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.

To Techguy512
 

If this fits my 300SD 1981, I would like to have one. How do I pay?


tyl604@aol.com
Adrain Taylor

I think I will buy one too. How do we pay? :D

Faq
 

Here are some answers to some questions that have come up several times:

1) This is not a kit of parts. Most people will not be able to handle the soldering of the small SMT parts. For this reason it comes assembled and tested, with user instructions.

2) Paypal is preferred. Alternate arrangements can be made, just PM if needed. My paypal ID is rlm_0513@earthlink.net (that's with an underscore between 'm' and '0')

Thanks,

For those wondering about the 84 and 85 models and the tach amp, my 300 CDT (production date 01/84 - must've been one of the first one off the line that year) there is NO tach amp under the cover on the assembly mounted inside the DS fender. If you remove the cover and look at the sockets, the #3 socket (they are numbered) doesn't have the metal female pin in it.

On this style the tach gets its signal from a different sensor mounted at the BACK of the engine in the bellhousing of the transmission - just behind and below the bottom of the oil filter assembly. The sensor is held in place by an allen-head screw and it takes a 5mm allen key to remove it.

My sensor is bad and needs to be replaced. You can test it by unplugging the 2-wire cable connector at the center of the hood opening just in front of the cowl. If you follow the wire from the sensor up to the connector it is easy to find.

With the car off, connect an Ohm meter to the two make pins of the cable connector. It should measure 1.9k Ohms +/- .2k ohms (that is 1900 Ohms +/- 200 ohms). If that measures OK, then start the car and use a volt meter to measure the voltage across those same tow pins. At normal idle speed of 750-800 RPMs it shoud measure ~4V +/- .1V.

If EITHER of these measurements is outside these value ranges, it needs to be replaced. Mine measured 2024 ohms (in spec) but only a few millivolts at idle.

Tach Diagnostic Tool
 

A lot of folks are experiencing frustration when debugging their tach systems because of the "lack of feedback" that the system is able to give. Is it the amp or the sensor? Is it the sensor or the gauge?

I've taken one of the tach amps, and made it into a diagnostic tool by adding an LED and modifying the software a little bit (see photo).

To use the tool, plug it it into the tach amp socket. Turn the key to the glow position. The led will flash for 20 seconds. If your gauge works, it will indicate a constant 1200 RPM during this 20 second period. Once the LED stops flashing, start the car. If the amp is getting pulses from the sensor, it will flash, once per pulse. The speed of the flash will go up and down with engine speed.

So-

No flashes at glow = no power to amp
No flashes at idle = possible bad sensor / bad cable
Flashes at idle, but stops flashing at higher RPM = weak sensor input or sensor too far from pin
Flashes at glow and idle, but no needle movement at gauge = bad gauge or wiring

Tool is available for loan.

I'll take one... can I mail you a check?

The above is from this thread...the next quote is from this thread (post 22)

I measured mine and I get a reading (I think around 80 ohms) between 7 and 9, which should be correct, and which does your new amp use? Because I think I want one!

Check to be sure your car has an amp first
 

You need to first be sure that your car is equipped with a tach amp at all. There was a switch sometime in 1984/1985 to a different scheme that deleted the tach amp and used the EGR computer instead. This may not have been true for your car, as I don't have any experience with euros.

PM me your email address, and I'll forward installation instructions with photos. It talks about how to tell if you have an amp or not.

I didn't have a tach amp originally, I didn't have a tachometer originally. I do have the little receptacle for the amp. I got a tach from another diesel and installed it. I got an amp, ran a wire from pin 3 to the tach and gave the tach +12v. I've got +12v at pin 6 and GND at pin 2 (they were already in the receptacle) and the three crank sensor pins, 7, 8, & 9. But I don't know if the crank sensor pins are the same for the cars with and without tach amps...I would assume that the pinout of the receptacle would be the same, but that;s me assuming :rolleyes: As far as I can tell, the tach should be working...I don't know if its a bad gauge or bad amp or if the pins for the crank sensor aren't giving the amp the input that it requires...does the amp output AC mA? I've tried to measure with the car running, but never had a good ground other than the cigarette lighter, so the numbers were bouncy. I guess I should check the amp more thoroughly before deciding its defective...but I have no way to test the gauge.

You need to borrow the diagnostic tool.

The amp outputs a pulse train. One 3ms, 5V pulse per engine revolution. To properly see it, you need an oscilloscope. A DVM will only give you a coarse indication that SOME signal is present - not necessarily the proper signal.

The diagnostic tool sends a signal to the gauge (even before you start the engine) so you can see if the gauge and wiring are working.

Try to eliminate potential problem areas one at a time.....

I don't have the right kind of tach setup to benefit from this, but I am very impressed with your finished project. :notworthy:
I hope you will tackle the early style cruise amp next......:D:D

:idea2:

Here is my latest bizzare issue....I replaced the tach sensor pickup (boy was that fun :o, and yes, the old one was destroyed, frayed wire! ) and now I get a needle that "slightly" lifts off the peg at idle, and rises to maybe 1500rpm when the engine is at like 3000+....it moves smoothly, its just reading like 50% lower than it should be.... :dizzy2: The tach amp is good, I tried two different working ones that work fine on our 300D....with the same "weak needle" result on mine.

So I have gone from dead tach to smooth but inaccurate tach. :o Why o why is it so hard to fix these!?

Any ideas why its reading low like that? I just don't get it. Maybe I will need to borrow that diagnostic tool also....

Could the pickup have too much gap with the pin that it reads.......

Thats what I am currently looking into, it does look like it could be closer....I am going to try and get it as close to the pin as possible.

