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Tach ckt board repair ?

Kenny78

Kenny78

1/2 ton status
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Nov 7, 2007
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Location
Around Tulsa, Oklahoma
I bought a NOS Tach/fuel guage off of Ebay and its DOA. Seller immediately refunded my money plus shipping and told me to dispose of it. Good customer service if you ask me. The needle pegs down when it gets 12V but does not respond to the tach signal. I checked the wiring with my other tach cluster. My other tach board is a breadboard with microprocessors, resistors etc.

Upon further inspection, I am certain somebody at a dealership long ago took the ckt board off this one cause there was a nut missing on the plastic housing

http://www.instrumentsrus.com/gm.htm has new solid state boards with 2yr warranty for $175. He also states that this style board is inferior, dipped in epoxy?? and unrepairable.

Since I have nothing to lose with the board and can get a replacement, has anybody ever messed with one of these? How would you remove the epoxy? Think it might take the heat and melt without taking everything with it? Scratch it off? I'm not an electronics man, but I know my way around a DMM and understand most electronic concepts. I didnt find any cold joints although if I can remove the red crap there is a shiny joint with some extra flux where I'llstart chasing components

Thanks for looking if you were bored enough to read this whole novel

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There was a fella over on 67-72chevytrucks.com a couple years ago who went by necromancer who was well versed in the operation of these factory tachometers. He had refaced mine to a 7000 rpm and was able to calibrate it to be pretty incredibly accurate. I haven't seen him around in a very long time, but it may still be worth shooting him an email over there.

Nice score on that tach if you are able to repair it! Those medium duty tachs are hard to find, esspecially in a 5000rpm span -- Many only go to 4000.
 
There's a few guys on E-bay who offer rebuild/repair/calibration for the newer clusters. Maybe they can help you out...
 
Hey Russel, thanks for the tip, my registration is pending over there.

62, one of the guys on ebay is the link I posted. He installs a new board, no questions asked, for $175. I cant fault him for a sound busisness plan, but in the short term my $175 is going elsewhere on the truck.
 
My apprenticeship teacher is electrical maintainence man at the local refinery? I took it to class to ask if he had any experience with instruments or something, and he offered to take it to the instrumentation techs to see if they had any ideas! May not come of anything, but scratching it off with a knife isnt going to work, I know that.
 
That epoxy makes the boards pretty much unserviceable. It it were mine, I would look for guts from some other tach (Camaro?) to use behind that face and movement. Some scaling (i.e. resistor changes) would be needed if your donor had a different number of cylinders or a scale other than 5k rpm. Even the guts from an aftermarket tach might work. Essentially they are a frequency-to-voltage converter, which is pretty generic as long as you can get the scaling right.

I made a factory-styled tach out of an aftermarket tach by attaching a factory needle to the movement, making my own gauge face (to stick on the factory one) and rescaling it.
 
BTW -- I am an instrument tech in training! While we do definitely get into the theory behind gauges, most guys won't know the specifics of that particular gauge. However, you are more likely to find guys who love playing with electronics and have the equipment to troubleshoot them properly in my field.

I'd offer to take a boo at it (have access to the electronic's lab and all the state of the art equipment in there), but I don't know if I have enough know how to diagnose and repair it yet...
 
I am more interested in if they had any tricks to removing the epoxy. If they dont have any suggestions ormagic elixers, I might throw some aircraftstripper at it. If nothing else works, I might throw it your way Russel(though shipping to Alberta might make it not worthwhile)sothat you could play destructive testing .

Blue85, I havent seen any similar componented tachs on ebay(doubt theyd be resonably priced enough to gamble on) and although I enjoy messing around with electronics, by the time I bought an aftermarket tach and screwed it up I would be money ahead to go buy a new board from those guys for $175
 
I happen to have a factory tach here at my apartment right now. Mine works, so I'm not gonna do any destructive testing, but I'll take the cluster apart tomorrow and see if mine is epoxy coated or not.

If it isn't, I'll poke around and see what kind of components it is made of. Mine is probably 12 years newer than your model however, so I can't promise my board is the same as yours.

The other option would be to build a new digital tach circuit yourself. At school we play with LM2917 ICs. They are a frequency to voltage conversion IC, with a build in op-amp capable of handling up to 50mA. Calibrating it would probably wind up being mostly guess and check, but I do have access to equipment that can simulate the signal produced by a V8 engine's ignition coil. I don't know what kind of current the analog meter draws, but I'd be surprised if it was more than 50mA...
 
I have another tach also that is the breadboars type, and works, so I am like you in being careful with it. The processors is something I know nothing about but if you had a diagram I would be interested in seeing it.

Thanks Russel
 
Just type the IC's part number into google and you will find everything you could possibly want to know about it :)
 
If you build your own, be sure to give due care to the IGN coil interface circuit. The transients there are nasty and can make your circuit do all kinds of things it wouldn't do in simulation or on the bench. I would probably use an optocoupler through a resistor. A circuit like you propose is basically good for only a 1-point calibration. You depend on the linearity of the movement beyond that.
 
Aircraft stripper, heat, scraper wont touch this stuff. So, Ill probly just buy from the pros unless I can get up the nerve to attempt a replication of my other one.
 
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