I have a question concerning the hook up of a factory tach. I'm running a HEI as well as a MSD 6AL box. I just hooked up a factory tach that I bought many years ago and while it seems to work, it is reading too high (between 4000 and 5000 RPMs). I plugged the tach directly into the HEI distributor but am wondering if it is better to plug into the MSD box. In other words since a MSD makes multiple sparks will it cause a tach to read 4 to 5 time higher than normal? Maybe the tach lead in the MSD box compensates for this. Anyone know for sure before I go through the trouble of plugging in a back up factory tach or re-routing the wires to where the MSD box is. Allen allioop108@aol.com
Allen, Your tach is supposed to be connected to the 6AL box. The 6AL applies 450 volts to the coil. Get it off there before you fry it ou:
Ok Larry, I plugged the tach directly into the MSD box and the rpm dropped down to about a 1000 RPM's a little below. That's more like it. I know its still a tad bit high since the car sounds like its idling more or less at around 700 RPMs. Don't know about the tachs accuracy but I'll leave it for now and send my spare out for a rebuild and calibration. Thanks. Allen
The factory tachs are notoriously innaccurate. My factory tach dropped dead shortly after I purchased my car. I replaced it with an Autometer tach that fit right into the dash opening. I've always intended to get my factory tach serviced, but the Autometer is just so accurate, I've been procrastinating. :laugh:
