Center Diff Lock Gremlin

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Dec 11, 2003
the truck i just bought may have a cursed CDL :-\

i found out shortly after buying the truck that the CDL did not engage, and have been trying to track down the problem. here's where i'm currently at with it.....

nature of problem:
when i put the truck in 4L, i can hear the relay behind the kick panel clicking, but DO NOT hear the "whirring" noise of the actuator, and the CDL and ABS lights on the instrument cluster do not come on. The CDL does not engage.

background on truck:
bone stock '93, no F/R Lockers, no CDL switch. 155K miles.

what i know so far:
* Checked both fuses for CDL. they are fine.
* Shorted the "Transfer Switch Indicator" per instructions on Slee's site, and the ABS and CDL lights came on, so i know that the bulbs are good.
* Checked the "Transfer Switch Indicator" switch on the front of the t-case. i pulled it and checked the resistance. it showed no difference between open and closed (greater than 1000Kohm) so i replaced it.
* Checked the switch on the back of the t-case and it showed greater than 1000Kohm when open, and between 0 and 400ohm when closed. occasionally it would show as little as .3ohm, but mostly in the hundreds. does this indicate that it is bad?
* Put the truck on a lift in 4L, and physically verified that the CDL is not engaged by trying to turn the wheels independently.

Given what i know for a fact is working, can i make any assumptions as to what is definately OK. i would like to be able to skip diagnostics on parts of the system, if i can infer from what i know IS WORKING. I'm not so good at reading electrical diagrams.

What direction should i move now on this one.

thanks in advance,
Sounds like that rear switch is bad. You should read pretty much the same accross the closed switch as you do when you short the leads of your meter. However a digital meter isn't the best tool for checking switches. The prefered tool is a sweep meter. With a sweep meter you can see the switch actuate. With a good switch the meter will sweep from open to closed in a smooth movement. If the switch is the least questionable the sweep movement will be jittery. Don't lnow where you would find an analog meter though ;).
if i were to put in the dash mounted CDL button, that would overide the rear switch on the t-case, right? i know a lot of you guys remove that rear switch in the t-case anyway. WITHOUT the Dash CDL button, is that rear switch on the t-case what actually tells the CDL actuator to engage? and the front switch on the t-case tells the ecu, dash lights, etc, once CDL is engaged.

am i understanding this right.

i'm earning my :beer:
on this one.

Yes, you are correct in thinking that the L4 position switch is input to the center diff lock control relay. The CDL dash switch would provide the necessary functionality other than your transmission shift points would be effected. Search the archives for the "pin 7" modification.

There are actually three sensor switches on the transfer. The front one is the center diff lock indicator, the other two are located on the rear extension housing. The one on the drivers side is the Neutral Position switch and on the passenger side is the L4 Position switch. For each of these switches I see no harm in removing the connector and shorting the terminals in order to diagnose any problem. Just do one at a time, listen for sounds and lights turning on. Obviously you can't have the transfer in neutral at the same time it is in low. :doh:

Each of these switches should indicate either infinite ohms or zero, anything in between would be unstable. I was able to temporarily repair my rear locker indicator switch by taking it out and squirting WD40 in it while repeatedly working it for ten minutes. This repair lasted for a year until I had to do it again. It was easy to get at that one and I was satisfied knowing I will have to do it again but was not worth the $50 for a new switch.

You can also use a little 9V battery directly to the CDL motor to see if it works. Check your service manual for the correct pins.
I second Phil's suggestions.

I would attempt to power the actuator (motor) manually and see if the diff will lock. It is possible that the systrem has atrophied from lack of use.
thanks for the help. :cheers:

i'm gonna try the trick with the 9V direct next to see if the actuator is working at all.

if it is "atrophied from lack of use" is this a repair or replace?
and we're just talking about the actuator being atrophied, not the whole system right?
The atrophy would affect any combination of the moving parts. The actuator, shift fork, shift shaft, ETC.
is this a fairly common problem for a 11 year old LC or am i just lucky? 8)
It seems to manifest it's self in vehicles that have never had the switch thrown by previous owner(s)
:D to look on the bright side, i guess the PO wasn't lying about never taking it off-road.

thanks to everyone that has been helping me troubleshoot this. i feel like i'm getting closer. :cheers:
well this weekend i took all three switches out of the t-case and cleaned and tested them. they all check out, but no CDL :(

are the 4L Switch and Transfer Indicator Switch identical parts?


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