Bench test a tachometer?
 

I still haven't figured out my tach, is there a way to bench test it?

I have tried runing a JUMPER wire direct from the CDI to the tach - with no results.

There is continuity in the wire from the CDI to the tach.

I have also tried 3 different CDI boxes, no change the engine runs but NO tach??

I do not have another tach to swap out and check, I am hoping there is a way to bench test a tach?

Thanks
Shawn

Shawn, tach in my 83 went south recently. Don't know how to bench tach, but I used digi-meter to check for trigger at tach. What year car??

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Robert Stoll
83 SC
83 944

Robert
It is a 77 911. How exactly did you test fro trigger at tach? I have a digital multimeter I but am not a great electrician, give me details if you don't mind. And numbers that I should be reading too?

Shawn

Shawn, my meter has a setting for reading frequency pulses. I didnt look for a specific reading, just a rise or spike when the engine was revved. At first I tried checking just at the tach leads, but Warren said the tach must be hooked up and read at the TD and Ground spades. There is a resistor in the tach that must be in the circuit to get the reading. If you get the swing when revving the engine, tach is probably bad. There are usually plenty to be had on ebay. Or Pelican will refurbish for about $100.00. At John Walker's suggestion, I got one from a salvage yard. Campbell/Nelson in Edmonds, Wa. Ph 425-771-4931, ask for Pat. Good service.

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Robert Stoll
83 SC
83 944

Shawn,

If you can handle a soldering iron and pliers/cutters to hook up a very simple circuit consisting of a 'wall-wart' transformer, switch, reed relay, and four diodes ... to provide a 60 Hz and 120 Hz pulse train to test your tach. at 1200 rpm and 2400 rpm. To just test it at 1200 rpm only, you can leave off 3 diodes and the switch!

If you want to try this, let me know, and I will draw up a circuit and parts list, and scan it and post it later. Canada does use 60 Hz power like the USA, right? It doesn't really matter, it only affects the calculations for the test rpms.

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

Warren, shoot me a copy of that please. Trying to repair my other tach. All of the common components check ok. There is one device, maybe op amp, that I havent been able to cross reference. Any ideas? Eight legged IC.

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Robert Stoll
83 SC
83 944

Warren

Sounds great please post that. or email me

Robert, how much did your tach run you? I am in Canada so there are no shops close by. The part that confuses me, is the Tach worked when I took the engine out, nd then nothing. But I may have fried it or something on re-instal

Shawn

[This message has been edited by davis911s (edited 09-03-2001).]

My tach was $100.00 freight prepaid. It does sound like something common, since your speedo and tach are out. Do you have the 12 volt supply to tach. Also check continuity on your ground.

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Robert Stoll
83 SC
83 944

Yes 12V to tach and grounds are all tested and good. Confusing isn't it?

Thanks all
Shawn

Thanks for the wiring diagram Warren. But for some damn reason I am sure it is NOT my gauge but some friggin wire or something somewhere. Damn!

I have good ground
Continuity from the CDI to the tach
12V at the Tach
Used 3 seperate CDI boxes ALL the same.

I might end up putting a bounty out on this one.

6 pack of beer or something to the first person to figure this out. Haha

Shawn

Can someone post the diagram for how to build this simple tach tester?

this is how I do it.

https://ashlocktech.com/store/ols/products/tach-test

Why waste your money, when a DIY effort is simple!

https://www.electro-tech-online.com/threads/rpm-signal-generator.156262/

Agreed: I bought a signal generator for 20 bucks. No need for any super-fancy bells or whistles, just a square wave generator. It should do +/- 5 V and be able to adjust the offset to get 0 - 10V. Good enough!
You can also use it to drive your CDI and test the ignition :cool: