Oil temp switch and head temp sensor

[The Glademister]

On my race car I've been using the head temperature sensor on the OEM temperature gauge (from the center console) and that appears to working fine. I'm tearing the engine down and removed the "taco plate" with the oil temperature switch. And from what I can tell, that is just a switch for the idiot light in the OEM instrument cluster (large gauge). But now I'm looking at the diagrams and the temperature gauge is listed as an oil temperature gauge.

So what's the correct hookup? I'm not using the instrument cluster so whatever sensor is associated with the oil light could be hooked up to an aftermarket gauge if it is a sensor and not just a switch.

And just to be clear, the third sensor on the engine, oil pressure, which is actually just a switch, I've got wired to a yellow light on my racing panel. That works great but it would be nice to have a proper gauge but that's a retro fit to a oil pressure sender. I know that was discussed in my build thread and to do that requires cutting a hole in the engine tin. I'd like to explore that again if anyone has a part number or link to share.

[Dave at Pelican Parts]

The temp sensor is a thermistor. It changes resistance with heat. It is grounded through its threads. It is not just a switch.

The light in the combo gauge is an oil pressure light, and it is triggered by the pressure switch near the distributor. If you want a pressure gauge as well as a light, you need to use a dual sender.

The head temp sender on the car was used by the stock fuel injection system. It is also a thermistor, but the temperature range is quite different from the oil temp one. I don't remember the resistance ranges, but I'm a bit surprised that you'd get sensible readings from the CHT plugged into the oil temp gauge.

The stock CHT sensor isn't in a great place for keeping track of the engine's health. Generally by the time you see much movement from it, it's too late to react to it. It is mostly intended to provide a steady signal to the FI, not to monitor instant changes in engine condition. The thermocouple senders that go under the spark plug are much better for monitoring the engine health, as they react a lot quicker and are in a better location. Check aircraftspruce.com for high-quality instruments; the common VDO CHT gauge and sender are not that great.

--DD

[The Glademister]

Originally Posted by Dave at Pelican Parts

The temp sensor is a thermistor. It changes resistance with heat. It is grounded through its threads. It is not just a switch.

The light in the combo gauge is an oil pressure light, and it is triggered by the pressure switch near the distributor. If you want a pressure gauge as well as a light, you need to use a dual sender.

So I had the correct wiring there. Dual sender was the term I was looking for.

Originally Posted by Dave at Pelican Parts

The head temp sender on the car was used by the stock fuel injection system. It is also a thermistor, but the temperature range is quite different from the oil temp one. I don't remember the resistance ranges, but I'm a bit surprised that you'd get sensible readings from the CHT plugged into the oil temp gauge.

Then I had the OEM temp gauge wired to the wrong sensor. It should have been connected to the sensor in the "taco plate". Makes me wonder just how hot that engine got?

Originally Posted by Dave at Pelican Parts

The stock CHT sensor isn't in a great place for keeping track of the engine's health. Generally by the time you see much movement from it, it's too late to react to it. It is mostly intended to provide a steady signal to the FI, not to monitor instant changes in engine condition. The thermocouple senders that go under the spark plug are much better for monitoring the engine health, as they react a lot quicker and are in a better location. Check aircraftspruce.com for high-quality instruments; the common VDO CHT gauge and sender are not that great.

--DD

[TheCabinetmaker]

Originally Posted by The Glademister

So I had the correct wiring there. Dual sender was the term I was looking for.



Then I had the OEM temp gauge wired to the wrong sensor. It should have been connected to the sensor in the "taco plate". Makes me wonder just how hot that engine got?

the dual sendor is for oil pressure. it has both a switch for the idiot light (set at some low value so it normally wont come on at idle, and it also is a sendor for the pressure gage. it is supposed to mounted via a hose to the oil pressure port near the distributor base. in cars with out an oil pressure gage, a simple switch is used in this location, only for the idiot light. the dual sendor was for cars equipped with both idiot light (pressure) and pressure gage.


the head temp was as far as I know never was supplied a gauge on the 4 cylinder cars, the head temp sendor on four cylinder cars is for the fuel injection enrichment, not for a gage. It is a semiconductor device and thus decreases in resistance as temperature increases, the opposite of the oil temperature sendor which is a resistive device that increases resistance as temperature goes UP.

Most all aftermarket head temp sendors are of the thermocouple type, which produce a small voltage (millivolts) that increases with temperature increase, Thermocouples are extremely accurate.

the factory option for an oil Temperature gage included the sendor on the so called taco plate at the bottom of the sump, this was only offered as a gage sendor, and does not activate any idiot light.

the other idiot light on stock cars besides the oil pressure light is the alternator warning light, indicating when current is flowing from the battery indicating a drain on the battery (lamp lit)

[John Rogers]

On my 914 race car both with the big four and the six conversion I used the following:

- Temp sensor on front left spark plug. This will tell you of a lean condition very quickly.
- Oil temp sensor from a factory type sender.
- Oil pressure AND a huge red light from a dual pressure sender. The light was a red trailer running light from Pep Boys.
- Transmission temp from Jim Patrick in Phoenix that took the place of the speedo drive gear.

[The Glademister]

Thank you Cabinetmaker and John. The dual oil pressure sender definitely sounds like the way to go. From what I can tell you've got to extend the orifice to get the sender away from tins and distributor. What's the most common method? Hard line or flex hose? Is there preferred length to either method?

[jamcleod]

Originally Posted by The Glademister

What's the most common method? Hard line or flex hose? Is there preferred length to either method?

Many people say that you can use a flex hose from a hand squeeze cartridge grease gun so you could find that hose at "any" home improvement/hardware store

[porschetub]

Originally Posted by jamcleod

Many people say that you can use a flex hose from a hand squeeze cartridge grease gun so you could find that hose at "any" home improvement/hardware store

Yea you could do that cause I think the ends of those are 1/8''npt (someone correct me ) but from memory you could visit a VDO shop and buy a stand piece or raised tee with the right thread and go from there,make sure you refit or modify the rubber seal around it so you don't lose cooling air.

[Dave at Pelican Parts]

Don't put anything with mass on the end of a stand-piece pipe. Because a pipe threaded into the case makes for a nice lever arm, and the weight of the sender or whatever will swing back and forth at the end of that lever arm. It's not good for aluminum to be stressed like that...

A VDO shop should be able to steer you right, though.

--DD

[John Rogers]

I have seen several hot rodded 914-4s that used a piece of flex hose and the end with the sender was fastened to the front right sheet metal to prevent movement.

[TheCabinetmaker]

As stated, a 1/8th inch Pipe thread will work, use sealant or tape on the threads (use tape very carefully, none must get past the threads into the oil system, as it could cause a plugged oil passage, some folks don't trust tape at all for this reason and prefer the paste / liquid pipe sealants).

As Stated, don't use a metal pipe to extend the sendor above the dizzie. I have seen too many fail , fracture at the threads just above the engine case. Instead use a rubber hose. some are sold for this purpose (VDO$$$$) or improvise with an oil rated hose as suggested.

The sendor is often clamped in a similar way the spark coil is, with a band and then attached either to fan house, or often piggy backed to on the spark coil clamp hold down screws. There are kits sold with the hose and clamp needed. However the clamp can be homemade with an thin, inch wide strap of aluminum wrapped around the sendor, leaving a tab, with the over lapping strap ends bent to one side so you can drill one hole thru both overlapping ends to secure the band and sendor to the motor.

Easy as pie.