Another No-Start Thread... Bad ECM ??
 

98 D1, ran great yesterday as usual, left it in the driveway instead of garaging and last night it poured something awful, and this morning... no start. I can hear the fuel pump spin up and get a good squirt out of the pressure test valve, but am not getting any spark. The engine compartment was pretty wet, but still no spark after an afternoon drying out in the sun. All fuses intact. Noticed that the CEL stays on while cranking, but the Ultragauge is not reading any trouble codes. What I did notice however is that the UG doesn't fire up and communicate when I put the key in the Run position as it normally does, in order to get the UG working I have to unplug & replug, then it goes through its discover routine. Without unplugging first, it remains dark. Weird. However, once the UG lights up, I am able to read temps, bat V, static O2 voltage, etc. Took apart the ECM and let it dry out in the sun for a few hours, tho there were no obvious signs of dampness or corrosion. Is there a more definitive test for the ECM, as well as a test for the crank position sensor ? Any help and insight would be most appreciated, thanks -

Have you checked fuses? There are several for the ECU through the MFU and one for the Data Link Connector.

I sure did, thank you for the suggestion, why start with the hard stuff. It's weird, UG stays completely dormant until I unplug and plug it back in again, then it goes through a full read discovery process from scratch. Normally, it springs to life when the key hits the run position.

Just was under there staring at the CKS, even after a day in the sun the backside of the engine is still drenched. Noticed that one corner of the cowl rain gutter over the firewall is missing it's caulking, probably root cause right there.

Once the UG fires up, it appears to communicate. At least, with the information that is available in the run position with no start. However, the normal reading for my TPS is usually 12%, and it is reading zero at this time. fires up, it appears to communicate. At least, with the information that is available in the run position with no start. However, the normal reading for my TPS is usually 12%, and it is reading zero at this time. Is that normal ? Never noticed before, because I never had the need to look that closely when the truck was not running

Supposed to be normal as the TPS is zeroed by the ECU no matter what it's actual position is. Reality is a whole nutha matter with these trucks. If it where me I'd have a heat gun on the ECU, it's connector to see if anything comes back alive. The UG is kinda the give-away, that it isn't just firing up like normal. It gets power but only after an unplug. You know what, check your MFU for water, the ECU is fused through it. I wonder if it's not waking the ECU.....?

I'll poke around there tomorrow, thanks. Had the ecm apart earlier and it all looked clean and dry, still hit it with my wife's hair dryer for a while. No difference. Meanwhile, the mfu seems to do all the things one would expect a mfu to be doing

checked a few more things, and got a bit more detail... by chance, I accidentally fatfingered the "up" button on the UG while it was dark & the key in the run position, and it lit up. Tried this a bunch of times, and sure enough, with the key in the Run position the UG is dark and lifeless but when the up button on the rear is pressed, it is instantly at the correct display and is showing a heartbeat with the ECM. Everything that is readable in a no-start condition seems to make sense, temps, O2 volts, trims, etc, including the TPS. Maybe was looking at the wrong reading earlier, but the TPS comes up at 12% and the reading tracks the position of the pedal. ECM appears alive ? No freekin spark tho.

How's the crank sensor wires look?

Might want to unplug it see if water is in the sensor, blow it out with compressed air

Well, back on the road. Took a shotgun approach, as it's my daily driver and not a science experiment, so not exactly sure what the root cause was. Blew out the crank position sensor as Tom suggested, took a hard look at the ECM connectors as well and found that the lowermost pins of the bottom connector were all nasty and corroded so got that all cleaned up, opened up the ECM and baked it in the sun for a few hours, and all the while had a few box fans drying up the engine compartment, which was still covered in moisture this morning. After putting everything back together again, it fired right up and everything behaved normally once again. Thanks for the suggestions, looks like the old hooptie will live to fight another day. Question... should I be thinking about a proactive crank position sensor replacement in the near future ??

Glad u figured it out

Best think would have been to do one of those at a time so you know the cause for when it happens again,

I would at least unplug things slather with dielectric grease and plug back in, helps keep out water and corrosion, buy the big tube

Btw crank sensor and wiring should be heat shielded

What about the UG, is it behaving like it was before all the hoopla? X2 on the dielectric, not the most water tight connectors on these things.