Questions regarding 4WD not engaging on a 1991 Chevy K1500 pickup

Hi everyone,
My 4WD stopped engaging on my 1991 Chevy K1500 Pickup. I noticed the 4WD light was not coming on and I ran the truck up on the bottom of a
steep bank to check if it was not in 4WD or if it was just the 4WD indicator light. Only the rear wheels were spinning and it definitely was not in four wheel drive.
I had just used the 4WD a few days ago when I ran the front of the truck up on ramps so that I could spray some rust inhibitor and undercoating on it. The wax based rust inhibitor and waxed based black undercoating were made by www.valugard.net and are approved by GM and Chrysler. The rust inhibitor is a "cavity wax" that is sprayed inside of doors and body panels but you can also spray it underneath the vehicle if you want. It's supposed to be safe for electrical connections but I guess I could have sprayed some on a connector and caused a problem. I'm sure I did not get any of the actual black undercoating on any electrical connectors, but perhaps I got some of the cavity wax on the connectors. I had also blew everything off underneath with some compressed air, but I did not notice any connectors near the air nozzle. I'm not sure if something happened or if it is just coincidence. I also greased the fitting on the front drive shaft, but I don't see how that would contribute to a problem like this.
After I backed the truck down off the ramps I parked it I left it in 4WD (I forgot I even had it in 4WD). Today I took the truck for a short trip to the park and when I got there, I noticed that the selector lever was in 4WD but the truck did not feel like it was in 4WD on my trip. When leaving the park, I move the selector to 2WD and drove for a while and then moved it to 4WD so see if it would engage and it would not go into 4WD. Neither the 2WD or the 4WD light show up on the selector console on the floor.
The GM factory manual said to check the A/C-HTR fuse. I checked the A/ C-HTR fuse and it was fine. However, I had a 25 amp fuse labeled 4WD and when I checked it, it was blown. I had a 15 amp fuse in a drawer and I put that in just to see if it would get things working. However, that fuse was blown almost immediately.
I was going to get some 25 amp fuses from NAPA tomorrow and see if those fuses would work, but I suspect that those fuses will blow as well.
The manual says if it's not the fuse then it's the "transfer case switch" or the transfer case linkage is disconnected or not adjusted properly. The manual also says it could be a faulty transfer case or front differential, but I'm guessing it's probably the switch, provided the proper 25 amp fuse keeps blowing tomorrow.
How many switches are on the transfer case ? The manual says it's on the left upper side of the transfer case. I think the 4WD indicator light switch is a completely separate switch from the switch that actually engages the 4WD. Can I check the switch with an Ohms meter to verify if it is faulty or not ?
The transfer case housing is aluminum so I hope it's not a nightmare getting the old switch out without stripping threads. The switch that I saw looks very hard to get to.
If it's not the switch or a mechanical problem with the transfer case or front differential, what else could it be ?
My only guess would be wiring or a rusted or faulty connection. I seriously doubt it's a mechanical problem with the transfer case or front differential, I've changed all the fluids at recommended intervals, and have never had any noise or shifting problems with the drive-train. There is about 100,000 miles on the truck now.
I would appreciate any feedback, advise, or similar experiences anyone could relate.
Thanks John
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<snip>
In the other group you were advised to replace the fuse and if it blows to check the actuator. I do not disagree with this. However, I would disconnect the actuator FIRST, then replace the fuse and check the actuator for voltage. I do believe the key needs to be in the "ON" position for this, so don't forget to turn the key to the "ON" position. If you do not have any voltage, check the fuse again. If the fuse is good then you will have to check all connections to, and possibly replace, the Tcase switch assembly. Verify that the switch does not function by direct testing the terminals. It would suck to replace the switch to find out later that a wire somehow broke or that one of the connectors was faulty.
(Remember, the actuator could have blown the fuse and you made it up the ramp with the diff unlocked.)
As for the Tcase switch: I wouldn't worry about stripping any bolts. Just don't go gorilla on them. :)
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Thanks for the additional info on top of the other chevy truck forum Nightcrawler.
Seems like good advise to disconnect the actuator first, I don't know if it would blow the fuse without the key being on but better safe than sorry.
As I understand it, you can see the front axle actuator move if you take it out and apply 12 volts. However, can you also verify the transfer case switch in the same way ?
I hope it's not the front axle actuator since those are about $100 bucks from what I have seen on-line, at least at NAPA. I would probably go with a NAPA or GM / Acdelco. I purchased a dealer coolant temperature switch once and they expected me to cut the connector off since it would no longer fit my truck an then splice the wires with those cheap wire crimps which I do not like. I went to napa and the connector on their switch fit fine with no modifications and it's been working fine for years. I just mention that to note that after your vehicle gets some years on it, a sensor or switch from a quality after- market source may fit better than what you can get from the dealer.
I'm pretty sure I was in 4WD going up on the ramps. The driveway is loose gravel and I was never able to make it up on the ramps without 4WD. Perhaps I could have made it getting a running start first, but I've never done that, I always just engaged the 4WD. I also think I remember seeing the 4WD indicator light before going up the ramps.
I'm mainly concerned with getting the switches out of those aluminum housings. Aluminum does not rust but still corrodes. Whatever switch is bad, I've got an aluminum housing to deal with. I will apply a rust buster or penetrating oil first.
Is there any chance I could put a 25 amp 4WD fuse back in an never have any other problems ? I just tried the 15 amp fuse for a quick test but perhaps the stock 25 amp fuse will do the job. I guess I'm dreaming there, something caused the original 25 amp fuse to blow but perhaps I can find a loose or corroded connection I can clean up or a bare wire I can put a heat shrink or electricians tape on that will be easier than replacing the switches.
The switch on the front axle is easier to get to and replace but expensive. The switch on the transfer case is hard to get to but cheap. And of course my 4WD would have to go out right before winter :-)
Thanks again, John
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This is possible. Though, if the actuator is shorted be prepared for a spark. If you have an ohm-meter you may always test the actuator with it.