(I am out in the garage typing this.... :D :D) Now thats live-forum-ing! :D ;)


On a cool note, I figured out that the 1 and 1/4" deep socket (27mm) works on my crank pulley bolt to rotate the engine! How awesome! I can't believe I've had it that long and never realized it was the right size for the crank pulley! Its because I usually only think in/use metric....never occurred to me that it was roughly 27mm.... :dizzy2:

Can you put me in line for borrowing the diagnostic tool?
For some reason, I thought the amp output was in amps, and increased as the engine speed increased...could just be something I made up.

Is it a 6 point? 1 1/4" is just shy of 32mm...careful, don't want to round THAT bolt.

Yes, when I said 1 and 1/4" I actually meant 1 and 1/16" :D Wups. :D ;)

It is a 6pt....works great.

Well, I got the pickup as close to the crank pin as I could, but it still reads the same....barely anything at idle, and 1/2ish of actual rpms when revved up. :o

Could it have anything to do with the fact that I have a gasser tach in the car? My original had a dead clock so I swapped it with one from a 280se....they have the same (large 2-pin) connector etc...the only difference is that the gasser one goes to 7k instead of 6k.....

Techguy? Anyone?

I think I have it figured out, the gasser tach is designed to read the pulses from the ignition system of the 6cyl gasser (280), which would happen more frequently than the crank pulse of a diesel, which is why it reads less than half of my actual RPM......sooo, I will be doing surgery on my cluster tomorrow and taking the working clock from my gasser tach unit, and inserting it into the original diesel tach with a dead clock that I took out of my car a long long time ago, then reinstalling that back into the car....hopefully it solves the problem!! I know the sender and amp work now, since the needle does move.... :D After my modifications tomorrow it should work again.... :D

FYI the meter movements are the same on all models. The only difference is the circuit board on the back & the face. It might be easier to change out the board.

PM your address to me. Should be ready to go Tuesday or Wednesday....

Yes, but the gasser ones measure the signal differently...so they are not interchangeable.......The part numbers are different also.

The gasser one says "21000 lmp" on it, while the diesel tach says "6000 lmp"

Gasser one also says 902/29/8 while the diesel one says 902/29/5

Interesting stuff.... :D

Ah yes, sometimes I mix up my denominators too ;) 1-1/16" should be dead on..

I tried a gasser tach in mine and got no movement, so you're ahead of me...I guess you're finding that its hard to find a w123 diesel tach for less than $100? I lucked out on mine, it wasn't too expensive, but I'm finding that it may not work :rolleyes:

Good luck.

It probably depends on the gasser tach, I got mine off a junk yard 1984 280SE euro.....the 8cyl ones are probably configured to read even more pulses per second equivalence....so on a diesel they don't register anything.

My car (that I am fixing this on) is actually a W126....our W123's tach works fine....annoyingly....

Can you publish the pinouts and signaling methods for the tach? Mine is working at the moment, but I don't think its very accurate, I'd like to be able to use my oscilloscope and signal generator to test it...


Also, if anyone has similar info on the cruise amp, I may be interested in making a similar product, since mine has been malfunctioning...

Fix the clock
 

If you haven't already done the above, the easiest task is to replace the two capacitors in the clock. About $2 at radio shack. Do a search, there is a detailed thread on this repair. Did mine and the clock works fine, my tach is still not working but I'm sure it's the amplifier under the hood.

Charles

This is the best thread EVAR about the pinouts and signaling of the stock VDO amp.

OldPokey did a great job on this one......

http://www.peachparts.com/shopforum/diesel-discussion/206808-how-tach-amp-works.html

How did you measure the input pulses on the microcontroller? ADC with the sensor interface described, measuring for changes in voltage?

I'm going to have to build a circuit that can generate test pulses since my diy signal generator can only generate square, sine, or triangle waves...a 555 should do it

Im in for one also....Send me an email..
Dan
Thank you

What exactly are you trying to do? Are you trying to debug your system, or are you doing this for fun and learning?

Little bit of both, turns out my tach is a few hundred rpm off like i suspected, threw together a circuit that outputted 50 3ms pulses per second, and my tach was a bit off from the 3000rpm line... Not sure if its a constant difference or if it varies with speed, but it does explain my unusually low idle (on the tach)...

Let me know if you want any help. I seem to remember seeing a trimpot on the backside of the gauge itself. I never messed with it, but it might be there for gross calibration.

The microcontroller I'm using is quite flexible. I can change the program to output any RPM I want, and hold it there for any length of time, and step it through various RPM values.

if you give me say 5 different values of "pulses per second" that you want, I can write a short program that will output and dwell on each value for 10-15 seconds. You can then check the needle indication for each, and see how your tach performs through the range. Then, if the trimpot actually turns out to be for calibration, you can make minor adjustments to bring the gauge into better calibration.

Let me know if interested.

I'd like to post how happy I am with my new tach amp from Bob. The install time was a whole three minutes. My tach now works like a champ. It's kind of strange and exciting seeing smooth needle movement from that side of the cluster.

I replaced my Tach Amp with Bob's self testing unit in under 2 min. It was quick, it was easy and best of all it works great. 1 more issues put to rest on the way back to all systems GO. :)

83 300 sd crank sender location?
 

thanks to techguy! I now know the amp is working as well as the tach...can anyone tell/show me where the crank sender is on the 83 300sd? can't find it? thanks for any help! glider

I've bought one from techguy. :) I may have to borrow that diagnostic tool if the new amp does not fix the problem.

The sensor is on the edge of the vibration damper on the crankshaft nose. Follow the white wire that goes into the tach amp and you'll see it.