What is the best way to trouble shoot the tcs system on a 70 LT1?

[Alfred W.]

I have a 70 LT1that I would like some help troubleshooting. On cold start, after warmup the engine speed slows down, but
it does now speed up in 3rd or 4th gear with clutch in. Where do I start checking? How can I check the trans switch, vac. solenoid, ???? Thanks Al

[Pat M.]

Re: What is the best way to trouble shoot the tcs system on a 70 LT1?

Hi Al. Assuming the 70 LT1 TCS system is the same as the 70 base engine system, which I have, here are my thoughts.

The fact that your car operates correctly during warmup suggests to me that the TCS switch in the head, the relay near the driver's side firewall, and the vacuum solenoid are all working properly. If any of these three were malfunctioning, I don't believe the system would work at warmup. That would leave the transmission switch.

Short of simply trying a new switch, years ago I tested mine on the car connecting a common test light, although I honestly don't remember exactly how I connected it now. If I recall correctly the test light would be normally on, and would then go off in 3rd and 4th, which makes sense because the solenoid has to lose power to allow vacuum advance.

Good luck

[Vinnie P.]

Re: What is the best way to trouble shoot the tcs system on a 70 LT1?

Al...Winter 2014, V40 #3, issue of The Restorer had an excellent article by Erich Meyer on the 1972 TCS system. However, I have no clue as to whether the 1970 system is the same. If you would like a copy send me an email restorereditor@ncrs.org...
Vinnie

[Mike F.]

Re: What is the best way to trouble shoot the tcs system on a 70 LT1?

From your description ("after warmup the engine speed slows down") I can't tell if your TCS system is working or not? Are you misinterpreting the choke opening up fully?

With the engine at full operating temperature and idling at 800rpm, the TCS solenoid (on the manifold) is closed when the transmission is in 1st, 2nd, or any Neutral area of the shift pattern. When shifted into 3rd or 4th gear, the solenoid opens and the rpm should increase about 400/500rpm (up to 1200/1300rpm) or so.

On a fully functioning TCS system, if the transmission switch is disconnected/unplugged, the TCS solenoid stays open all of the time providing full time manifold vacuum to the distributor.

Step one, with your car at full operating temp and at 800rpm, unplug the transmission switch. Does the rpm increase? It should, even with the car in neutral. If it does, the rest of the system is functioning correctly, just replace the transmission switch. If the rpm doesn't increase, you have a problem elsewhere in the system. (The transmission switch may still be bad in conjunction with another component.)

Step two if the rpm hasn't increased from the test above. Leave the transmission switch unplugged, engine running at idle, transmission in neutral. Unplug the TCS solenoid (on the manifold) and run a 12v jumper wire to the solenoid and to ground. This will test the solenoid to see if it is opening and closing. If it opens, the rpm will increase and you know the solenoid is working. (If the solenoid does not open (no increase in rpm) you will need a new solenoid to continue.)

If the TCS solenoid functions properly in the test above, the only two components left are the TCS relay (located on the firewall) and the Temp sender in the passenger side head. I don't have a wiring diagram in front of me for better TCS relay instructions, but more often than not the TCS relay (moving parts) has failed.

The Temp sender could have gone bad, but is the least likely part to fail (no moving parts).

There could always be a wiring issue as well, but again, typically the moving parts fail first. The first two tests above should get you most of the way there.

[Terry M.]

Re: What is the best way to trouble shoot the tcs system on a 70 LT1?

From your description ("after warmup the engine speed slows down") I can't tell if your TCS system is working or not? Are you misinterpreting the choke opening up fully?

With the engine at full operating temperature and idling at 800rpm, the TCS solenoid (on the manifold) is closed when the transmission is in 1st, 2nd, or any Neutral area of the shift pattern. When shifted into 3rd or 4th gear, the solenoid opens and the rpm should increase about 400/500rpm (up to 1200/1300rpm) or so.

On a fully functioning TCS system, if the transmission switch is disconnected/unplugged, the TCS solenoid stays open all of the time providing full time manifold vacuum to the distributor.

Step one, with your car at full operating temp and at 800rpm, unplug the transmission switch. Does the rpm increase? It should, even with the car in neutral. If it does, the rest of the system is functioning correctly, just replace the transmission switch. If the rpm doesn't increase, you have a problem elsewhere in the system. (The transmission switch may still be bad in conjunction with another component.)

Step two if the rpm hasn't increased from the test above. Leave the transmission switch unplugged, engine running at idle, transmission in neutral. Unplug the TCS solenoid (on the manifold) and run a 12v jumper wire to the solenoid and to ground. This will test the solenoid to see if it is opening and closing. If it opens, the rpm will increase and you know the solenoid is working. (If the solenoid does not open (no increase in rpm) you will need a new solenoid to continue.)

If the TCS solenoid functions properly in the test above, the only two components left are the TCS relay (located on the firewall) and the Temp sender in the passenger side head. I don't have a wiring diagram in front of me for better TCS relay instructions, but more often than not the TCS relay (moving parts) has failed.

The Temp sender could have gone bad, but is the least likely part to fail (no moving parts).

There could always be a wiring issue as well, but again, typically the moving parts fail first. The first two tests above should get you most of the way there.

Actually, there is a moving part in the temperature sensor, and in the transmission switch.

You can test the function of the temperature portion of the system (less the sensor in the head) by grounding the green wire that goes to the temperature sensor. Some 1970 engines have a connector in the green wire, and some only have the connector at the temperature switch. Wherever you can get at that green wire, run a jumper to ground (the engine block or brackets should be fine) and see if the engine RPM increases. If so your solenoid is good. if not, the solenoid could be bad.

Check for 12 volts (more or less) at the solenoid (you will need a meter or a test light), and then jumper the other side of the solenoid to ground. That should make the engine RPM increase unless the solenoid is bad or the vacuum lines are plugged or miss-routed.

I am not sure how to test the transmission switch without the electrical diagram of the TCS system. I believe that diagram is in the 1970 Corvette Supplement (May 1970 Chevrolet Service News), but I don't keep a copy of that on hand, and I am too lazy to dig it out.