Ohm specs for 355 cat ecu thrmocouple?

What should I be reading on a good one? Trying to help a friend out...

 

Thanks Dave.

Yeah, so if the thermocouple unit is cold and in proper working order, should you get continuity when probing the 2 wires with a tester?

 

A thermocouple is a battery not a resistance. It produces a voltage at certain temperatures and we find that the voltage change is very small (couple of dozen microvolts per deg C).

http://www.picotech.com/applications/thermocouple.html

Based on the charts given in link, looks like you are wanting something in the 30-60 mV range to fool the Exhaust computer. This is a low voltage in a noisy environment.

The 94-95 exhaust computer is supposed to inform you when the cat gets to 1724+/-68 dF (940+/-20 dC) and tell you to slow down. The 96-99 exhaust computer uses 1720+/-86dF (938+/-30 dC). If the exhaust computer sees 1778dF for more than a couple seconds it cuts power to that bank. This is about 50dF change and results ina a change of thermocouple output of only a few millivolts.

I wish I knew what kind of thermocouple these cars utilize.

 

Warm I am reading between 9 and 10.5 on the left and right, and 8.5 on the center thermocouple.

 

Ohms? Volts? I'm curious about the units in case I run into this in the future.

 

Hugh,
as Mitch explained, an ohmic measurement is not relevant for a thermocouple check ... except if you get extra high reading which may indicates it is broken.
A thermocouple is basicaly the welding of 2 different metallic and conductive wires, so you should read a low resistance value attesting that the 2 wires are not broken and are connected. So your measurement are fine.
If you want to check it works, I would suggest to measure the voltage at the output of the cat "ECU". You should read #0.5V @ 300°C and #4.5V @ 1100°C with an almost linear curve in between.

 

There we go. Nice.

 

MV

 

With that method, how do I separate/confirm the proper operation of the cat ECU from the thermocouple?

 

You can try to swap the cat "ECU" ...
If the continuity of your thermocouples is OK, they should be just fine. I cannot imagine any other failure mode of a thermocouple than a broken wire.

If you really want to check the thermocouples themselves, you can use a DVM, some of them have a thermocouple input which will tell you they are alive, whatever the type they are.

 

Got it, continuity present, then "okay". I have a fancy Fluke "true rms" meter...but god help me to figure out if I have that feature.... I am not an electrical engineer....