Had an interesting hot restart problem with the Isuzu pickup (1995) this past week. Occasionally when hot, the engine would start (i.e. starter would spin and engine would fire) but would not continue running when the ignition was turned back to Run. This had happened once before, years ago, when a fan belt broke and I assumed that the battery had run so low that the ECM was misbehaving.

I demonstrated to myself that the truck would not run with the alternator unhooked. I then took the alternator tomy local rebuilder who tested it and found no problems. He also vehemently disputed my claim that the engine would not run without the alternator, stating “no car on the road does that”.

Long story short, last night I chased the wiring diagram and found that the alternator light lead drives a ‘Charge Relay’ that grounds a lead on the ECM through the NC contact. Apparently, this caused a race between the alternator output/engine speed/ECM that the ECM was occasionally losing when the alternator was hot.

Since the truck is worth only a few times the cost of a rebuilt alternator, I disassembled the fuse box and removed the grounding lead from the relay NC contact. Now the truck truly does what all other cars on the road do; i.e., runs with the alternator unhooked.