Tach problems

[Nate355RS]

Well, I have everything running good on my LT1 and now I'm just trying to fix little things that are annoying me, like my tach. The tach is working fine it just reads between 500 to 2000 RPMs too high. I'm thinking that it's because of the MSD 6al box, and the fact that the tach output is not hooked directly to the tach. My problem is I can't seem to figure out which wire to hook to the box. Right now I'm pretty sure it is a white wire, but I don't know if I need to get it from the PCM or the coil pack or somewhere else. Could someone help me out with maybe a pic and/or description of where the wire that goes to tach is located? Also, if anyone thinks that my problem is because of something else I'm all ears, right now I'm just assuming that hooking the tach straight to the MSD box will fix it.

[Morley]

Don't bet on the box being the cause. Third gen tachs are junk, they start off reading 50-100 RPM too high, then they get worse and worse as time goes by.

[Nate355RS]

I've read that they are notorious for reading way off but I guess I'm not sure if it's the electrical signal that it's recieving is to blame or the tach itself. The higher up in the RPM range the engine is the higher off the tach becomes, like at 850 RPM the tach says 1000 RPM, and at 3000 RPM the tach says around 4500. I've verified this through dataloging so it seems to be working just getting a higher signal maybe because of the MSD, which I have heard does happen. If I hook the tach to the box and it still doesn't work than I'll say hell with it and just get a autometer, but I'd like to see if I could fix it first. Trying to preserve the sleeper look

[Morley]

If the data log shows the correct RPM, suspect the tach is to blame...And that is how the factory tach look when they go bad...They may read right or close to it up to about 1k RPM's, then they get further and further off as the RPM's rise.
There is a fix for this. There is a large resistor on the tach's circuit board, repalce it with a variable resistor (reostat or potentiometer), keep adjusting the resistance and checking tach readings against the ECM's tach reading until they match.

[Nate355RS]

I know a very marginal amount about about electronics, I can soder and desoder without damaging curcits but that is about it. If I go to say Radio Shack, and tell them I need a reostat or potentiometer variable resistor are they going to know what I am talking about, and is this a pretty straight forward fix or should I ask my more electronicly inclined friends if they can help me out? I would like to fix it myself but if it is a rather sensitive job I think my pride would get over asking for help if it'll save me a couple hundred on a new tach.

[Blue Bowtie]

In my experience, Radio Shack will be clueless about anything you might ask regarding electronics. They have long since shifted focus from a hobbyist store to a consumer electronics retail outlet. it seems their latest emphasis is on cordless and cellular phones, and they can probably tell you anything you might want to know about the features and functions of those, providing it is not a technical qestion. They're really up on the games included with the phones, but won't likely be able to distinguish a capacitor from a coin cell.

You need to find a regular electronics supplier.