temperature sender wire broke

[Jim K.]

I have a 71 454 365hp. The wire on the temperature sender on the right hand side of the block broke off. No big deal, I can fix it fairly easily. But while it was off, I was warming up the engine, and I noticed that the temp guage was working! What does this temperature sender do? Where is the real temp sender? I am confused? Jim

[William C.]

Re: temperature sender wire broke

I believe that is an overtemp sensor that opens the distributor vacuum advance to engine vacuum. It is covered in the service manual if you do some digging in the emissions section.

[Ed Jennings]

Re: temperature sender wire broke

I owned one of those cars in the early 70's and if memory serves, that sensor operates the TCS (spark control)system. It kills your vacuum advance. I took mine off and threw it away in order to frustrate future NCRS members, and to make the vacuum advance work properly.

[Jim T.]

Re: temperature sender wire broke

Jim I have a 70 350 with essentially the same temperature senders,one in the left head is the only one connected to the temp guage. The temp sender you have the broken wire on is what is identified by previous posts. This sender more or less monitors the temp of the engine and when it gets to hot it will permit full vacuum advance until the temp is reduced. I bypassed the vacuum control device on my 70 Corvette about a month after I bought it new and have been using full vacuum, not ported vacuum to my distributor's vacuum advance since. Better gas mileage in town was the biggest benefit. Correct vacuum advance also contributes to operating temperature.

[Jack H.]

For '71 only....

the emissions control system is call CEC (combined emissions control) and while similar to earlier and later TCS (transmission controlled spark) systems, has a life of its own....

The item in your engine's RH cylinder head is not a temp sender, it's a thermal switch. Your temp sender is located in the LH cylinder head back by the #5 cylinder on BB engines (SB engines have the temp sender located between the #1 and #3 cylinders).

The thermal switch is a single throw, double pole device wired with both terminals daisy chained together. When coolant temperature is low (below normal operating temp), the switch closes and gives an input to your CEC control relays (on firewall ahead of drivers seat) to engage the CEC solenoid on the carb to give you full vac advance and a slightly higher engine curb idle to speed engine warm up and improve cold engine driveability.

When the engine reaches 'normal' coolant temp, the thermal switch opens and the CEC solenoid on the carb is controlled by other design considerations (what gear are you in? has the engine JUST been started and 15-30 seconds hasn't expired?).

Should the engine 'begin' to enter overheat conditions (stop & go traffic in summer heat) the thermal switch again closes. This tells the CEC control relays to fire the CEC solenoid and give you full vac advance + slightly higher curb idle to suck more air through the radiator and bring down the engine's coolant temp.

I can testify that this last aspect of CEC operation (overheat) which we do NOT test at NCRS in either Flight or Performance Verification actually works! Several years ago, our chapter participated in a local July 4th parade along with several other non-NCRS Corvette clubs. It was a stop & go affair lasting about 2-hours....

I saw at least three nice looking older Corvettes blow their radiator caps and geyser to the side of the parade route (embarrasing for the owners). I watch my '71 temp gauge climb and enter the red zone. I did nothing.

BINGO, the CEC solenoid snapped in, engine RPM increased slightly and the coolant temperature began to fall all by itself. When it dropped out of the temp gauge's red zone, CLICK, the CEC solenoid released. I watched this cold/hot/cold process repeat 3-4 times during the parade (don't think I'll play that game again) and the car performed admirably--as designers intended!!!!