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truck randomly goes 'dark' total lock out

1170 Views 4 Replies 2 Participants Last post by  LAclassic
I have a 95 Classic but I'm posting here because electrically it seems to have more in common with the 95 Disco than pre-95 RRs.

6 months ago my truck started intermittently going into lock out mode. This was always preceded by an alternator whine coming through the speakers and sometimes a noticeable blip where the power drops out for a second. After this happens when I turn the truck off it goes into full lock down mode as if the alarm is immobilizing the systems. No power to anything but the clock.

Letting the truck sit for awhile would work to reset and unlock it but to reset quickly I started disconnecting the pos and neg terminals from the battery for a few seconds after this would happen. Just disconnecting the neg terminal would not cure it for some reason. I would usually be good for a few days or longer until it happened again.

I have disconnected the alarm relay and alarm ecu yet the problem persists. Other things I've done: new alternator, battery, IACV (three times), plugs and wires, cap and rotor, new MFU, checked and double checked all connections, fuses, relays, and grounds.

For the past several days the problem has become worse and more dangerous. Now the truck is going dark, full lockout, while I'm driving. So far I've been lucky in that it has "come back to life" while coasting trying to get to the side of the road. As if it knows I'm coasting through an intersection during rush hour.

I have searched the forums, memorized the RAVE electrical guide, and talked to others but nobody has offered anything beyond what I've already done.

What would cause the truck to immobilize other than the alarm? The MFU is new. A bad ECM? I purchased a good used ecm but haven't installed it. It's a 95 so I'm pretty sure I don't have the alarm spider deal that the 96 on trucks have.

If anyone here has any insight it will be appreciated. I'm ready to park it in the driveway for good.
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I've checked the ground wires and the two leads on the fusebox from the alternator and battery. They have continuity and I cleaned the connections although all were in good shape it seemed. I'll look at them again. One thing I haven't done is check to see if there is power at the under bonnet fuse box when it's locked down like this. I assumed there was and the power was stopping at the alarm.

The alarm does do more than interrupt the crank circuit. At least I think it does. The radio needs the code to reset it after this happens but then again, that would happen if the power was cut to the under bonnet fuses. But...if the cable from the alternator was dodgey wouldn't the cable from the battery keep everything from going dead and the radio going into reset mode afterwards? As I'm typing this I realize I need to check the ground for the under bonnet fuse box. Is there one? I don't see it in RAVE. I think I'll take the whole thing out and re clean all those connections.
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Stupid question about the fuse box ground. By "lock out" I mean it's as if the alarm was locking out the electrics. No power to anything and the radio code needed to be reset after it came back to life.

I changed out the mfu and the problem has gone away.

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