Random stalling when hot (FIXED)

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SportRam00

Senior Member
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Apr 1, 2016
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Location
Joppa, MD
Ram Year
2000
Engine
Magnum 360
Hey everyone. I wanted to post this as more of an informational post rather than help me with my problem post...

I've been fighting with my truck for the last month or so. Truck would start fine when cold. Truck periodically would drive fine with proper airflow (ie. no traffic/no stop and go) when ambient temps were low (70*F or lower) outside. It would drive fine all winter and most of spring. No stalling problems what so ever.

Issue would appear, weather was warm to hot (anywhere between 75-90*F). Truck would warm up, then randomly shut off (like when you turn the key off). Issue would happen while driving or while idling at operating temps for upwards of 20 minutes. I found at times, I could press on the right most PCM connector (C1) to get everything to wake up again. Truck would start and drive until the next occurrence of the problem.

Symptoms:

1. Engine shutdown at high ambient temps after warm up to normal operating temperatures.
2. NO BUS across odometer. Gauges dead. (This is key. Tells you that the cluster lost communication with PCM)
3. No start immediately after shut off
4. During no start period, fuel pump would not prime.
5. After a cool down period of anywhere from 5-10 minutes, turn key to run, hear fuel pump prime, truck starts... only to die again a few minutes later.
6. 5v supply voltage to TPS reference wire (violet/white wire) would show good voltage, then drop to approx. .5v when engine shutdown.
7. Could apply pressure to right PCM connector (C1 black connector) and fire truck up.

Diagnostics:

1. NO BUS clues you into the fact that there is something wrong with the PCM communicating to the gauge cluster. NO BUS IS KEY. If you don't see NO BUS then it could be a spark or fuel issue and the rest of this post won't help.
2. The 5v supply drop means the PCM is not able to supply 5v power to sensor network or there is a short on that circuit. Backprobe or pierce the violet/white wire and test with a multimeter.
3. Battery tests good (either take it to a parts store to test or test with multimeter)
4. Verify good power and ground to PCM and no short on sensor network though wire harness using multimeter (again backprobe or pierce wires. don't jam your test lead into the connector pins...). There are many ground points on our trucks. They are located on the PS pump, under the battery tray, and the ground straps for the engine. Use this link for a good test procedure: Test Diags
5. Verify your ASD and fuel pump relays in the PDC are good. You can find testing procedures somewhere out there, or you could do a swap test with known to be good relays.
6. After running the tests above to make sure your battery is good, your fuel pump and ASD relays are good, your 5v sensors aren't shorted and your power/grounds and wiring harness are all good, the next place to turn is the PCM.

Crank sensor

Tried replacing the crank sensor since everywhere you look, that's what people say to check first. Although, looking back at my testing mention previously, I should have known that was a dead end since I didn't find any shorts on the wire harness or sensor network. A bad crank sensor will cause a no start condition. My testing did not find any shorted wires/sensors on the 5v supply network from the PCM, so crank was probably good. I replaced it anyway... Starting to throw parts at the problem... Not good. The NO BUS gives you a clue that the crank sensor may not be at fault because the PCM does not need that sensor to come online to communicate with the cluster. If you are thinking crank sensor at least make sure the wiring to it is good. Visually inspect the wires to the sensor since they tend to get pinched, especially if you had a trans put in. You can test the crank sensor with a multimeter by back probing the connection and spinning the crank by hand to see if it registers voltage as the crank turns.... Easier said than done. An extra set of hands helps with this.

Other Modules

There is some evidence out there that other modules could be dying and resulting in the NO BUS. CTM and ABS are the main culprits... However, I would think you would see other issues to signal they are failing (ABS light being on for example). Fuel pump could also be shorting out, but again, I would think the symptoms would be slightly different (ie. feels like the truck is running out of gas). Same with a bad ignition coil, cap/rotor, wires (engine misfiring).

PCM and the FIX

You could order a new PCM from one of a dozen of so companies. You could try the junkyard... I tried both. Ordered a reman from A1Cardone($$). Turned out the unit was faulty and also had to find someone to program the VIN (more $). The junkyard one was a last gasp attempt to find a working unit. Unit was SKIM enabled so the truck would start then immediately shut off. Screw all that... I was done.

So.. I took my old PCM and took the cover plate off. I connected it to my truck and turned the key to run. I started poking at and around the C1 connector and found a spot where the truck would come alive (ASD/Fuel Pump relay click).

The inside of the PCM is submerged in a silicon potting compound. I cut this away around the area that seemed pressure sensitive. In that area I found a small red rectangular part, known as a "step-up transformer". I lightly touched the transformer and truck would start. Touch it again, it would stall.

Disconnected the PCM and use a multimeter to probe around the transformer. Found out that the top corner solder joint on the circuit lead had a MICRO crack by testing for continuity on the circuit board while poking the part with my finger. Took a soldering iron and re-flowed all the solder connections for this transformer. Tested both sides for continuity between the top/bottom left and top/bottom right. Verified good. Tested continuity from transformer out to a trace probe point. Good. Plugged in unit, truck fired up.

Total fix time: 1 hour (mostly to scrape the stupid potting mix off. BE CAREFUL when doing this. Don't want to damage the board!)

Total DIAG time: days... The diagnosis part is where you really narrow down this type of issue, lose your mind, and go bankrupt...

Informative links I used:

1. Of course Ram Forum. Good stuff here.

2. Test Diags

3. Information Video (This guy goes over every chip that fails on these PCMs. Turns out the first one he points to was the one that was bad on my PCM.)

Lesson Learned: Don't throw parts (or even worse let someone else throw parts) at a problem in frustration! I started to and stopped myself. Don't be scared to open the PCM and poke around. The worst that happens is you have to spend $150-$300 for a new one (you may still have to anyway). A little testing and soldering goes a LONG way. If I listened to my initial thoughts and test results I could have saved even more time and money.

Hope this helps everyone with their NO BUS/Stalling when hot/No start issues.

Good Luck!
 

Yeret

The Village Drunk
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Under the hood fixing/breaking something.
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5.9 Magnum
Tricky but really neat. I had a '93 Honda Accord that used to have the exact same problem. I sold the car for, like, 300 bucks a few years back and never resolved the problem but I'm still 95% positive that the issue was a cracked solder joint in the main relay which supplies power to the fuel pump.

I haven't soldered in years but I'm still pretty sure I can do it, LOL. Maybe it's best that I hope that I don't actually have to. :crazy:
 
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SportRam00

SportRam00

Senior Member
Joined
Apr 1, 2016
Posts
142
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Location
Joppa, MD
Ram Year
2000
Engine
Magnum 360
LOL. Best to hope you don't have to. But, with all the heat under the hood hitting these PCM's, it just leads to an early death. My "old" PCM was actually a Mopar Reman. So the original had already been replaced sometime in the trucks life...

Heating and cooling cycles are not friendly to electronics. In my case the truck would run until the bad solder joint expanded and pushed the contact off the board.These PCMs are also exposed to the weather to some degree as well where moisture can get in and form corrosion. The under hood location is not a great place. I'm somewhat contemplating moving the PCM to inside the cab. Just need to find a place on the firewall to go through...
 
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