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The other day, I got in my car to start it. As I turned the key, the tach started moving upward. Then I noticed the battery was dead. I could hear a faint, high-pitched sound from behind the dash. The tach pegged all the way to the right and stayed there.

So I disconnected the tach wire from the distributor and started the car. No electrical fires. I'm suspecting a shorted-out tach filter. Has this happened to anyone else?

BTW: It's a '79, so the tach is electric, not mechanical.

TedH

It happened to me when my tach filter went south. Take a look at your tach filter wiring. The wire that enters the filter through what looks like an epoxied end had snapped off. There are schematics with parts recommended available. I have a copy of the schematic from a fellow forum member and I can forward a .pdf to you if you would like one. Just let me know.

I dug the diagram out of my electricals book and found that it is a bit hard to decipher. Parts list to create a 6Db filter:

1K ohm resistor
0.33 uF capacitor
Wires
Connectors
Solder
Shrink Tube

All available at your local Radio Shack or equivalent.

Assembly:

Solder/Attach the 'Tach' wire that runs into your distributor to one end of the 1K ohm resistor. Solder/Attach the tach filter wire that goes through your firewall to the other end of the resistor. As a ground for the filter, run a wire off of this wire to the 0.33 uF capacitor. Connect your ground wire that attaches to the tin piece on your intake manifold. Shrink-wrap the wires/components and install. Sounds too easy but I tried this and it works.

As a footnote, my tach filter lasted about a year before it came apart. I'm no electrician so that is probably the cause. I have since obtained another homemade unit from a fellow forum member that had an electrical engineer make one. Don't be surprised if your 'creation' looks like a glorified 'T'. That's all there is to it. Fairly simple and I spent less than $5 to build my first one.

Good luck!

:seeya


[Modified by TedH, 10:56 AM 12/19/2002]

LABulldog

Mine did the same thing when I ran my battery down due to a bad alternator. When I jumped the battery off and started the car, my needle pegged wide open. Man I was pissed. But, after checking on the Forum, I found that some members had experienced the same thing and for some, just putting a few miles on the car caused the needle to correct itself. Well, I tried that (what did I have to lose!) and voila! my needle jumped back to its normal position and it has been working fine since (8 months and counting). Hope this works for you. Obviously the easiest and cheapest fix. Good luck!

Smokehouse69

Yes, the bitter cold of the South Texas winter probably caused the wire to break! :D
If you ask me really nicely, I Might volunteer to make you a new tach filter, if that's really your problem. But, it sounds like to me that the actual circuit board behind the dash went south on you. I think the amount of actual current that comes from the tach terminal on your distributor is tiny, I don't think it is enough to pull your battery down like that, although I could be wrong.
One thing you could try is to remove the tach-filter from the circuit and plug the lead directly into your tach terminal on the distributor. The tach should still work, but it will most likely be jumpy an inaccurate. If that's the case then you'll need a replacement for the tach-filter.
I really think it is more likely that the actual tach circuit board has gone south and the tach is toast. Zip sells a new circuit board, that is of a more modern design and should work perfectly.

page62

Do I get to choose one of these responses?

I'll pick "reconnect it and drive it some" :jester

'79ProwlerOrange

mine does it everytime i start it. and my battery is fine. i just give a tap of the gas getting it up to about 2000rpm and the tach comes back. im not going to fix it because im switching to autometers in the not so far future. :cheers:

BobJ

SHIFT!!!! :D