You may always try the bone yard.

This makes me wonder, though. You did state that you drove it without changing it back to 2wd, right? Usually you would know right away if this was happening.

That's a good idea. Regardless, they make re-threading kits that work quite well.

Yes. Fuses do blow for no apparent reason. Checking the connections is not a bad idea, anyway. It is better to do preventative maintenance when you don't need the 4wd.

Obviously. I used my truck only for traveling up and down a 700 dirt road to my house one winter after the road had a trench dug down the center of it and was not properly backfilled. Can you say mud bog? When I wanted to take it out of 4wd the TCCM failed. I guess I didn't really want to make that trip, anyway. :) I sure wasn't going to use the work truck...
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Hi Nightcrawler,
One of my new 25 amp 4WD fuses blue immediately when I tried it.
I pulled the connector off the front axle actuator, installed a new fuse, and with the key on and the selector in 4WD, I am getting 12 volts on my multimeter at the front axle actuator feed connector, so the front axle actuator is getting voltage. I guess this means the switches on the transfer case and the wiring up to the front axle is in all likelihood OK, since I am getting 12 volts at the actuator ? If the selector is in 2WD, my multimeter said it was getting .02 volts, I guess some bleeds through anyway or it's just the meter.
I was going to pull the front axle actuator out, plug it in and hit the key to see if it will move just be be sure, my guess is that it will just blow a fuse which would indicate that it's bad. The fuse did not blow with the actuator Un-plugged and the key on while I was testing for voltage.
There appear to be two switches on the axle housing. One which according to a parts picture I have is the front axle actuator, it is mounted horizontally on the front axle and it has a black connector. The other switch or sensor is above the front axle actuator and looks smaller. The second smaller switch has a white connector and is more out of sight and on top of the axle housing whereas the axle actuator seems easy to get to. What does the second switch or sensor do ?
Both connectors are mounted to a thin piece of stamped steel that is screwed to a small platform on the passengers side aluminum axle tube with a single screw . Will the black connector for the axle actuator slide off of this steel mount ? It looks very rusted and would not seem to come off when I pulled on it unless there is a trick ?
When I take the old axle actuator out, it appears the wire will twist around and around unless I can get the connector at the other end, off the mount. That wont matter if the part is bad, which my gut tells me it is, but I still have to get the thing off in order to replace it. I don't want to break the steel clip or a plastic connector and I'm not crazy about spraying it with a rust buster since it's an electrical connection.
Any other feedback would be appreciated.
Thanks John
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I was able to wiggle the actuator connector off the stamped metal support, it looked worse (more rusted and stuck) than it really was. I took the 4WD actuator out, connected it back up, installed a new fuse, hit the key, selected 4WD and the actuator did not move. It also blew the fuse once again.
I picked up a new 4WD actuator from NAPA-Balkamp for $62.00 which beats dealer list of around $212.00.
I'm going to install it next time I get a little time and the weather is decent. If it does not work I will check back but I suspect it will work.
Thanks again, John
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Well, I guess I got back to you to late. I think you've found the culprit. :)
Let us know how things go.

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Oh, and for the record. That "other" sensor is a 4wd switch that indicates if the differential is engaged or not.
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I installed the new actuator and the 4WD is working again :-)
The 4WD light is now on, and of course I tested the truck on a hill to actually makes sure the 4WD is working. However, the light to indicate 2WD is not working. I suspect that it must be a burned out bulb since the 4WD light is working. The light may have been burned out for a long time since I never pay it much attention. I may replace it when I get time.
As long as the truck goes down the road in 2WD and I have 4WD when I need it, I'm in good shape.
Perhaps I did not really have 4WD when I went up the ramps, or perhaps the 4WD actuator just had one more push left in it. I"m just glad I found this & got it taken care of before I really needed the 4WD this winter.
Thanks again for the help nightcrawler. John
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Good to hear.
I guess the lamp might be one of those rainy day projects if you have a garage/ dry car port.

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