tach problem

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tach problem

Postby spirit » Sun Mar 31, 2013 10:07 am

OK Guys, Got a problem with the tach. It quiet working. 86 GT with 4.9 and Caddie trans. What do you think?
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Re: tach problem

Postby Emc209i » Mon Apr 01, 2013 12:41 am

No idea Jack. But the transmission will have nothing to do with the tachometer, it only controls the speedometer.

If the engine uses a distributor, the tachometer feed comes off the ignition module, and through a tach filter. If it uses coil packs, the coil module sends the signal.

Completely theoretical, no idea what you're using.
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Re: tach problem

Postby Chug » Mon Apr 01, 2013 11:59 am

You'll have to chase down the tach feed that goes to the car and make sure all your connections from the ignition source (where the signal comes from) to the car's wiring. If all that is intact and clean then you may need to take out the cluster and clean the terminals in the back, you may have lost connection there. Drew-py has the later of these 2 issues with his car.
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Re: tach problem

Postby CowsPatoot » Mon Apr 01, 2013 8:41 pm

With the 4.9, the tach gets it's signal the same way it does from a stock Fiero....white wire (this may be different color in your custom harness) goes from the coil to the tach filter to pin C3 on the C500, then from there it goes straight to the instrument cluster.

So...one of the following things is bad.
1. Coil quit sending a signal (unlikely...never heard of this happening from a running car)
2. Tach filter has gone bad
3. Tach has gone bad
4. wire has lost connection somewhere along the line

I would start by doing a continuity test between the tach filter and the pin on the back of the tach. That will eliminate the wiring from the equation (assuming it shows proper continuity). From there....well, I have spare parts laying around, so my next step would be to swap the tach filter, then the tach. Keep in mind that your tach has been modified to read properly with the v8...a stock v6 tach will read 33% high, a stock 4 cyl tack will read double.
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Re: tach problem

Postby spirit » Wed Apr 03, 2013 7:54 am

Thanks Guys. I am as dumb as the handles on a wheelbarrow when it comes to anything electric. I do know that there was a dioed added to the tach when the swap was done. I'll check that out.
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Re: tach problem

Postby Emc209i » Wed Apr 03, 2013 11:02 pm

Here's where you'll find the resistor then:

http://www.fieros.de/en/articles/tach.html
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Re: tach problem

Postby Trinten » Sat Apr 06, 2013 10:01 am

Well, apparently I forgot to set this part of the forum as a "parent". I might move this under electrical. Does that seem like a logical place to put stuff like this?
1986 Pontiac Fiero GT, 355 cubic inch small block Chevy putting down 371 HP and 381 ft/lbs torque at the wheels, with more to come!
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