Some authorities say to use ohms to test the CHT Sensor, other authorities say use volts. Which is best and/or what do you suggest. Where would you tie into the system to get your readings? I am tying into the system with my ohm meter with parallel connections with the two wires leaving the CHT and the two wire going to the engine controlling unit; at the connector bracket that holds both the CHT connector and the LH knock sensor connector. Page 303-01B-15 of the 2000 LS Workshop Manual.
Ohms vs. Volts
- Thread starter bputty
- Start date
joegr
Dedicated LVC Member
Please don't do that, ever! You could damage the PCM and/or your meter.
The PCM is a voltage/current source. The ohm meter is also a current source and should never ever be connected to a power source. At best your reading will be meaningless, at worst, you will cause damage. Only ever connect your ohm meter to a passive resistance.
Measure voltage with the PCM connected to the CHT with the key on. Put your Volt meter across the two CHT wires while it is connected to the PCM and the PCM is powered. Compare this to the chart in the manual.
To measure resistance, you would disconnect the CHT from the PCM and measure resistance between the two CHT wires from the CHT.
Since the service manual only gives voltages and not resistance, it implies that the CHT is not a purely resistive device, so resistance measurements may not be helpful.
You never said why you think something is wrong with the CHT or the EOT. Why do you think something is wrong there? If it is due to overheating indications, the odds are really good that it really is overheating, and you are trying to shoot the messenger.
The CHT is likely just a thermistor (thermal resistor). A change in temperature equates to a change in resistance. So unless you have a rigorous test procedure or some kind of known thermal resistance curve all you can measure for is shorts/opens.
So, if there's no voltage coming out of the sensor, it's open and bad. If the voltage coming out of the sensor is equal to the voltage going into it, then it's shorted out.
With the sensor disconnected, you should have a ohm reading within a certain range that it works at. But unless they tell you that it has X ohms and Y temperature then you can't definitively test it. It's also worth nothing that these things very rarely fail, and especially fail in a way that it's open or closed.
Like Joe said, I wouldn't doubt the CHT sensor if it passes the basic checks. No clue what those should be. You'd need the powertrain manual.
So, if there's no voltage coming out of the sensor, it's open and bad. If the voltage coming out of the sensor is equal to the voltage going into it, then it's shorted out.
With the sensor disconnected, you should have a ohm reading within a certain range that it works at. But unless they tell you that it has X ohms and Y temperature then you can't definitively test it. It's also worth nothing that these things very rarely fail, and especially fail in a way that it's open or closed.
Like Joe said, I wouldn't doubt the CHT sensor if it passes the basic checks. No clue what those should be. You'd need the powertrain manual.
joegr
Dedicated LVC Member
Probably, but it could be a diode junction type thermal sensor instead.
I am pretty sure that the CHT is ground referenced. The PCM injects a current on one leg, and the other is grounded. A reading of max reference voltage (5V ?) would indicate open, and a reading of zero volts would indicate a short. It doesn't really matter as either reading would point to a bad sensor, bad wiring, or a bad PCM. There are other tests to do to narrow down which of the three it is.
The problem here is the buffered temperature gauge. It can go from the midway point to the H and back to the midway point very quickly making some think that the reading is false because the engine couldn't get that hot or cold that quickly. What they don't realize is that you may be at 230 degrees at the midway point and only 250 degrees at the "H" point. *
If this is what is going on here, using an OBDII reader to read the actual temperature should make it clear.
*: These are made up numbers. I have not verified what the actual numbers are.
FDR
Dedicated LVC Member
On my 02 V8, I have seen the gauge sit in the middle from 190 to 228. I've watched the temperature and gauge while warming up, but I'm not about to intentionally test the upper limit.
cammerfe
Dedicated LVC Member
You often bring a smile to my face. (This is a note to Joe after reading one of his posts.)
KS
KS