72 TCS Relay Attachment

[Erich M.]

I need to replace my TCS relay which is located under the dash just to the left side of the center console instrument cluster. Most of my dash is apart. The radio is out, the center gage cluster is loose and removed from the slotted support bracket under the center console. I can see the relay but cannot see to which part it is attached. I have a new relay PN 3996297. The relay has a right angle bracket coming off the back of it with one mounting hole.

How do I gain access to the relay mounting bolt. I can't see the bolt with the interior parts that I currently have removed (not even with a mirror). Do I need to remove the dash to gain access?

72 LT1 Coupe 5 Star Bowtie

[Edward J.]

Re: 72 TCS Relay Attachment

Erich,I believe the relay is under the dash pad on the drivers side, the only way to remove is to remove the dash pad. Are you sure its bad?

[Erich M.]

Re: 72 TCS Relay Attachment

I believe problem is due either to a bad relay or a continuity problem in the black wire comming off the trans switch and running to the relay. This is based on the following tests which were done with the ignition circuit powered but the motor not running. If I replace the relay and the problem goes away then I know that continuity in the black wire feed is not the problem.

1. With ingnition off, could not get any air flow through TCS solenoid when blowing or sucking air through the vacuum source hose connected to the carb.

2. With ignition circuit powered but engine cold, I could get air flow through the solenoid when blowing or sucking air through the vacuum source side of the solenoid. This means that the TCS solenoid is getting power, working properly, and activating vacuum advance when the engine is cold as it should.

3. As a double check, I disconnected the TCS Temperature connector and the solenoid would not permit any vacuum to pass through it when the ignition circuit was powered and the transmission was not in 4th gear. This test simulates a normally warm engine operating environment where the solenoid is to permit vacuum advance only in 4th gear.

Tests #2 and #3 indicate that the TCS temperature overide switch is allowing vacuum advance when the engine is cold as it should.


4. Next I performed a continuity check on the transmission TCS switch. When the transmission is in any gear other than 4th, there is an open circuit between the switch terminal and ground. When the trans is put in 4th, I get a closed circuit from the switch terminal to ground. This means that transmission TCS switch is working properly. Howerver I couldn't get any vacuum air flow through the solenoid with the trans in 4th and the termperature connector disconnected. I waited several minutes.

As I understand it, when the trans is in 4th, the circuit completes through the trans switch and the TCS relay to power the solenoid. This has not happened.

I am the original owner of this car and I don't think it ever delivered vacuum advance the way it was supposed to through the TCS system. I have much of the interior apart now and I can see the PN on the relay. Interestingly, the connector attached to the relay has white tape wrapped around the outside of it. I wonder if this was some kind of marker during manufacturing that flagged a malfuncioning component.

[Patrick H.]

Re: 72 TCS Relay Attachment

Erich,

Also realize that the function of the relay is to give a 20-30 second delay before allowing the TCS solenoid to activate when the car is placed in 4th gear. I am not sure if this is "testable" without the car running; I'm sure that Jack would know. However, if you expect the relay to immediately provide power I don't think it will and you need to wait for the timer.

Patrick

[Edward J.]

Re: 72 TCS Relay Attachment

Erich, a simple test of the system is to ground the wire on the transmission switch while car is running and wait for the 20 sec. delay, you should hear a slight RPM increase, if this doesn't work next to the solenoid on intake and and use a test light one side is power the other is ground, using the test light this will act as a ground path for the solenoid just by touching the ground wire on solenoid.

I believe the switch on trans. is the most common failed item.

[Erich M.]

Re: 72 TCS Relay Attachment

I performed a ground/continuity test on the transmission switch. I works correctly. It has an open circuit to ground in all gears except 4th. In 4th gear the switch closes to ground.

[Edward J.]

Re: 72 TCS Relay Attachment

Erich, isn't that the way the tcs works,4 gear only? other than the cold start-up.

[Erich M.]

Re: 72 TCS Relay Attachment

Yes Ed, thats correct. The TCS should close to ground in 4th gear and the current then goes to the relay which should close the circuit after about 30 seconds. My TCS switch closes to ground in 4th gear and it works correctly.

If you look at my test precedure, I get vacuum advance when the engine is cold and no vacuum advance under normal operating termperatures when not in 4th gear. This is as it should be. The solenoid is working properly. What is not happening is that the solenoid is not activating for vacuum advance when in 4th gear and under normal engine operating temperature. Even after waiting 30 or more seconds.

Thanks for the input. I have to remove the upper dash anyhow in order to replace my speakers so I'll probably get a better look at how the relay attaches when the dash is removed.

[Patrick H.]

Re: 72 TCS Relay Attachment

If you look at my test precedure, I get vacuum advance when the engine is cold and no vacuum advance under normal operating termperatures when not in 4th gear. This is as it should be. The solenoid is working properly. What is not happening is that the solenoid is not activating for vacuum advance when in 4th gear and under normal engine operating temperature. Even after waiting 30 or more seconds.

One more item -

mine did not work because the sender in the right hand head had corrosion on the terminal and end of the wire. Be sure that the wire is in good shape, and that the sender doesn't have teflon tape or thread sealant on it. It must be "bare" as it enters the block and the connections must be clean. Otherwise this too will cause it not to function in 4th gear.

Patrick