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CAN Resistance Questions

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Hi everyone! I'm working on a 2012 Chevrolet Colorado that has been swapped with a Gen 3 LS and is running an E40 PCM. Current Performance made the swap harness for the truck and tested and approved the harness before it went out to the owner. The harness does have the GM LAN Hi and Lo wires to the PCM and TCM. The truck is throwing a U0100 code for Lost communication with ECM. I understand that the data from the ECM will not be accepted if the voltages on the CAN Hi and Lo don't match, the resistance doesn't match the bus, or the system voltage is wrong. Ironically, the truck runs and drives well aside from some bugs likely associated with the code.

My question is when I ohm the harness, PCM, TCM, and OBD port, what ohm reading am I looking for? Is it 120 ohms or 60? What should the control modules read when unplugged?

The numbers I got were:

OBD port PCM / TCM plugged in: 60 ohms (multimeter set to 200 ohm scale)

OBD port PCM / TCM unplugged: 120 ohms (multimeter set to 200 ohm scale)

PCM connector: 120 ohms (multimeter set to 200 ohm scale)

PCM unplugged: 120 ohms (multimeter set to 200 ohm scale)

TCM connector: 33.3 ohms (multimeter set to 200 ohm scale)

TCM unplugged: 60 ohms (multimeter set to 20k ohm scale)

The data seems to be all over the place. This is my first real world application of the CAN course so maybe someone with more experience can chime in.

Where are the resistors typically found in an OE application?

A CAN bus should have a 120ohm resistor at each end, so if you measure between CAN H & CAN L with all devices connected you should have 60ohms. The resistors may either be inside one of the devices at each end or sometimes inside a connector on the end of the bus. You should not have 33ohm anywhere, that would suggest there are three termination resistors.

You can find where the resistors are by watching resistance as you unplug one device at a time, if the resistance changes when you unplug a device then it has a resistor inside.

The fact that you have 60ohms with the TCM unplugged and 33.3ohms with the TCM plugged in provides pretty clear evidence that there is an internal resistor within the TCM along with two additional resistors in the harness.

There are two possible paths forward:

1) IF the resistor within the TCM is controllable, you may be able to simply deactivate it - either by software or by a dip switch or something similar.

2) If it is not, you will need to remove the terminating resistor in the harness at the end the TCM is connected to. (You also should double check that the TCM is the last device on the chain, though if there are only 2 devices this is somewhat moot). Your wiring harness supplier should be able to provide documentation to show you where the resistors are and also the relation of the TCM to the other devices on the CAN circuit.

-Mark

3 resistors would give ~40ohms.

Measuring 33ohms could mean there are actually 4 resistors.

Or that some installed resistors are less than 120 ohms (often 100 ohm resistor are used).