2000 Discovery Interemittent 12v loss to ECM

[Billman]

2000 Fleetwood Discovery 37V. 275 ISB.

I'm experiencing intermittent power loss to the ecm. Fault code 434. Cleaned and checked all batteries and connections I could find. Wiggled every harness I could get my hands on. Problem happens under all conditions. Just sitting in the driveway in neutral with the brake on. Engine stumbles, dash lights come on then off, CEL sets.

Fuses are good in battery compartment that supply unswitched battery power. Harness seems ok. Cannon plug to ecm doesn't look corroded or have any loose wires that I can see. Next I will check connectors on ecm.

Contacted Fleetwood through customer service in search of wiring diagrams.

Any help is appreciated
Thanks in advance

[EngineerMike]

This is rare, but check the pins on the two plugs going into the ECM. All should be neat & orderly like little soldiers lined up, all same height. Occasionally when making up the plug, one or more pins will be made up short, i.e. not pushed all the way in, so contact may be made but over time might become intermittent. If you find a short one, you may be able to snap it fully "home" from the wire side so it seats well like the others from now on. Since you'll have the plugs out anyway from your description this additional check shouldn't take much time. If you can't get your head in there to see, take a light & a camera that can focus in very close, might need to use the "macro" setting which usually looks like a flower.

[Billman]

Thanks Mike

I didn't think the ECM connectors would be the problem, but I checked anyway. Both sides were clean. I did check the cannon plug again. This time I looked at the female side with a mirror and thought I may have found something. It looked a little corroded. I put some di-electric grease on the plug, but 6 minutes after I started it, it stumbled and set the light. What I thought may have been corrosion on the plug was probably grease applied by the factory.

I did get a response from Fleetwood today directing me to Freightliner. They apparently get the chassis in running condition before the body is attached. Freightliner sent me the wiring diagrams for the chassis. I explained the problem and the fault code. His answer was I was headed in the right direction with the unswitched battery supply circuit from the battery to the ecm. Again I grabbed all the harness I could from one end to the other.

I put a voltmeter on the batteries to observe when it stumbles. Never went below 14 volts on either with it stumbling.

The only thing I did this season before this began happening was to service the air dryer. Only thing related to 12v on the dryer is the heater. I can't imagine a heater problem would cause this, but I will disconnect it next time I'm under it.

Another possibility Freightliner said was the 135 amp circuit breaker feeding the cab. I changed it but nothing.

I did change the lift pump because of its age when this started last week. It was original. I don't have a pressure gauge on it yet, but I think this is electrical.

The only other weird thing that's happening is sometimes on restart after it is warm, I can get the CEL to light during crank. Is there a crankshaft position sensor?

If a fault code is set, wouldn't the CEL light come on by itself? Why would dash lights come on as if you switched off the key? Because the dash lights come on also, I'm not so sure this is an unswitched battery supply circuit problem. Could it be an internal ECM problem? Maybe losing power or ground internally?

Thanks again
Billy

[RV Mech Tech]

Billman - are you sure this is a loss of power to the ECM? or power being cut off to the ignition or fuel system and the engine stalling and the computer turning on the CEL (service engine light?) ? - when something is wrong with the engine the sensors in/on the powertrain will signal the computer that something is wrong ( data coming back to the computer from the sensors is above or below the designated electrical/digital parameters) and the computer will turn on the service engine light, computer light, CEL etc. and indicate a trouble code- how did you retreive a trouble code if the power is cut off to the computer? - if the power is off the the ECM a scan tool will indicate "loss of pwer and or signal to ECM" - and you will not be able to retreive a trouble code until power is restored - or is the fault code a 'hard' code that comes up when power is restored? - if this is the case trace the wiring diagrams for the power circuit to the computer, find out which terminal is the power and when it cuts out again have a voltmeter ready and disconnect the harness plug to the computer and check for power at that terminal- if there is no power then back track from there according to your wiring diagram- also be carefull what you stick into the harness plug- you can damage those terminals inside if you stick something larger into them (such as a multimeter prong) so use something the same size such as a pin and then clip your voltmeter on to whatever you stick in there - be carefull not to touch any other terminals - use a magnifying glass to check the terminals in both the harness plug and the computer connector - these things are so small that you may not see a defect- you mentioned your dash cuts out - the computer power supply is usually a 'dedicated' circuit (nothing else is connected to that circuit) but it may go to a power terminal or connection that may be defective - does anything else stop working as well as the engine?- check to see if all other items powered by the chassis battery operate o.k or does something else cut out at the same time?-this may give you a clue on where to look -other big trouble spots are the fuse box and bulkhead connectors (wiring harness connector through the firewall from the engine compartment into the interior)- (check for power at the ECM fuse as well) - you may have a bent pin or terminal in one of them -I have seen the same thing with extreme temperatures affecting electrical connections -works/doesn;t work etc.)- you are going about it correctly by getting the wiring diagrams - when you are checking do not connect the multimeter to any terminal anywhere unless you know what that wire/circuit does and if its a powered wire, or a ground etc. - that's the first thing they teach you when you go through training to be a tech for cars or RV's - - you can easilly damage the computer or an electronic control module by touching the wrong terminal etc. - wiring problems can be difficult or easy to solve and if you take your time and follow the wiring diagrams you will more than likely find the problem and solve it - and you will be saving yourself mucho dollars$$ !- let us know what you find.

