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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....
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Bob '82 300D Petrol B-G Metallic |
#2
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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.
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1983 300D-Turbo - Deep Blue w Palomino MB Tex (total loss in fire 1/5/09 RIP) 1995 E320 W124 Polar White/Grey Mushroom MB Tex 2005 F150 Supercrew - Arizona Beige - Lear topper 1985 Piaggio Vespa T5 - Black and Chrome www.cphilip.com |
#3
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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.
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Bob '82 300D Petrol B-G Metallic |
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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? 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. It'd also not be bad to have a spare....
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-diesel is not just a fuel, its a way of life- '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k) |
#5
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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)
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Bob '82 300D Petrol B-G Metallic |
#6
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Quote:
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.
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1982 300CD Turbo (Otis, "ups & downs") parts for sale 2003 TJ with Hemi (to go anywhere, quickly) sold 2001 Excursion Powerstroke (to go dependably) 1970 Mustang 428SCJ (to go fast) 1962 Corvette LS1 (to go in style) 2001 Schwinn Grape Krate 10spd (if all else fails) |
#7
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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
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2009 Kia Sedona 2009 Honda Odyssey EX-L 12006 Jetta Pumpe Duse (insert Mercedes here) Husband, Father, sometimes friend =) |
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That is very impressive Techguy! Your forum name says it all I guess.
Alas, I have a non-working '85. I hope you buy one just like it soon........ Good job Bob!
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Jimmy L. '05 Acura TL 6MT 2001 ML430 My Spare Gone: '95 E300 188K "Batmobile" Texas Unfriendly Black '85 300TD 235K "The Wagon" Texas Friendly White '80 240D 154K "China" Scar engine installed '81 300TD 240K "Smash" '80 240D 230K "The Squash" '81 240D 293K"Scar" Rear ended harder than Elton John |
#9
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Quote:
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.
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83 SD 84 CD Last edited by toomany MBZ; 04-19-2008 at 06:29 PM. Reason: grammer |
#10
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Quote:
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-diesel is not just a fuel, its a way of life- '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k) |
#11
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I am on the garage pc... ......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.... *casts money into the pit.
Replacing that thing does not look like fun.....oh well, its worth it for a working gauge.
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-diesel is not just a fuel, its a way of life- '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k) |
#12
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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.
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#13
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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.
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-diesel is not just a fuel, its a way of life- '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k) |
#14
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Quote:
Manfred 63 E type coupe 73 IH 125C 83 300D 84 BMW R100 91 Chevy G20 w/6.5 NA |
#15
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Quote:
__________________
-diesel is not just a fuel, its a way of life- '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k) |
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