Hi Bob,

I just came across this thread and I too have tach problems. My current tach does not work and jumps around when I use other electric devices throughout the car (i.e. fan, windows, etc.). I bought a new cap from ******** az which did not help at all. What should I check first? Should I go ahead and get the new board you offer or do I need to use the tool to check out the problem? Thanks a bunch!

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Scott

1983 300D Turbo

Scott, I would do the following:

1) gently squeeze the female contacts in the socket into a very slight 'oval' shape. The objective is to help them make better contact with the pins.

2) Check the crank sensor at the harmonic balancer. Check the clearance between the pin and the sensor face (should be 1-2 mm), and check the sensor itself for damage.

If this still doesn't help I'd go for the tool.

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Bob
'82 300D Petrol B-G Metallic

You can also check the sensor's millivolt output by using a voltmeter at the amp socket. The higher the RPM the more voltage it should generate.

Bob,
Do you have an illustration of the location of the crank sensor? This is my first diesel benz and I've had the car for about 6 months. Thanks.

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Scott

1983 300D Turbo

I just ordered one. Crossing my fingers that this tach amp will solve my issues! Thanks!

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1985 300SD - 167k miles
1992 F350 7.3 (Soon to be converted to 2 tank Veggy/WMO)

Bob, got any of those tach amps left?
I reviewed a bunch of confirmed fixes on Identifix which all involved delicate soldering on the circuit board. Seems like for the price, might be a more durable repair to replace.
Also, you mentioned a trimpot on the back of the tach. Does the speedo have one as well?
I mention it because my speedometer reads about 5 mph over actual speed. Same 5 off at 20 as at 70. This leads me to believe that there is an adjustment somewhere.

Bob I would like to purchase one . I need one for my Merc 1980, 300SD Turbo Automatic.
If possible I need the product in India, if not then I can have it sent to someone in the USA. Kindly let me know of the possibilities. Regards, Yashpal

Yashpali - I've shipped to Canada, but not yet to India. I'm willing to give it a shot if you are....

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Bob
'82 300D Petrol B-G Metallic

Dear Bob,
Thanks for the reply. I would be obliged if the product can be shipped to India.
Mailing Add:
Yashpal Amarsingh
House No:5417, Sector 38 (West)
Chandigarh. 160014 (UT)
India.
How do we work out the payment mode.
Else you could ship the same to my brother in North Carolina and he could do the rest.
Mush obliged, waiting or your reply.
Regards,
Yashpal

I just wanted to say that I bought one of Techguy's tach amps and it worked perfectly. I would recommend it to anyone that wants to get the best out there.

Frank1943

and have been very pleased with it. Mine works perfectly, also..

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Mine: 1987 MB 420 SEL - 62K (Dec 2019)
Mine:1991 MB 350SDL - 244K (Aug 2011)- Totalled 9/14/2016
Mine:1981 MB 300SD - 326K mi (July 1997)-sold
Wife's: 2008 MB C300 Sport - 92K mi (April 2009)

This has been the cure I have been searching for for 10 years (honest). It works and works well and provides much needed diagnostic info if needed...

Is it possible to modify your basic design to work with an aftermarket tach? I know those sense rpm by being grounded 2, 3, or 4 times per engine revolution for 4, 6, and 8 cylinder petrol engines respectively.
I'd just really like to run a gauge that matches the rest of the instrument cluster without having to spend big money on a VDO setup.

It's possible, but it really depends on the aftermarket tach you're talking about. Is it 4, 6 or 8 pulses per rev? What are the input signal requirements (amplitude, duration, etc.)

I'd be willing to look at it......

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Bob
'82 300D Petrol B-G Metallic

Well, aftermarket tachs have a switch to choose 4, 6, or 8cylinders for four stroke petrol engines. The number of spark plugs fired per revolution would be half the number of cylinders, so 2, 3, or 4 pulses per revolution. All aftermarket tachs take their signal from the switched negative side of the coil, which would be alternately grounded and then have 12v present. Since you have to supply 12v and ground to the tach itself, I assumed it watched for the signal wire to be grounded, but I suppose it could be looking at the presence of voltage there, or it could be the combination. I have an old Sunpro sitting around I can test this on and get back to you. Any aftermarket tach would work the same way though, so the findings would apply pretty universally.