[Billman]

Thanks Tech

No. I am not sure I am losing power to the ECM. I'm going by what I have read as a definition of fault code 434. 'Less than 6.2v to ECM'. I have also seen the code could be set if the power is shut down before the computer has time to store proper data. I read that somewhere on the internet. Maybe shutting the master switch off less than 'x' amount of seconds after the key is switched off. Don't know for sure.

I undestand if the computer receives a signal outside the parameters, it will set a code and set the CEL. I'm getting some dash lights along with the CEL.

I checked for codes through the switch at the base of the column. I don't have Cummins software, but connected a Nexiq Prolink that said same thing. It would be easier if the engine stalled to pinpoint the problem and trace the wire but this is intermittent and the engine never stops running. Engine stumbles for a spilt second, dash lights come on, and the CEL lights. Like an ignition switch problem. That's where I'm leading to I think.

All 5 ECM fuses and harness to ECM seem fine. Checking and cleaning all connections that I can find.

I'm thinking there is nothing wrong with the 'dedicated' circuit to the ecm.

I'm not a new mechanic, just new to RV's

Thanks again

[J Walker]

Billman, You are doing an excellent job digging into this problem. I have a similar 2000 Freightliner chassis except that mine is XCS and yours is XC I believe. The wonderful electronic controls can turn into a nightmare if something goes wrong. I would say to drive it to Gaffney, SC for repair but it seems that it is not drivable.

[RV Mech Tech]

Billy - if the ECM has a chance to store a trouble code then it is unlikely the computer is at fault (in most cases) and it sounds to me that the problem could be temperature related since the engine will run and then quit - any electrical connection at any voltage will heat up if the connection is loose or engine heat will affect a part. or connector etc.- as I said backtrack from the computer connector if no power is found at the power supply terminal/pin - as I mentioned the dedicated power supply circuit may not have anything anything else connected to it but it may receive its power from a common power terminal which is directly connected to the battery power- chassis manufactures will use these common terminals/posts to attach several power circuits to them (you can see these on cars as a convenient battery boost terminal on the fenderwell) - you said that the ECM code says loss of power "less than 6.2 volts to ECM" - this sounds like a bad connection and more than likely is a power circuit but could also be a bad ground -check the ECM ground- most ECMs ground through the harness wiring but I have found that the ground teminal pin leads to a short wire that is then grounded to the frame on some of the older RV's but the newer ones will ground the ECM directly to the battery-check your wiring diagram for this- also as we all know the chassis are shipped to the RV manufacture and are called 'incomplete chassis' and there are wires that have to be attached once the coach body is on the frame - you would think that the ECM connections should already be attached but this is not always the case - I have seen grounding wires attached to the plastic steering column peices or under dash heater/AC plastic cases and power wires attached with undersized terminals that were overcrimped and falling off with the same result - low voltage so check for that as well- let us know what you find.

[Billman]

Still have problem...

I spoke to Freightliner Tech. Traced wiring diagrams with him. Tech is leaning towards faulty Cummins ECM.

I then spoke to Cummins Tech. Traced wiring diagrams. He doubts ECM is at fault. Very rare he says. He’s leaning towards Freightliner problem. Blames battery harness like code describes.

I have checked the battery harness but incorrectly according to Cummins. Voltage drop test and ohmmeter is wrong way to do it. I need to ‘load’ the wiring. Makes sense I thought. I then completely removed the harness from the chassis, removed all tape and tubing, inspected all wires and connections, and loaded it with 5-6 amps. No problem found. I did find another relay that is in the harness that sends 12v to ECM when key is switched on. All seems fine.

Ignition switch is new and I did try a different battery.

I no longer need to run the engine to have the problem happen. I had the key on for a few minutes and heard relays click under the dash and the lights came on and the CEL set. Spent the next couple of days disassembling and checking the 2 fuse boxes, relays, and fuses under the dash with no luck.

Tonight I disconnected the 23 pin cannon plug at the engine and turned the key on. Problem doesn’t happen. Reconnected cannon plug and removed the relay I found in the battery harness that supplies 12v to the ECM. Problem doesn’t happen. Replaced relay with a known good one. Problem happens.

After countless hours I’m making progress. Slow, but progress.
I now need to find what is on that circuit along with the ECM before I can go further.

I’m open to any and all suggestions.
Thanks