Tach Problems

[jamesroy9289]

I just took out a year old aftermarket tach and replaced it with a tach dash. But I realized that the rpm's are off when driving down the highway now. The tach reads about a thousand rpm higher than my aftermarket tach read. I have 4.10 gears with an automatic and now it says I'm doing over 5 grand going down the highway. Is there anything I can do to my tach or do I have to replace it with a new one?

[STOCKISH]

Your tach dash for a v6 or v8?

[jamesroy9289]

The tach dash I got came out of a grain truck with a big block in it.

[PCHRODS]

What kind of distibutor do you have? Some aftermarket distributors require a tach signal adapter to function with a factory tach, the signal is different for oe vs. Aftermarket!!
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[jamesroy9289]

I have a standard hei distributer, but it has a hypertech coil in it. I'm kind of clueless. The only thing I can think to do is buy a new tach.......

[PCHRODS]

Coil shouldn't matter, it's an hei which should put out the right signal, the tach may be for a 6 cyl, it is a GM tach right? if you know 100% it's an 8 cyl tach then it should work. If it is an original tach and it's reading too high it sounds as if you got a 6 banger tach.
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[jamesroy9289]

Okay, well thanx for the information. I'll have to tear it apart this weekend and start looking around for another tach.

[too much stuff]

If it is not too faded, there should be a stamp on back of the tach head for 6 cyl or 8 cyl. There is also a small hole that has an adjuster in it. I have heard of people tuning them that way, but i have never done it. Keep in mind, these were never real accurate from day 1. It shouldn't be off by 1000 rpms though. Good luck.

[jamesroy9289]

ya I couldn't remember so I looked in my pictures I took and it was an 8 cylinder tach. I'll try adjusting it a little to see if it helps, thanx

[72BlckButy]

I thought there was a different wiring harness for a standard coil vs. HEI. Am I wrong?

[PCHRODS]

Originally Posted by 72BlckButy

I thought there was a different wiring harness for a standard coil vs. HEI. Am I wrong?

You are correct on the harnesses, but keep in mind hei wasnt an option on our trucks if I recall correctly, that didnt arrive until the later body trucks. Either way the tach still gets it's signal from the coil regardless of distributor technically from the pick up that triggers the coil. So I'm leaning towards him trying to adjust the tach manually, you could use a higher end timing light with a rpm readout to get it close, the factory tachs are never spot on but close enough to give you a safe read out. Good luck trying to get it dialed in be sure to let us know the outcome as I am curious to what happens!!
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[PCHRODS]

Another thing to look at!! Make sure the coil is getting a full 12v alot of time on originally equipped points vehicles they had a resistor wire on the coil feed that dropped the voyage down to 7 volts, you disnt say anything about sputtering issues so this has probably been long sense removed! Just a thought!!
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[gonzo6869]

You said you have 4.11 gears and an automatic; I am going to guess a th350. You can always do the math and find out if it’s the tach dash that’s off or the old one, maybe a bit of both.

Check out this link and enter your information and you'll get your correct RPM and Speed,

http://www.pszweb.com/car/gears.htm

[jamesroy9289]

well thanx for the tips guys, the only thing I can think of is that I didn't buy a harness for the tach. It had the female connector on the back of the tach when I got it so I just made a harness for the tach using some regular wire hooked up to the distributer and fuze box. I didn't think this would affect it since it's still getting power, what do you guys think?