Tach in '77 reading roughly double rpm, HELP!!!

[IDIDNTDOIT]

I restored a '77 3/4 ton 4X4 suburban (one of only three I have seen with factory 4-spd). I put in a factory tach from a '73. I didn't know to get the filter on the distributer.... A friend, that is a GM wrench, said I needed one but they are obsolete from GM so I orderd one from Classic Gauges. My tach reads roughly double the rpm's the engine is turning, I tried it with the filter, w/o the filter, tried reversing the wires.... I don't know what else to do????
This is driving me crazy, and It also scares the hell out of any one that drives it!!!

Thanks for the help,

Bill

[Captkaos]

I assume you rewired the truck to make it work, or pulled everthing out to make it work as you can't put a 73-76 cluster in a 76-77 truck. Second I would ask if it came out of a 6 cylinder truck?

[LEEVON]

Time to get a 4 banger! Sorry, I couldn't resist...

[IDIDNTDOIT]

Captkaos: I honestly don't remember what we all did as this became an 8 year deal with me scrapping out over 10 pu's and burbs. It started Innocent enough and snow balled till all I used of the original truck was the frame, axles, tranny/transfercase, driveshfts, and the gravel pan by the front bumper.... OH, and the vin tag!! The truck I took it from had a v-8 in it, but I never saw it run, maybe it read wrong in it too. Any way to covert a 6cyl one to v-8? How do I tell? I think we just used the tach and put it in my cluster, the truck still has the glass fuse style harness, but we were able to get a new style cruise to work by polishing the spot on the back of the speedo the vss flashes. We had to use the small gas gauge too. My friend Mike did most of that and he really is very sharp and one of the best GM master tech's in the state.

Thanks

Bill

[IDIDNTDOIT]

LEEVON: Sure... pick on the new guy.... Actually the old v-8 was so tired that a 4banger would have been an upgrade. Even in low when I would park it on a hill it would start to roll the motor over and head down hill!!! I had a mildly built 327 sitting in the corner for waaaayyy too long that was supposed to go in a hot rod that wound up getting sold, soooooo, wala, new home!!! It has a little too much cam but having the 4-speed helps, and that lope at idle at the stop light is impressive along with my hood being the height of most cars roof...

Thanks

Bill

[77gmcjoe]

I had a similar experience with the first tach I put in my '77. I'd bet that the tach is going bad.

P.S. I f I'm not mistaken, the filter you mentioned is only there for the purpose of preventing any possible radio interference, so it shouldn't have any effect on the operation of the tach.

[mhicks]

Bound to be a way to create a circuit to divide the pulses in half and install between the dist and the tach. May be one available to do that already on some website.

[Captkaos]

Filter was for interference. In general 73-75 tachs can't be rebuilt, but I was thinking someone on here said they fixed someones.

[Prong]

You might try http://www.delcobatteries-denver.com/
They've rebuilt 2 factory tachs for me and they might be able to help you out. Expect to spend ~$100 or so depending on what's wrong with it.

[us111]

Hello,only help i can be just to elimanate something
is the filter like has already been said is just for
noise interference,(make it bounce around a tiny bit)

[Mickey_D]

Are there any potentiometers on the circuit board of the tach? I've got an Autometer tach sitting here on my desk and it has three in it. One of them actually came apart (the adjustable part) and was giving the previous owner symptoms like you describe. I haven't been able to find any replacements for mine yet (even calling Digikey didn't come up helpful), but I'm hopeful I can fix it as it's a nice tach otherwise.

[IDIDNTDOIT]

It will be a while before I have time to pull the dash, thanks for all the ideas, guys.

Bill

[Twisted78SS]

This guys has a good reputation with some friends who have had work done.

http://www.tachman.com/

I have no personal experience, but might be able to tell you whether or not its worth trying or doable.

[jdmaffeo]

I have the same problem. I talked to Gary (The Tachman) and he said that the board is shot. He has new boards that are adjustable. $225 plus shipping. About a week turnaround.