I've searched posts until I'm blue in the face and havn't come across a combo of malfunctions like mine. I have a 93 D350 2wd with the A518 that has been dropping the speedometer and alternator with increasing frequency. Cruise has never worked. After driving several miles the speedo drops to zip, od kicks out of course, and the ammeter shows discharge. Go home, throw a multimeter on the batt and yep, no charge. Every time I turn the motor off and restart, everything works fine while stationary. Once rolling again the speedo and alternator die after a mile or two. I've replaced the VSS cleaned every connector, ground, PCM connector and wiggled every inch of harness on the truck. I just went for a 5 mile spin and it's ok for now but I don't trust it.

Ideas? Crank sensor? Wiring? PCM? What do the alternator and speedo have in common? Thanks.

Mike Allred
Vanceboro, NC

 

Mike, did you check the fuseable links? IIRC, they're on the drivers side close to the firewall. I think there should be 3, orange in color.

 

Check the Crank sensor and the wires to it. No crank signal to the PCM and the Alt, Speedo, and OD controls wont work.

Check the sensor gap with a non-magnetic gauge (think brass feeler gauge or a doubled over matchbook cover) .040" is the spec. Make sure the wire aren't chaffed or broken wher it runs up the front of the engine and across the head.

 

I had something very similar going on. Speedo intermittent, not doing anything or jumping around. Kicking out of overdrive. Headlights dim, bright, dim, bright.

Standing in front of the radiator with the hood up, just to the right of radiator ON TOP of the sheet metal, there is a ground for the headlight circuit - among other things evidently. That's what was loose on mine ('93 w250 a518), causing everything to freak out. Tightened it and everything was right. Haven't had a problem since.

 

Thanks for the quick replies fellas. I cruised another 25 trouble free miles but I do suspect the crank sensor wiring might be the culprit. In my wire wiggling, I was able to get the alternator to quit as I was tugging in the crank sensor area. I need to spend an hour of **** obsessive ground terminal cleaning / checking. These old truck are beginning to remind me of the equally old airplanes I work on --- corrosion, marginal continuity, questionable terminal ends. Ain't it great!