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Old 08-13-2003, 06:18 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I worked a CAN problem in an ML and can tell you a number of things about its characteristics.

The first thing to do is to measure the resistance between CAN L and CAN H on the 16pin OBDII connector. The two wires are daisy chained between all the controll units (about seven if I recall) Where they go in on both LOW and HIGH there is high kohm resistance in the controller except for two terminator resistors of 120 ohms a piece. These terminator resistors are in the ME controller and the IC controller (Inst cluster). If the system is electrically right there will be 60 ohms across those two wires anywhere you care to measure it. But the easiest place is at the 16pin OBDII connector.

So to recap, if one were to take every controller that has CAN L and CAN H connections and measure the resistance across the two terminals while disconnected from the system one would get high kohm resistance on every one except ME and IC. Measuring these will give 120ohms each. Measuring anywhere from wire to wire there should be the resistance of two 120ohms resistors in parallel or 60 ohms.

The car I had would not shift and it had one of the wires CAN L or CAN H severed where the bundle passed from the electrical center to under the dash. Depending on what that first resistance is will determine what you are looking for. A shorted controlller, any controller would make the resistance less than sixty and interfere with communications. With a short just disconnect every controller till the resistance chnages.

My car had 120 ohms instead of sixty and I didn't at the time know where the terminators were. The I was reading was the IC which we found by disconnecting untill we had open circuit when disconnecting the IC. We still should have seen 120 ohms with the ME terminator engaged but one of the wires being broken left that side open.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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