Head Temp Sensor Bad, Can I ground the wire? Options

[ericread]

I have determined the head temp sensor is faulty. In order to get to a sensible idle speed (under 2k RPM) can I ground the sensor wire directly to the engine, or do I need to put a 300 Ohm resister in line to protect the brain?

This is only a temporary fix until I get the replacement sensor.

Eric Read

[pbanders]

If you've got to run the car w/o a working CHT, I'd try a 100 ohm resistor, which would simulate the car being fully warmed up. Be aware that when cold, you'll be running very lean, it may not even start. You may want to try a 2K to 3K ohm pot, wired to the driver compartment, that you can adjust on the fly.

AMENDED: Just don't do this. My testing shows it will be way too lean when cold. Why risk damage to your motor over a $20 part?

[jshaddvw]

I have determined the head temp sensor is faulty. In order to get to a sensible idle speed (under 2k RPM) can I ground the sensor wire directly to the engine, or do I need to put a 300 Ohm resister in line to protect the brain?

This is only a temporary fix until I get the replacement sensor.

Eric Read

you have to create some sort of resistance for the engine to run. it will not run if the sensor wire is directly grounded. i have done this before with a test light. it was a worst case scenerio. but it worked. the light bulb in the test light created enough resistance so i could drive the car home. it ran like dog poop all the way. a resistor would create the same effect maybe a little better. if you have to drive that way then so be it but the sensor is not very expensive i would pick up one asap so it will run right.

[realred914]

dead short wont hurt the brain, but you will be much too lean, maybe not even be able to run

[pbanders]

dead short wont hurt the brain, but you will be much too lean, maybe not even be able to run

You know, I'm going to test this, but perhaps not. Once the engine is fully warmed up, when the value of the CHT drops below about 300 ohms, the ECU cylinder temperature compensation circuit doesn't lean out the mixture any more. In theory, you should be able to ground it and there would be no impact on the mixture. In practice, I know that on a warmed up motor, the CHT is at about 50 ohms. I'll try this on my bench simulator and see if it works. See:

http://members.rennlist.com/pbanders/ecu.htm#CTC

Where you'll definitely be too lean is when the motor is cold, and you're trying to start it, if you have the CHT grounded. May not even start.

[pbanders]

FYI, yep, I just went out and tested this, and there's no difference in the injection pulse width between substituting a 50 ohm resistor for the CHT or grounding it. I don't have time right now to do it, but I'll do a full evaluation across the CHT's resistance range for each type of ECU, and see how closely it matches the pSpice simulation I did in my ECU page. I'll post the results here and update the page, too.

BTW, for the specific conditions I was testing, with the CHT circuit substituted with a "cold" resistance value, the injection pulse was about twice as long as when the CHT is shorted. That's more than enough difference to make it nearly impossible to start a cold engine when the CHT is shorted. I doubt you're going to be able to fake a CHT by shorting it and suffering through the warm-up period, it's more likely you'll never get it started because it will be too lean. Just don't do it.

[ericread]

FYI, yep, I just went out and tested this, and there's no difference in the injection pulse width between substituting a 50 ohm resistor for the CHT or grounding it. I don't have time right now to do it, but I'll do a full evaluation across the CHT's resistance range for each type of ECU, and see how closely it matches the pSpice simulation I did in my ECU page. I'll post the results here and update the page, too.

BTW, for the specific conditions I was testing, with the CHT circuit substituted with a "cold" resistance value, the injection pulse was about twice as long as when the CHT is shorted. That's more than enough difference to make it nearly impossible to start a cold engine when the CHT is shorted. I doubt you're going to be able to fake a CHT by shorting it and suffering through the warm-up period, it's more likely you'll never get it started because it will be too lean. Just don't do it.

Thanks Brad - Great information!!!

(IMG:style_emoticons/default/beerchug.gif)

Eric