What's the difference between these two temperature sensors?

[Ho Hum 74]

Hi everyone,

I have a 74 911 and rebuilding the engine which has a 76 or later case (I know this because of the oil bypass mod incorporated).

I want to replace my temperature sensor and have noticed that my current engine has the later style temperature sensor:

911-606-112-00 Late Style Temperature Sending Unit, without numbered gauge (requires 1 J-123-007-30 sealing ring), 911 (1977-89)

My car has the numbered gauge so I have ordered part number 901-641-632-00 Early Style Oil Temp Sender

Question: My car was reading abnormally low oil temperatures. Could using the newer style sensor produce this? What are the differences (in voltage output or calibration) between the two?

I have also removed a part from the chain cover housing: 911-617-117-00 Thermotime Switch on chain cover, 911 (1970-83), 911 Turbo (1976-85).

What is the switch used for?

Thanks in advance.

Tristan

[Bill Verburg]

Question: My car was reading abnormally low oil temperatures. Could using the newer style sensor produce this? What are the differences (in voltage output or calibration) between the two?

Yes, the older gage need the older sensor. When the older gauge is fed a signal from a newer sensor it wil read very low.

Seems to me that the thermo time switch was used to energize the 7th injector on some cold starts. But it's been a long time since worked w/ that system and I could easily be misremembering.

[Jim Sims]

In 1974 the thermotime switch provider power to the cold start valve (the 7th injector) to enrichen the mixture for starting. It only operated (sent power to the cold start valve) when the engine was cold and only while cranking the starter. If a CIS fuel injection system is being used it should be retained and correctly wired up because not having the cold start system working will make the car very difficult to start. Jim

[Ho Hum 74]

Seems to me that the thermo time switch was used to energize the 7th injector on some cold starts. But it's been a long time since worked w/ that system and I could easily be misremembering.

Bill, Jim,

Thanks for the response. Very interesting, the previous owner wired the headlamp washer switch on the dash to inject fuel for richening the start. He told me to press it 3 times prior to starting. I guess this is the switch that makes that happen. I wonder how that system works. I have a MFI system and I'm unsure how this 7th injector operates or where it even is. Thanks for pointing this out.

Tristan

[Jim Sims]

Both the CIS and the MFI are "mechanical" fuel injection systems; is there a belt driven (driven off the driver's side camshaft) mechanical fuel pump? If not, you have a CIS system. Jim

[Ho Hum 74]

Jim,

I definitely have MFI. What is this cold start valve and where is it located?

Thanks
Tristan

[Jim Sims]

I only know a few of the MFI details but I believe the cold start valve solenoid is mounted on the top of the fuel filter assembly on the driver's side of the engine compartment. A 4.5mm x 9.5 mm hose comes off it; tees and goes to spray nozzles (6 of them?) mounted in the sides of the stacks above the throttle bodies. Perhaps someone more familiar with the system will confirm. Jim