rostra electronic cruise

I have a rostra electronic cruise control on a 76 chevy blazer (hei ignition) and apparently have noise traveling up my tach wire tripping my cruise offline. When the tach wire is hooked up it trips every few seconds I have even had the cruise turn on and accelerate all by itself. When I disconnected the tach wire from the dist. I could still see a rpm reading on my digital gauge intermittantly and suppose this is what is causing the problem. Any ideas on where this noise is coming from and how do I filter it out. thanks

If the vehicle has an automatic transmission, disconnect the tach wire and your done. It is only astually required for some manual transmission vehicles. It also acts as an over-rev protection but isn't really necessary for automatics. Also check to make sure the cruise ground is into a clean and paint free area for a good ground connection. If the vehicle is a manual transmission, still disconnect the tach wire and set the cruise dip switches for an automatic transmission. Be sure to unplu the unit to accept the change in dip switches. You must then install a clutch switch on the clutch pedal. You can sometimes use a factory switch or Rostra has a couple of ones 250-3172 and 250-4206 (universal) to help. This clutch switch will open the violet brake switch wire when the clutch is depressed and disengage the cruise. The noise you are reffering to is a voltage spike that intermittently occurs on HEI ignition systems. It happens more often as the vehicle ages.sparky

The HEI system on this truck is brand new and the thing I dont understand is that it worked fine for a year and a half before this showed up also I get this noise even when the tach wire is disconnected from the distributor so that would tell me this is either RFI or EMI and I just need to get some ideas of where to look. Thank you for your response

SCS 4365 cruise would cure that, no tach signal needed. and dead on speed holding too.

First off the rostra unit did have an alternative for over rev protection unlike this SCS 4365 the light green wire goes to the cold side of the neutral safety switch and if the transmission gets knocked into neutral the criuse drops out unlike this SCS 4365 which has no over rev protection that I can see . Second the rostra unit holds my speed right at the assigned speed even better now that I got rid of the noise on the tach wire. Third I bought this from a company that has given me first rate tech support the few times I have needed it (Dakota Digital) and the rostra integrates well into their digital dash they sent me a new control unit  just in case it was the unit before the warrenty ran out. And fourth I was looking for assistance with this unit not advertising for another unit. Thanks for all who responded working with Dakota Digital I got it fixed and working.