tachometer illumination interference

Here's a wierd problem.   I installed an aftermarket tachometer in a 1991 cutlass. I looked up the tach signal wire and tried to use that signal, but no good. the tach got no responce. I read on the12volt that it was possible to tap into the tach signal via the injector wires. Everyone keeps saying that one color will be the same on all the injectors, while the other will be unique. Well, all my wire colors are the same for each injector (pink and blue). So I decided to just pick one and see if it made the tach needle go. Well it did, even though it was reading about 500rpm below what it should have been. Anyway, I got the signal so I figuered everything is fine. hooked up the illumination wire to the headlight switch - i know i did this correctly, beause i used the same wire for my pioneer stereo dimmer and that works fine.    Now everytime I turn on the headlights, the tach signal disappears. Do you think switching to the opposite wire on the injector will fix this, or is it something else. I don't have any ideas yet...

I guess that would be a good place to start. I can't think of anything you could harm by doing that, but I'm not a mechanic. That is quite a strange problem.Nik

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And the problem was.....     Bad ground.   For as much time as I spend on this site, I never thought it would happen to me. Quite possibly the most stupid mistake one could make. Thanks for your help. On a different problem related to this tach... Upon fixing the ground problem and testing the tach again, I realized that the needle would jump from zero to 500rpm when I started the car, then if I hit the gas peddal, the neddle would jump for a second, then settle right back down to around 500rpm, regardless of what the engine was reving at.   I tried switching the tach signal wire to the other wire on the injector, and got no signal at all, so I guess I had the right wire to begin with. This 'injector' technique is obviously not working for my car, so can anyone suggest where I should look? I tried the white wire at the distibutor and the injector wire so far.

OK. I'm going crazy trying to find a tach signal. I have all the wiring diagrams from my car so I am sure I am tapping the right wires. So far I have tried: 1. Both white wires at the ign. coil (one is listed as 'IC' and the other is listed as 'tach signal') 2. Both fuel injector wires. One gave me no reading while the other gave me a steady 500rpm reading that would jump when i hit the gas, but would drop back to 500rpm instantly even if the engine was running fast. 3.   tried wrapping the signal wire for the tachometer a couple times around a spark plug wire. - no reading. Can anyone think of any suggestions for how to get a signal. this is a 1991 Cutlass Supreme Sedan, 3.1L v6 I am using a sunpro tachometer. Thanks

also tried a purple w/ white wire in the same harness as the 2 white wires on the coil packs.. this was listed on www.bulldogsecurity.com    ... no result at all.

note to Moderator - You can delete this topic if you want to. I've been ruling out some possibilites, and also bought a DMM. I started a similar, more straight forward topic on the general disscusion board relating to this problem I am having.   Thanks