[dv657172]

1972 GTO, auto with non-rally guages

I found an unconnected temp sending unit(?) on the back of PS cylinder head.

The service manual says the temperature sending unit is on the side of the PS cylinder head which I have and is connected although not working as telltale light doesnt come on at startup despite bulb being good.

Was the sending unit in the picture installed by Pontiac or a previous owner for a guage as car does not have rally guages or is it for something else?

Thanks
DV

[DavesGTO]

Never seen that before




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[camerjeff]

That is a TCS Switch (Transmission Controlled Spark) it limits vacuum advance until a set engine coolant temp is reached. I know they were used in the 1973 and later year emissions systems. But I don't know if they were used in the 1972 model year.

[dv657172]

Thanks camerjeff, that makes sense as the TCS system is installed but basically bypassed.

The engine is actually a Nov 71 production date and the heads are "7M5" L061 for the DS and K161 for the PS so they must of started using it in 72.

[DavesGTO]

Mine is an Oct 71 build, that sending unit is connected to the temp light. Before I converted to factory gauges that sensor was all there was. There should be a plug near the front of the driver side head. Another sending unit goes on there for the gauge.


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[dv657172]

Thanks DavesGTO (good name, mine too). Will check the harness near there for a connector whether for TCS or temp light.

Based on the 72 service manual (Fig.12-51), the water temp switch is on the side of the head which I have connected and matches the pic you sent although as mentioned earlier, my telltale temp light isnt working...

So if the sending unit is on the back of the head (as in my pic), any idea why there is another water temperature switch on the side of the head (as in your pic and on my car also)?.

[DavesGTO]

Sending unit could be bad, if harness is original and the printed circuit both 50 plus years old could be the problem. Never saw a sender hanging out the back of the head like that.




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[Kenth]

Originally Posted by dv657172

1972 GTO, auto with non-rally guages

I found an unconnected temp sending unit(?) on the back of PS cylinder head.

The service manual says the temperature sending unit is on the side of the PS cylinder head which I have and is connected although not working as telltale light doesnt come on at startup despite bulb being good.

Was the sending unit in the picture installed by Pontiac or a previous owner for a guage as car does not have rally guages or is it for something else?

Thanks
DV

Looks like a misplaced knocksensor, originally used on much later egines.

JMHO

[Peter Serio]

That reminds me of a sensor that Ive seen before on other GM cars. It senses metal temperature. It may have something to do with a delay for the blower motor if the temperatures are too cold, The blower does not turn on for a time to wait until you at least have some hot water going into the heater core.

It works in combination with a relay.

That, or it may have something to do with the AC compressor. Not allowing it to turn on if the temperatures are below freezing?

[dv657172]

Thanks Peter and Kenth. It looks like I definitely have some testing and investigation to do once the shop gets back above freezing next week. Car does have AC from the factory

[Peter Serio]

There is another sensor very similar to that one only it senses air temperature. It was behind the glovebox on 1978 and 1979 GM cars. and I recall it held back power from going to the blower motor during very cold start ups. I do recall seeing them, it had 2 flat tabs on it that plug into the dash wiring harness. It was never connected to anything except the wiring plug-in and those were always inside of the car. It just got a sense of really cold air temp or not, for it to delay + power to the blower motor the car would have had to be parked outside overnight in below freezing temperatures.

I can't recall one of those sensors ever going bad but I am pretty sure it must have given some time, maybe 5 or 10 minutes for the coolant to warm up. It's been a long time since Ive seen one.

[Kenth]

I have found in the 1972 Pontiac Service Manual pages 6D-5 to 6D-9 is a desciption of the "Controlled Combustion System", S.C.S./C.E.C./T.C.S. and figure 6D-8 shows the S.C.S. Temperature Transmission Switch. Looks like your switch.

In your car the S.C.S. (not T.C.S.) would be used and the temperature sensor at the back of the head may be a part of this system. It will provide vacuum spark advance when the speed exeeds 38 m.p.h. or with engine temperature below 95° and above 230°.
The S.C.S. (Speed Controlled Spark) was a 1972 one-year only system with a special plunger for the special speedo gear.

HTH

[dv657172]

Kenth

It does look like a TCS transmission switch as you and cammerjeff indicate.

The manual says that all engines except 4 spd manual use the Speed Control Spark Advance (S.C.S). which is controlled by speed and the TCS transmission switch is used for manual transmission cars

There is also mention that cold engine operation (below 95 degrees) and hot engine operation (above 230 degrees) will provide vacuum advance but would seem to use the temp switch located between cylinders 6 & 8 for that determination.

While the PS 7M5 head is original to the engine and the 455 YC block is code and casting date consistent for the car, the engine VIN (Pontiac plant) doesnt match the car vin (Lakewood, GA plant) so maybe the engine was replaced with one from a manual trans car at some point? Dont know. I've owned it since June 1987 and am only the second owner

[dv657172]

Actually while the sending unit does look like the TCS transmission switch that switch is for a manual transmission car and is mounted on the transmission cover case see 7D-22 part number 77.

[tjs72lemans]

My 72 Lemans has the TCS sender on the front/top area of intake manifold where it is affected by the temps as it warms. The temp sender for the non rally gauge is in the passenger rear side of head. I have nothing like you have at rear of head. My factory AC has the blower motor switch sensor on the passenger side firewall. I think someone added another sender for after market gauges for temp reading on yours.

[dv657172]

tjs72lemans

I tend to agree as it does not appear to be TCS/SCS related given its location and I have not been able to see any other need for one from the factory