1994 Dodge RAM 2500

5.9 gas engine

Here is the problem.

I have had the truck suddenly quit 3 times at speed whileunder load, with no prior warning signs. Each time the dash temp gauge wasshowing temps well within the operating range.

The first time I was hauling a load of patio blocks – about3,000 lbs. About an hour into the trip the engine just quit. This wasaccompanied by a loud backfire through the exhaust. The truck would not restartalthough it would crank. Had it towed to a dealer near my destination who firsttried replacing the coil. It started and seemed to run fine. I unloaded thepatio blocks and drove home no problems. I drove it around empty for a month orso, no problems.

The second time was a month or so later. I was towing aboat, about 7,000 lbs total trailer weight. About an hour into the journey, “bang,”loud backfire through the exhaust and the truck is dead. Fortunately theshoulder was wide. A farmer living nearby towed the whole rig onto hisproperty. The truck would crank but not start. I went home, got a new coil,came back (truck would still crank but not start) and installed the coil. Thetruck started and again, all seemed fine. A friend used his truck to tow theboat the rest of the way to the marina for launching. In the fall, pulling theboat home, about an hour or so into the trip, and the same thing happened. Thistime I had a spare coil and was back on the road in minutes. Towed the boathome (about 20 minutes more) with no problems.

I kept the replaced coils and tested them. They checked outto be within specs.

I have replaced the plugs and plug wires. Thinking that itmay have something to do with running lean under load, I have replaced the fuelpump unit and associated filters as well as the fuel lines since they are easyto get to with the fuel tank out.

The only other things my research tells me could lead tosuch a sudden quitting with no prior signs of roughness or poor running isfailure of either the crank position sender or the cam position sender.However, since the truck starts and seems to run fine when not loaded heavily(even at 65+ mph) I don’t understand how the crank or cam position sensor couldonly malfunction under load.
Suggestions -- or better yet, what is wrong??

Thanks.

 

Probably a failing crank sensor, it gets hot (and hotter when the engine is heavily loaded) and decides to stop working.....

 

Ahhh -- OK, sounds like I need to get my wrists and knuckles bruised. Thanks for the response. Have any special hints for making the replacement a little easier than it seems like it will be?

Thanks.

 

+1 on suspecting crank sensor. If you have big hands, it is more difficult. I put a piece of plywood across engine when I put mine back in after installing tranny so I could lay across the engine. I used a 1/4" allen head and u joint on a ratchet I believe to remove/install.

 

Yeah, I used the plywood "assistant" when I put the new plug wires in. I even put a foam pad on the plywood - wuss that I am.

 

I am going to steal that idea.......

 

OK - equipped with my plywood, rubber pad, various extensions, universals, etc. I dug deep and pulled the crank sensor. I could just touch one of the bolts reaching up from the bottom but did not have enough reach left over to do anything. I could see both bolts from the bottom. Taking the heat shield off helped a lot. In my case, the bolts were 1/4" - 6mm allen bolts with no hex. The new one goes in tomorrow.

I won't know if this solves my problem for a month or so - when I pull the boat about 60 miles to the marina I keep it in during the summer.

Now lets say the problem is a shaky crank sensor that gets hot and starts dropping signal causing the shutdown relay to activate. Why is it that replacing the coil appeared to reset everything. The last time it happened I was ready and had a coil and tools and it was about 5 minutes. Although I did not think to do it, I'll bet that just unplugging the coil connections and plugging them back in would have "fixed" it. Does the ECM "hold" the shutdown relay open after it determines the crank sensor is sending the wrong info, and then everything resets when you unplug one of the ignition pieces??? I can't figure how else it would work.

 

My guess is, had you just parked the truck for five minutes, and smoked a cigarette, it would have started right back up as well.

 

The crank sensor can cause the memory in the PCM to reset when it shorts. There is certain amount of overload protection built in to the PCM. Sometimes enough, somtimes not. The 5V reference output encompasses several things and any one of them can lock the PCM down. Typically anything ASD related like coils and injectors won't do that. They're protection is usually fused because it's 12V output. O2 sensors will also cause the same issue and I have found quite a few that have rubbed wiring if the teather comes loose, usually the downstream one. Crank sensors are the worst offenders though.

 

Thank you so much for this explanation. I have a couple of books that discuss/explain modern computer controlled auto systems - but none of them have much discussion on what happens when this or that sensor fails to perform or the types of failures and the related symptoms. I suppose this would be asking a lot since there are so many variations. While I am crawling around and replacing things, I'll check the O2 sensors. Thanks again.