Q: truck dies at start up on 1993 Isuzu Pickup

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My truck dies immediately when i try to start it and won't idle. I took the charging relay out and the truck starts and idles normally but the battery will not charge. When I replaced a new relay the truck dies at start up. I had the alternator checked ( it was bad) so I replaced the alternator by a shop and the problem still exists. I don't know if there is an in line fuse or not. If it is an in line fuse, could it cause this problem? The shop told me that it could be the ECM. Of course I'm not so sure that their diagnosis is correct. If it is the ECM, will it tell me what's wrong with a full diagnostic? Is it the relay, or something else? I don't understand why the truck will run without the charging relay. Could it be the fuel pump relay.? Has anyone else had this same problem? If so, please help.
(2) Answers
I would go to this site and look up the circuitry for the relay that you are talking about. then you will know how that relaly can cause such a problem and go from there
i had the exact problem with my truch, and it's the vacuum system, but luckliy i figured out a way to fix it, get to the fuse box under your hood and take out the ten a fuse for ecm then your truck should idle and run fine, thanks jake