Tach hookup

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valiant1966

valiant1966

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I bought an HEI unit from TrailBeast and it is working great. Just now hooked up a Sun Super Tach II (I know, it's cheap but only for tuning, not racing) and when I set it on 8 CYL, I seem to only get half the rpm's. I am certain all the wires are correct. Am I missing something? When I put it on 4 CYL, it seems correct. Thanks. Dennis
 
How exactly did you hook this up? You have another engine you can try it on?
 
Red wire to an Accessory hot. Black wire to a GOOD ground(several things are grounded here and they all work fine). The green wire is hooked to the white wire on the GM coil(HEI remember?). White wire is not hooked to anything. It's for lighting.

The tach has a switch on the back(as do most) where one can select 4,6,8 cylinders. If I set it on 4, I get about 900 rpm's and if I set it to 8, I get 450 rpm's. These coincide with my ancient dwell/tach tune-up gauge. The car is an automatic so it is set to 900 in neutral. I almost wanna believe the switch is backwards but thought it might have something to do with the HEI which has worked great for me.

Only thing I have available to hook to currently is newer stuff but don't really wanna hack into them. Everything is a pain right now with 10 inches of snow on the ground and the cold like so many others on here I'm sure. Just thought I'd play in the garage for awhile. Thanks. Dennis
 
Do ya happen to know which is negative on the HEI coil? There are are four wires coming out of it and they are paired up in two's. There is a white (current tach hookup) and red coming out of one terminal and a black and red coming out of another terminal. The black and red wires go to the electronic distro pigtail. Dennis
 
4? You talking about the module or the coil?

The coil has 4 terminals but electrically actually only two. I thought you might have gotten it onto the coil POS by mistake. The coil terminals are what can I say "across." That is, each pair that is closest together are opposite ends of the coil. Look very carefully at the bottom diagram. NEG is bottom left and top right if you follow

Dont know if this helps

http://www.truckforum.org/forums/at...500-5-7l-tbi-no-fuel-1989-gm-hei-coil-sep.jpg
 

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I'm talking about the coil. Sorry for the confusion but that was the best way to describe the wiring to it. Here is a better way. Pictures worth a thousand words? Green wire to white is my tach. By the way, thanks for the help. Probably should let good enough go that it is working but kinda like for things to be "right".

Module2_zpsed2cbd46.jpg
 
As near as I can tell that should be "right." I don't like the way GM did that coil, IE one terminal across from the other, as it makes it more difficult in cases like this
 
This is off topic, the gray coil to center distributor cable could be made shorter, avoiding the loop and reducing the parallel path with other plug wires.
 
Kit, thanks for pointing that out. I was aware of it when I installed the HEI this summer but wasn't sure where I wanted to locate the coil so I left it long and never got back to it. You have a good point tho. I need to fix that. Thanks. Dennis
 
You are welcome. I think you problem is in the tach. The components are likely switched at the selector switch, if 4cyl selection works, go with that, since your car runs well.
 
It does sound like an internal problem with the tach.
Just so you know, the white wire off the coil IS one of the coil grounds.

If I were in your position, I might try connecting it to the black wire on the coil that goes to the module.
It may interfere with the module signal, but I might try it just by skinning a small spot in the insulation and touch the green tach wire to it while it was running.
 
For all who tried to help, thanks. I finally packed it up and returned it telling them it did not function properly. They sent me another and it works as it should. Yeah! Thanks. Dennis
 
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