94 c280 ECM

[jeff280c]

Hi... I am one of the fortunate 94 - 97 c280 owners on the forum.

Recently, the 280 has been dying after RPM drops to 600 after first starting her up and the in the morning and after the car has sat for 30 minutes or more.

If it dies, I can restart a couple times and it operates fine. No problems while I am driving or sitting at a stop light and idling.

Dealer says: Found ECM is pulling voltage through cam adjuster and when cam adjuster is activated the module loses power causing vehicle to die. They want to replace the ECM.

Is this a correct diagnosis?

Past service on the car: Engine wiring harness, fuel pump, fuel filter, fuel check valve, crank shaft sensor, EGR valve, correct spark plugs boots and wires and overload relay.

Thanks for your help,

Jeff

[Matt L]

If the problem always occurs, replacing the ECM will tell if it is the problem. If it is intermittent, it's a very expensive part to throw at the car and you still won't be sure if it helped.

If your dealer is honest, they will remove the new ECM if it doesn't help. You can't buy one to test, as they won't let you return it, but they could put it back in stock if they're doing the work.

[Arthur Dalton]

< >>

Doubt it..

Whats the cam advacing at that rpm for???

Pull the cam advance plug and see if fault re-appears.

[jeff280c]

Thanks for the reply Arthur. I agree with you and it would not be the first wrong diagnosis from this dealer. Dealer stated that when they pulled the cam advance plug that the issue went away.

If pulling the cam advance plug makes the problem go away, what is your thoughts on the trouble? Something I did notice when I happen to be outside the car and the engine was on the Verge of dying, is the engine was making a lot of noise. Like it was back firing through the intake but not the exhaust.

[Arthur Dalton]

If cam advance is getting demand at low rpm, then all criteria inputs to the ECU have to be verified. It could also be a bad wire harness short on that model/chassis.

Any codes????????

[jeff280c]

Thanks Arthur, I didn't check codes prior to taking it to the dealer. I will check to see if there is any. The engine wiring harness was replaced 2 months ago. Is the cam advance wiring in the engine wiring harness?

[Arthur Dalton]

I noticed the wire harness change after I wrote that ...

..anyway..the ECU has inputs that must be met to satisfy cam advance criteria..so if those are screwed up, the ECU just responndes to those inputs..[ meaning just b/c they unplugged the cam sol and the problem went away does not mean the ECU is bad b/c the ECU may be simply sending a signal to the sol b/c something is incorrectly telling it to.]

...so codes will help.

The basic principle is cam advance demand criteria is both RPM and LOAD condition influenced.
But , in N or P , it is ONLY RPM demanded [ taking LOAD out of the input equation]...sooooooo... a simple test I would do first is to take a cam sol voltage test [ test lamp] with the car in P and increase the RPM to 3K...that should be cut-in spec on that chassis/ system for ECU demand from TN signal.

If that works as it should, then I do not suspect the ECU as giving incorrect demand to cam sol, but rather an incorrect load signal is getting to the ECU that is out of wack .............and the ECU is simply passing that info on to the cam sol.

[jeff280c]

Thank s Arthur.. I will try the test. Weird how I don't have the problem when sitting in drive and idling at stops after driving normally around town. But turn the car off and come back 30 minutes later and have it die at low idle in park before engaging drive or reverse.

[Arthur Dalton]

If it is dying in P before any gear selection , then it may be a bad ECU..that is what my test is for and why I mentioned it as a first test.

You may also have a cam advance mechanical failure...again, a voltage test will tell that.

When one is actually testing the cam sol advance system , one unplugs the connector and then brings independent 12v power to the sol while at idle and looks for the engine to run rough with power [ as in your condition..the engine will not run correctly w/cam advance @ low rpm, so if we force/ induce cam advance @ idle and the engine runs like crap, we know the advance mechanism is working, it is just a TEST].. ...do that a few times and you should see the change every time you turn power On/Off to the sol.
It is possible that the cam is advanced w/o power, but I doubt that b/c your guy stated it cleared when they unplugged it.
Do the test as I posted before getting beyond the basics...I can not diagnose before doing the sequential testings.

[jeff280c]

Thank you Arthur.. I will give that a try as they want $1300 for a new ECM.

[Arthur Dalton]

I would also charge the battery over-night just to be sure you do not have a low voltage condition effecting the ECU triggers..

[jeff280c]

Something I just remembered from the first time this issue started. The LCD's in the instrument cluster were blinking on and off rapidly along with a clicking noise coming from dash forward as the engine was on its way to dying. I first thought a bad relay near the firewall was causing the problem.

[Arthur Dalton]

Spin all the fuses in their holders to clean the connections....common Benz fault and a possible to your problem........

[jeff280c]

I have spade type fuses, but that is a great point... I will get to cleaning connections tomorrow.. relays as well. Thank you Arthur.

[whipplem104]

something I noticed while reading this was that you just had the harness repl.
Did the problem start after the harness? It may be possible that someone connected it wrong. Also, if it is unplugged and plugged back in after start does it dye? I always start with what was done last. History diagnosis!