4x4 not engaging

[jcdubbu]

i have a 1990 chevy 3/4 ton pickup. it a 4x4 with automatic hubs, a auto tranny and a 350 engine. ok now my problem.....the truck will not shift into 4wd, (found out in a muddy hole when i was getting towed out). the actuator has been replaced in it twice, in the past two years. my understanding is, that the fuse is supposed to blow when the actator goes bad. is this true? if so my fuse is not blown (have tested it with a test light). is there anyway to test the actuator outside of the vehicle? is there a linkage adjustment that may be out? maybe the first thing i should do is check for power at the connection on the front housing. if power is present what else can i check for? any advice would be greatly appreciated.....roads r getting slick here in Montana!

[ChevyWonTon]

I'm not particularly knowledgable about the mechanicals in these 4WDs, but I don't think that's the correct understanding about the purpose of the fuse for the actuator.

If the actuator fails in such a way that it is drawing more than normal current, then the wiring can become heated and cause more drastic problems due to the extra current (which causes heat). The fuse is there to insure that the wires never see more current than they are designed to safely handle, and if the actuator fails this way, it CAN cause the fuse to blow too.

On the other hand, if the actuator just fails from some internal problem and simply stops drawing any current because of the failure, then the fuse will likely not even know it has failed. An example might be if the actuator gets hot from heavy loads or external heat (exhaust, maybe?) and the internal wiring in the motor winding just opens up suddenly. The actuator doesn't work anymore, but there is no extra load being drawn to blow the fuse.

In the case of your actuator, this may be the case. You mention getting pulled out of the mud, so my first thought is that the actuator gets muddy from your use of the truck and you shift in and out of 4WD during these times to only engage it when you really need it. If this is the case, the layer of mud may be holding the extra heat inside the unit from the extra usage, and you end up having the actuator just "burn up" inside itself.

Obviously, these are all just guesses, but based on what you've said and what I can understand about the system, you should look closer at these factors and decide if you need to somehow add protection for the actuator - maybe a shield to keep mud off of it.

On the other hand, the first replacement might have just been faulty. But at some point in the multiple replacements, we all have to decide if we have a cause at work making it fail over and over again and look for the cause.

Good luck!

[JaVeRo]

The mid 90's model trucks used a thermal actuator. I believe your truck will have the same but I could be wrong. It should have a blue and a black wire going to it and engages by heating a liquid into a gas creating pressure and forcing the plunger to engage. If your front differential was under water it could have cooled off enough to disengage.

Most of the trucks that had the actuator replaced are using the motor driven actuator similar to the newer trucks. If yours has this then there will be three wires. Black is ground, Blue is 12v when the transfer case is in gear, and the brown wire was added when the upgrade was done and will have constant 12v with the key on.

It would be easier to diagnose if you can find out which actuator you have.

James

[383chevyZ71]

If you haven't upgraded to the newer style accuator you definely should. The accuator and wire harness is around $160 now ( I gave less than $100 for mine a few years ago but the have went up). Also I've heard on the older style accuators you should use them fairly often or the will go out, if not driving in 4 wheel drive just kick it in and out regularly.

[jcdubbu]

ok after checking into the problem, i found that i have no power to my actuator when engauged into 4wd, check my fuse and its ok. also found that the actuator was faulty and replaced it..... traced actuator wires (2 wire system, blue and black) all the way back to the top of the transfer case, where i come to another switch. it is a check valve type switch and of course my local parts store does not have it in stock! ...my question is where is the power introduced into the 2 wires for the actuator? from what i can tell its a direct system from the switch on the trandfer case to the actator plug. does anyone know of where i can find and illustration of the 4wd system including electrical schematics?

[383chevyZ71]

The sensor on the transfer case sends the signal to the accuator. You can probably pull the sensor out and check the resistance with a volt meter with the ball on the end of the sensor out and pushed in to see if the sensor is bad. Also hope you upgraded to the newer style accuator. They are about $160 compared to $70 but well worth the money, as they don't go out and kick in immediately instead of having to wait like the thermal accuator.

[jcdubbu]

ok so i figured out that the sensor in the transfer case was bad and i had to order it, should be here in a few days.....as for the upgraded actuator? i bought the replace ment one from napa which was about 90 bucks. is the upgraded one motorized? does it require an additional wiring harness? if this is faster and doesn't go out as much i would deffinently go with the more expensive one, if its not to hard to change over!

[383chevyZ71]

The new style accuator is easy to install. There a used one on ebay right now just the motor item #150054775867 or you could probably find one at a salvage yard off a newer model if you didn't want to get a new one. That would just be the motor, you would still have to buy the harness which I the is around $25-30. With the harness will come a metal ball that is flat on two sides, just put it in the accuator hole flat side facing out and screw the new accuator motor in. The harness just plugs into the motor, then the old plug will plug into the new harness. There is one extra wire that you run from the harnes and splice into the one of the wires from the sensor on the transfer case ( not sure which wire I would have to check mine to see). Also definitely faster than the old style, only takes maybe 1 second.to kick in and out.

[JaVeRo]

The upgrade has three wires, Black, Blue, And Brown.

James

[jcdubbu]

thanks for all ur help guys! i really appericate it