guessing (based on physical condition also) that its dead. I doubt its the bracket not being close enough, I hand rotated the engine today...and it looks to be VERY close...so now I suppose I will order a new pickup.... :rolleyes: *casts money into the pit. :D

Replacing that thing does not look like fun.....oh well, its worth it for a working gauge.

An Ohms test has to be done with the engine off. With the engine running you need to measure millivolts and see if the voltage changes with RPM.

I also did a millivolts test while I was at it (did every test the meter can do) and all resulted in a "No Reading"/ 0 on the display....so I'm going to call it dead.

I had the same problem with my 300D. Sometimes it would work but mostly not. When testing it I would get the right ohm readings but apparently it was bad enough so that it didn't work with the vibrations from the engine while running. After getting a new amp from Bob It worked better but I still had problems with wild mood swings of the tach needle. After replacing the sensor which, after getting it out, turned out to be kind of messed up at the tip. With the new sensor the tach works great. I especially like that test swing. Lets me know right away that at least the amp and tach are good to go. Thanks Bob.


Manfred
63 E type coupe
73 IH 125C
83 300D
84 BMW R100
91 Chevy G20 w/6.5 NA

Awesome, I have ordered a new sensor pickup....so I'll get that installed later this week when it comes and if it works well, then I'll be all set....I am still contemplating that newer style amp though...we'll see how well the one in my car works first.....it worked fine when I tested it in our other two cars.