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

"John2005" <johnjmechanical (AT) yahoo (DOT) com> wrote

<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.

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

"John2005" <johnjmechanical (AT) yahoo (DOT) com> wrote


Oh, and for the record. That "other" sensor is a 4wd switch that indicates
if the differential is engaged or not.

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

Good to hear.

I guess the lamp might be one of those rainy day projects if you have
a garage/ dry car port.

"John2005" <johnjmechanical (AT) yahoo (DOT) com> wrote