My assumption is it's a different circuit pattern so I wouldn't recommend a switcheroo with the others. I think it's a double pole/double throw and the others are single pole/single throw. Did you check the wires on top of the gas tank? I had an intermittent problem where the engine would just cut out when I hit a bump. Finally stopped for good and I found the wires corroded off right where they meet the fuel pump. Too much road salt I guess. It would make and break depending on which way I twisted the wire.

Try putting voltage right to the fuel pump, and see if the engine runs then.
It would atleast take the pump out of the equation.

Went to the salvage yard, and grabbed a few relays. Now I can swap every single relay. I was going to check all the pump connections again tonight. The top of my tank is pretty nasty. Last year I had to replace the connector for the fuel pump, so it wouldn't surprise me if they are corroded at the tank. I just really did not want to drop the tank, since it never has a problem on an empty tank. I don't even think I have enough buckets for all the gas left in it. And the irony of the whole thing, the Trooper in the salvage yard had no rust. The engine was clean, it looked like it was driven into the salvage yard and parked! I asked if I they would sell the whole thing, but they won't. Now they can just kill me for all the parts.

I know you said you replaced the connector on the fuel pump, but are you positive that's not the problem again?

I had these same issues that presented themselves as alternator/battery problems and after a search through the archives came up with the fuel pump connector have issues. My boyfriend (RobG) talked to Jerry Lemond about it for hours and Jerry said he's seen the problem alot, even with replaced connectors. We ended up just sodering them together permanently and I haven't had a problem since. <img src="/forums/images/graemlins/confused.gif" alt="" />

Rob and Jerry also did some other troubleshooting to narrow it down to this. Maybe Rob will chime in if he remembers what he did - I know it involved jumping lots of wires around the fuse box.

Good luck.. stupid electrical gremlins are so annoying! <img src="/forums/images/graemlins/cheers.gif" alt="" />

"I don't even think I have enough buckets for all the gas left in it"

My personal recommendation? Cut a hole in the floor of your rig if you have to access the top of the tank. It's a whole lot safer than bucket's of gas sloshing around in the winter when static electricity can arc a spark and send you to glory. Just make sure your saw path doesn't cross over things like other wires and brake lines. As far as the connectors I just cut them off, soldered the wires, shrink wrapped them with the shrink tubing from Radio Shack, then gunked them up with marine grade silicone sealant. Those connectors are for the guys at the factory throwing the tank in as a unit. They're a problem on the road.

Okay, just looked into the archives! The alternator is the problem! I pulled the engine relay and it runs, that's right, it runs with the engine relay removed. The idiot lights get very dim, they do not go out entirely, but they get very dim, but it runs for more than a few seconds, I think it would run until the fuel pump runs the battery dead!! There is a cert tag with the new alternator that says the "L" circuit tested good, but apparently it does not work properly. There could be a short somewhere else, but I doubt it, I have checked every wire I could find associated with the charging system, and they all ohmed out good. They ordered an alternator that was only 50 amps, when my original was a 60. Do you suppose that this makes enough of a difference to keep it from running the fuel pump?! They told me the 60 was not available. I will have to take these fellows to task on this one. BAFFLING! <img src="/forums/images/graemlins/scared.gif" alt="" />

JMHO, never buy an alt. from chain stores. There are varying opinions on the subject..... but that happens to be mine. I always suggest people taking their bad alt. to a reputable local auto electric place to have it rebuilt. That or buy a new one from Isuzu if you have the coin. I think Smiley (or Captain Alt if you will) would probably agree with me on this one <img src="/forums/images/graemlins/lol.gif" alt="" />

I am digging in the archives right now to get some ammo to convince these guys at Napa that their alternator really is bad. I suspect that the next alternator from them will not be any different. It looks like I might be visiting the salvage yard to pick up an alternator, and then take it for a rebuild. I will not buy electrical components from Checker or Autozone, but I have always had pretty good luck with Napa, as a matter of fact, this is the first Napa electrical component that has given me trouble. After much digging in the archives, I have decided that the Isuzu engineers never heard of the K.I.S.S. principle. That is Keep It Simple Stupid! As an engineer I really try to employ this in all my designs. This relay system although effective when functioning properly, depends on far too many relays. Don't get me wrong, in spite of my threats to light my Trooper on fire, I love the stupid thing. Now I just have to convince the know-it-alls at the parts store that I know what I am talking about. This was certainly interesting, I never would have guessed that so many dependencies existed in the FI system. <img src="/forums/images/graemlins/shiner.gif" alt="" />

"it runs with the engine relay removed"

Now that's interesting...! All these little knowledge nuggets just lying around in the archives. 'Course now I'm going to wonder what circuit it defaults to... or maybe the default circuit is the cranking circuit that energizes the fuel pump off the battery, but then what makes or breaks that circuit with the relay removed... Yeah, k.i.s.s. sounds good...

I don't know for sure which relay it switches, but I am assuming that it switches between the two fuel pump relays depending on condition. I installed the $25 salvage yard alternator, problem solved! I am going to go back and get another one and keep it for a spare, probably take it to a reputable electrics shop and have it rebuilt, install it and keep the old one for a spare. With a Trooper running up on 20 years old, I could probably never have enough spare parts<img src="/forums/images/graemlins/notooth.gif" alt="" />! The fellows at Napa will not be too pleased with me. It is important to note that with the engine relay removed the battery is not charging, at least as far as I can tell. I assume that part of the function of the engine relay is to switch from battery power to alternator power. If you are curious about this, search the archives, there is a TON of great information about this specific problem. Thanks to the board for all the great info. I think this is one of the best BBS's out there! Thanks to everybody!!