Engine stalls, fault discovery
The start of this thread got me started on the right track, so I thought I'd share my discovery.
Symptoms: intermittent stall, particularly on hot days, with a/c on. Attempts to restart fail, crank - no run. Also, no fuel pump buzz, dash gauges i.e. fuel, temp. at zero. No Bus in the odometer, thought not right away. Code scanner also has "communication error". After a variable amount of time with engine stopped, (hood-up helps cool off faster), key turned to ON, once fuel pump buzz returns, gauges return, engine will start and run normal until next stall.
The start of this thread made the point that the +5 Vdc sensor bus (there are two, primary and secondary) had to be supplying power to make the sensors work for the engine to run. +5 Vdc primary appears at the PCM connector 1 (black), pin 17. +5Vdc secondary appears at on PCM connector 2 (white), and supplies only the transmission governor pressure sensor (aka gps).
I started off by checking the two ground posts behind the left and right headlights. All good there, though I did take the nut off of the right side to examine for corrosion (minor) and polished the tab with emory cloth.
Then using an insulation piercing probe (expensive T-pin, though it does less damage to the wire) and extra long test leads connected to a voltmeter. I connected the probe to the +5 wire on the throttle position sensor (TPS) and the (-) lead to the frame ground post (w/alligator clip). I put the volt meter on the windshield and closed the hood. Started the engine, turned on a/c, let it idle and hoped it would stall, it did! after about 15+ minutes. When it stalled, the +5 Vdc bus =0 volts.
Now I had to figure out which sensor was short-circuiting the +5 Vdc to ground. Now the engine is stalled, Key On. I opened the hood, probe still on the +5 Vdc wire, pulled the connectors from the throttle position sensor (TPS) and MAP sensor on the throttle body. Volt meter still shows zero. The other two sensors on this bus are more difficult to reach, the crank position sensor (CKS, down by the starter) and cam position sensor (CPS, by the distributor). To be thorough, I decided to check the powertrain control module (PCM) connectors (black, white, gray). I wiggled the wire bundles, no effect. I wiggled the connector housing of C1 (black) and suddenly heard the fuel pump buzz and the voltmeter read +5 Vdc!! Haa! now I had a physical location of failure! But the PCM had cooled enough that the +5Vdc voltage was steady.
I moved the probe to PCM C1 pin 17 and got +5Vdc. Moved the voltmeter (-) to the right-side ground post behind the headlight. Placed the voltmeter on the windshield, re-connected the throttle body sensors (2), closed the hood, started the engine with a/c on and let it idle. With the engine compartment pre-heated, it didn't take as long to stall the second time. Sure enough, engine stalled, +5 Vdc bus =0 (now measured at the PCM. Opened the hood, flexed (slightly) connector C1 up against the PCM housing and the voltage came back (and fuel pump buzzed). Flexed the connector slightly down against the PCM housing and the voltage =0. Now I'm certain that there is a heat related open circuit fault on the +5 Vdc line inside the PCM.
After removing the connector hood on C1 (after cutting away the tape with scissors, don't use a knife!) I repeated the test a third time and wiggled just the +5 Vdc wire (after the engine stalled and bus voltage=0). No effect. [I also tested the B+ (aka 12 Vdc) power input to the PCM, to make sure the PCM had the power supply it needed.] So now I'm certain the open circuit is not the wire connector. Now I'm confident the fault has to be inside the PCM, not something I want to take on the liability to fix (though possible to do, it's just solder).
I decided to use Cardone for repair and return service of my PCM at my local Car Quest shop. I've requested they return the PCM I've shipped to them so I don't have to pay extra for reprogramming. Won't know till I get it back if they paid any attention. This process will take longer than an exchange, though hopefully cheaper.
Btw, Dodge dealer "guessed" the PCM was at fault, no guarantee of repair, for a price quote of $1,200. Cardone will do R&R for $350 plus shipping.
Thanks for reading my post. Questions are welcome, though I may be slow to respond. (noob)
Good luck, I needed it and so will you
Disclaimers: past performance is no indicator of future success, your mileage may vary, batteries not included, all available factory manuals where used in the course of this diagnostic, I am not a professional. All complaints referred to Dewey, Cheatham & Howe.
