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 cause 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

On Tue, 20 Oct 2009 17:51:42 -0700 (PDT), John2005
<johnjmechanical (AT) yahoo (DOT) com> wrote:

Does this truck use a vacuum activated disconnect? If so check the
unit, there may be a 'vent' hole that you've clogged with the
rustproofing. Or you may have accidently pulled or damaged the tube.

PeterD wrote:
The 91's had the wax element actuator from the factory. The retrofit
kits use the newer motor style actuator.

--
Steve W.

Hi Steve,

One of my new 25 amp 4WD fuses blew 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 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

P.S. Can I get a direct fit replacement 4 WD actuator at the dealer ?
The plug from the dealer picture looks the same, but have heard that
there was some type of "retrofit" kit available. I don't want to have
to splice the thing in with some of the cheap wire end crimps, I just
want to take the old one out, screw the new one in, and then plug it
in the same way it unplugged. The reason I ask is because the dealer
sold me a coolant temperature sensor once and it needed to be spliced
per their instructions. I did not like it and wound up buying another
one from NAPA that just plugged right in the same way the old one came
out. It's been working fine for years.

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.

John

On Mon, 26 Oct 2009 15:27:01 -0700 (PDT), John2005
<johnjmechanical (AT) yahoo (DOT) com> wrote:

I didn't know there was a 2WD light...

You know Peter, I can't say I remember the last time I actually saw or
noticed the 2WD light, but my GM factory manual says the following in
the transfer case section...

From the manual...

When shifting into 4WD,

"A switch in the shifter mechanism causes the 2 wheel indicator light
to turn off, and the 4 high indicator light to turn on..."

My guess is the bulb burned out so long ago, I just never noticed.
It's not the kind of thing you pay much attention to since it's
basically on the floor. Hopefully I will not take the thing apart to
find there is no bulb there.

John