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  #1   IP: 137.103.82.227
Old 03-19-2020, 09:04 AM
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Tachometer Odd Issue

My VDO tach finally died after acting odd for a year or two. They are not cheap, so I took a chance on a FleaBay Chinese knockoff for $25.
It fits the 85mm hole exactly and has nice red lighting, so far so good. What wasn't good was it would sit at 0, sometimes go to 2500, or sometimes go to 3,000. All totally unrelated to what the engine was doing

Before sending the thing all the way back to China, I decided to Google-Fu some repair ideas and found out something interesting - these low RPM tachs that read to 3 or 4 thousand RPMs primarily go on diesel engines and primarily read off the alternator AC tap. That is a much cleaner waveform than what comes off the ignition coil. I could move the connection to the alternator, but a couple of other ideas found online looked promising. VDO themselves suggests using a diode in the line and another person used a 10K resistor. I found a 5K resistor on the boat and tried that and it worked
I'll have to calibrate it to my timing light/digital tach, but for now the suggested ratio setting seems very close, idle is about 800 RPM and full throttle about 2400-2600. You would go nuts using one of these in a car, the needle is damped enough that shifting at a certain RPM would always miss.
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Last edited by joe_db; 03-19-2020 at 09:11 AM.
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  #2   IP: 134.134.139.74
Old 03-19-2020, 12:55 PM
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Joe--

Did you move to the AC tap and add the resistor of did you keep the tach input on the coil?
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  #3   IP: 137.103.82.227
Old 03-19-2020, 01:19 PM
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It is on the coil. I wanted to see if that worked first and it seems to
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Old 03-19-2020, 01:57 PM
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thanks for the clarification.
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  #5   IP: 72.69.36.126
Old 03-20-2020, 07:23 AM
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Which eBay tach did you buy, and where did you put the resistor?
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Old 03-20-2020, 08:40 AM
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This one:
https://www.ebay.com/itm/85mm-Marine...8AAOSwX0VdQ98H

The resistor goes in the lead from the tach to the coil. 5K seems minimum to get a reading, I am going to try a 10K I found in my supplies too and see how that does.
Some VDO tachs suggest a diode, I have some and might give that a try as well just to see if it works.
Note that if you buy any Chinese stuff now, it might be a long time getting here! Do keep in mind about 5,000 vendors that come and go sell these things and similar, I am not really saying this one vendor or gauge is a great idea - way too soon to tell if it holds up. On the good side, if it doesn't I am only out about $22 or so.

Last edited by joe_db; 03-20-2020 at 08:55 AM.
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  #7   IP: 69.250.111.245
Old 01-07-2021, 11:04 PM
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OK...I am intrigued...
Bringing this back to the top.

I am not an electronics guru..My Dad is (long retired) a EE. He taught me to read resistors when I was a little kid, but it didn't stick.
Will something like what you put in your car when you convert from incandescent bulbs to LED in the blinkers be enough resistance to make the tach work on the coil? I will have to rebuild my gauge pod, but it is a brownie pan and a HDPE cutting board..not too much work.
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Last edited by sastanley; 01-07-2021 at 11:09 PM.
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  #8   IP: 138.207.177.95
Old 01-08-2021, 07:56 AM
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No, most car LED ballast resistors would not be anywhere close to 5,000 ohms. PM me your address and I'll send you some resistors, I have a ton sitting around.
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  #9   IP: 69.250.111.245
Old Yesterday, 11:53 AM
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Thanks, Joe! now I just need to order one of these and wait and see how it goes.
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