Mercedes E500 No start/No Comm

I definitely enjoyed this video. Always so satisfying when you can find the problem instead of running a new wire. There's nothing worse than taking a chance running a new wire and it comes back in a week with a different circuit that's down. Then you HAVE to find the damage and running the new wire was for naught. Also I love how this was fixed properly. The vast majority of shops would have thrown a computer in there!

I had a similar problem on an 09 Dodge Caliber 2.0 just last week. I learned a lesson on that one for sure. Same problem. No start/no comm/no 5v ref. When I went to check powers and grounds I thought I could save time by just going through the power distribution and ground distribution wiring diagrams. There are fewer pages and would save me from going through the mess of engine performance diagram pages. Especially since the PCM is usually split up onto multiple pages of the engine performance diagrams.



BUT... in the power distribution diagram, which is attached above, it shows power going to pin30 of a 'Main Relay' in the TIPM. It doesn't show the power going out of that relay and to the PCM! Since I was just looking for Powertrain Control Module I totally bypassed that and of course the Main Relay was the problem!

Because I didn't notice that I thought it was a 5volt ref issue. So I spent a bunch of time unplugging all the sensors on the reference circuit. Some of which were less than pleasant to get to... After I unplugged everything and still had no 5volt ref then I knew I must have missed something. That's when I pulled up the engine performance diagrams and noticed the power feeds from the Main Relay. Missing that the first time around cost an hour!

So ya, lesson learned... Power Distribution diagrams don't give the full picture. And don't try and be quick/lazy by only looking at them without also looking at the diagrams for the component itself...

Thanks man! And oh man, have I been there and done that!
I think in the case of difficult to get to sensors, an unplugged PCM resistance measurement on the reference and signal circuits works. Like I did in my Bad PCM videos on that Cobalt. Sorry no link at the moment.

Here is the part 1 of that video

ScannerDanner wrote: Thanks man! And oh man, have I been there and done that!
I think in the case of difficult to get to sensors, an unplugged PCM resistance measurement on the reference and signal circuits works. Like I did in my Bad PCM videos on that Cobalt. Sorry no link at the moment.

Got it: