99 diff lock problem- it's NOT actuator

I've got a 99 trd ext cab 3.4l, MT. Bought it a few months ago and the diff lock hasn't worked so I assumed it was the actuator and pulled it out. When i opened it up, it was pristine inside, all components freely turned, brushes were good, and contacts were great. Only thing at all I could think of was old grease, but even that was still in good shape.

So I cleaned it up, re-greased it, and put it back together on the bench. I used the bench test method in the FSM, and it operates great on the bench. So i took it back to the truck, plugged in the electrical connections but did NOT install the actuator in the diff. When i tried to actuate it with the switch on the dash, I got nothing, zip zilch. So I unplugged it and bench tested again, and it runs great on the bench.

Next I wanted to verify I had power at the plug and this is where I hit a snag. I have no power to the plug at all no matter what posiition the locker switch is in. So i proceeded to start testing the various components. I manually shifted the diff to the lock position, and the dash indicator lights up solid. When I throw it in 4wd, the light comes on. I don't have a separate indicator for 4 Lo and not sure if I'm supposed to.

I followed the troublshooting guide in the FSM and with connector to the 4wd module unplugged, I passed all the tests. I then verified all my wiring is intact to the rear by ohm'ing the connections at the diff to the connector at the 4wd module. Everything is intact. I've verified I have no blow fuses, and the relay clicks when I pull the 4wd fuse in and out. I'm stumped where to go next. My gut is telling me my 4wd module is flakey, but before i go spending a decent chunk on another module, thought I'd run it past you folks.

To my way of thinking, which has been wrong more times than I care to admit, the truck will refuse to lock the diff for many reasons (not in 4lo, going to fast, etc..), but it should try to unlock the diff anytime it's told to or it's not in 4lo. I was going to do the grey wire mod, but I'm holding back until I can troubleshoot this issue. Any tips on testing the 4wd ecm or what to check next, would be appreciated.

 

If the truck doesn't show 4lo it wont try, there's 2 detent sensors on the transfer case that need to be tripped to trigger the diff lock.

I've only changed a few in my life but they do fail, easiest way to diag is to jump the wires on the 4lo sensor and try to engage diff lock thereafter.

Should be a blue connector on the tcase.

 

So, is what you are saying is your 99 has problems, but the actuator ain't one?



Thank you! I'll show myself out....

 

I'll check out the transfer switch tomorrow, should be fairly easy to put the meter on and see if it's switching. As I said before, I can see that not allowing the diff to lock if it doesn't see 4lo, but would think it should at least unlock.

 

If the lack of sensing 4LO is what's causing your rear diff lock to not actuate, why not test it by doing the grey wire mod?

 

Do you get a flashing diff lock light on the dash at all? Have you tested the dash switch?
I know if the diff sensor (one that senses if it's locked or not) is not working, the dash light will continue to flash.

 

I have checked the switch, it passed. I do get the flashing diff light as well when I attempt to engage and it will go solid if I manually lock it.

Reason I haven't done grey mod yet is I want to isolate my issue so I didn't compound my problems.

 

i had a good actuator, and a flashing rear diff lock switch light. Did the grey wire mod and it fixed my problem and I now have a functional selectable rear locking diff.

 

Grey wire mod is on my list, but I really want to diagnose what's actually wrong with it before I go modifying the system. There's clearly something wrong.

Tested H4 and L4 switches, both are fine. Jumper across H4 switch and I can hear the relay click and 4wd light comes on. Jumper across L4 switch and no audible or visual changes happens, with or without H4 still no power to diff. Performed grey wire mod for the hell of it, didn't work at first, but tightened up connection and diff now works. Really would like to know why, but not going to question it after this many hours of troubleshooting.

 

All the grey wire does it sense for ground. So this means that your 4L circuit is not grounding that wire.

This wire connects to your Transfer L4 Detection Switch.

You said you had the FSM, but I have attached the wiring diagrams for the diff lock, and 4wd systems.

 

Thank you sir!

 

Hey just stumbled across this, I had a similar issue where I had a working locker before doing the grey wire mod, after I did it, the locker doesn’t work. Even after I reconnected everything in hopes that it would fix itself. Anyone have any insight on this?