70 Tcs

[Fred S.]

The TCS system on my 1970 LT1 is not working.
With a vacuum gage installed in the hose between the solenoid and the distributor there is a 3 PSI reading (vacuum advance).
This reading is constant in all gears with the temp overide switch connected or disconnected and the engine at temperature.
I have replaced the solenoid and checked all connections.
I have also checked the transmission switch with a VOM and it has continuity with the ball depressed and reads ~100 on the 10K scale (1,000,000 ohms) with the ball not depressed - this indicates that there is not infinite resistance so the switch may be bad.
I am not sure how to check the relay but I did remove the cover and the coil connections appeared to be intact.
I have followed the 1970 GM service manual section 6T and the diagnosis items.
If anyone has comments or suggestions it would be appreciated before I try to fins a new trans switch and/or relay.
Thanks.

[Gary S.]

Re: 70 Tcs

Relying on memory here (always a dangerous thing ) but doesn't the GM Service Manual give a troubleshooting guide for the TCS system? I recall a few years ago that Terry M. gave me this advice and my 72 Service Manual did carry that guide.

[Pat M.]

Re: 70 Tcs

Fred - when the solenoid has no power to it it allows vacuum advance to the distributor. This occurs when the engine is cold, hot, or when the car is in 3rd or 4th gear.

There are three "players" that govern whether the solenoid gets power (is "energized") or not - the trans switch, the relay (at the driver's side firewall), and the temp override switch in the passenger side head.

You can check the trans switch by placing a test light between the switch and the wire going to the engine - it should only NOT light (denying a ground and preventing energization of the solenoid ) in 3rd and 4th gear.

You can check the relay by using a test light to see if it allows a flow-through positive current (energizing the solenoid) only when the engine is warm (not cold or hot).

And you can check the temp override switch by disconnecting it when the engine is warm, which should deny vaccum advance.

Clear as mud?