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three questions about my 4 wheel drive on '94 gmc

03-12-2008, 11:43 AM
first off, when my truck is in four wheel drive i can go about 15-20 and no problems. i get up around 30 and it starts making a lound noise. kind of like a grinding noise but maybe a loud hovering. all the fluids are full so what would it be? also when i take it out going down road i can feel a vibration every couple seconds but when i let off gas it stops.
question number 2. why when im at a stop and my truck is in 4 wheel drive and i start to go just after engaging it the truck kinda slams into 4x4 when i start off?
and the third question. it takes about 5-10 seconds to engage into 4x4, any way to make it faster?

03-18-2008, 10:58 AM
in addition to the problem with the 4 wheel drive when i shift into drive or reverse from either or, if i step on the gas right away it revs then kicks into gear.

03-18-2008, 11:10 AM
Everything you described (even the slamming into gear, if you try to go when its not fully engaged it can do that sometimes) is something my '89 does as well. In terms of the delay in engaging the 4x4, GM was still trying to perfect the shift-on-the-fly technology. Remember, in 1994, the technology had only been around for 6 years. Before then, you had to manually go outside and lock in both of your front hubs. Your delay is actually quick compared to mine. Mine can take anywhere between 10-45 seconds to engage. Has been that way since the truck was brand new off the lot back in '89. Same goes for the noise. Its like a soft grinding sound almost. Like I said, my truck has done that since it was brand new, and that was 19 years/130,000 miles ago and no part of the 4x4 system has ever failed me. Remember, it's an old system that was still in the process of perfecting when your truck was built. Your best bet is to be patient with it. Wait for it to fully engage before taking off or trying to get yourself unstuck.

03-20-2008, 07:13 AM
the thing i dont like about it is the fact that i can hear the noise over my dual exhaust and radio, it makes me worried. i worked on a 1990 gmc sierra the other day and i wanted to see how the 4 wheel drive reacted on it, and sure enough soon as the lever hit 4wd it was in.

03-20-2008, 11:23 AM
Wow... thats suprising. I do wonder if there is some sort of modification thats out there that can speed up the engaging of the 4x4 system... It'd be nice honestly. But I'd recommend talking to someone who has some experience with working on 4x4's. If you find anything out post it. I'll do some research and post anything that I find on here.

03-23-2008, 12:28 AM
Okay, I know you're 4x4 isn't broke, but here's an article I found on how the 4x4 system works on our trucks, and will help to explain why it's so slow to engage. The way it works actually seems really dumb to me:

Weak link: Thermal Linear Actuator
Models affected: '88-'98 trucks and SUVs
What happens: An overly complex device called the Thermal Linear Actuator (TLA) fails, and four-wheel drive won't engage. See, when you engage the transfer case, the left-front axle begins to turn, and the TLA is engaged. The TLA creates an electrical current that heats an inert gas, causing it to expand and move a plunger. The left axle and intermediate shaft are connected by spider gears in the differential, and the thermalactuator, once heated enough, extends approximately 1 inch to slide the shift collar andfork over the intermediate shaft to connect it to the right-front axle. Other drawbacks to the TLA include the fact that four-wheel-drive engagement can slow considerably in cold weather and it can disengage if submerged in water.
Sturdy fix: Sure, you can replace the TLA with a new unit, but the hot ticket is toeliminate this horrible idea altogether and install the 4x4 Posi-Lok (shown). It allows thedriver to engage the shift collar and fork from inside the cab with simply a 1-inch pull onthe 4x4 Posi-Lok cable. As a bonus, it allows you to disengage the front axle when in low-range, thus allowing two-wheel-drive low-range operation.
Contact: 4x4 Posi-Lok.

EDIT: I just read something else. Some truck in those years came equipped with a vacuum operated actuator instead of the thermal linear actuator, which would make for a more rapid engagement of the 4x4 system because you don't have to wait for the inert gasses to heat up and move the actuator.

03-23-2008, 02:50 PM
how much would it cost for me to get the vaccum. i dont know if maybe all i would have to do would be to just install vaccum lines.

03-23-2008, 10:05 PM
I don't know, I'm planning to head to the auto parts store to see if I can see one and figure out if its feasable to change over from the TLA to the VLA, and how much it'd cost. Summit racing has available a manual pull cable that goes in place of the TLA so you can do it yourself, but I see that more for someone who does some serious 4x4-ing.

03-24-2008, 10:05 AM
i want it because in places where i instantly need 4wd i end up waiting for ever to get it.

03-24-2008, 11:18 AM
I know EXACTLY what you mean.

03-24-2008, 01:12 PM
i dont think it would be worth it if i had to change the transfer case and stuff like that. but it it was just intalling a vaccum switch and vaccum lines then i would do it

03-24-2008, 02:46 PM
i just talked to someone from napa auto parts and he said that they switched to the TLA because of problems with the vaccum. he told me to check my transfer case switch and if thats not the problem then change the front axle actuator. he told me it would make it about 3 seconds compared to 10 or more

03-24-2008, 10:03 PM
I'm thinking he has it reversed. My 1989 has the TLA and it's slow as hell, but my grandfather's 1996 has the vacuum operated and it's almost instant. I'm all confused now. Mine has always been slow, literally since day 1 back in 1989. The second year for the "shift on the fly" option.

03-25-2008, 09:51 AM
Never heard of this vaccum actuator. I think maybe that is confused with the electrical fix GM came out with. The 88 up models had the Thermal switch as OEM and that was a problem as they age. GM in about 95ish (been so long ago I don't recall all the particulars) came out with a retrofit kit. Its not cheap and requires a new wire harness from the switch to the shifter. The wire harness will depend on your model vehicle and the dealer will need the VIN to get you the right one.

There are other aftermarket parts out there but I know nothing about them. I always prefer to stick with GM parts.

The GM parts came with a new actuator, a special spacer. You ordered the harness depending on your vehicle (some had different wire connectors) from the factory.

03-25-2008, 07:50 PM
well maybe i will just leave it as is

06-13-2016, 07:42 PM
I have a 94 Gmc Sierra 4x4 used it 2 or three times since I bought it but like all of u it delays in the 4x4 engagement but I don't know how long it takes and don't feel it engage I pulled out a jacked up newer 4x4 the other day put mine in 4 low and when I was pullingin reverse no tire was spinning but after tugging back and fourth 4x4 engage and front tire started spinning and got him out but not the frist time like this its happen but grinds when changing it from 4L to 4h or from either one back to 2 high and whole time truck is in neutral does it do this cuz I'm in neutral can I engage and or disengage while in drive??

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