[unruhjonny]

I have been mulling over posting this here;
I am a member over on GMT400.com - a forum dedicated to the GMT400 (GM lingo akin to "A-body") for the 1988+ "OBS" trucks.

I am posting this here because I'd wager that people here have a broader knowledge base than those who focus on one vehicle type built over a relatively short period of time.

I recently came across a "4WD actuator harness upgrade" for front axle;
I was intrigued by this, because recently the truck (4wd with 'G80'-Limited slip) has been acting generally as a 2wd truck with a "one wheeler peeler";

Where I have been recently parking it, snow and ice tend to collect under the rear drivers tire, and the truck won't just get up and move - only the one tire will spin - it's rather embarasing for a cool (to me) 4wd truck.

In my minds eye, with the limited slip, both tires should have drive power at any given time;
Also, if the rear tire(s) are spinning the front axle should also be spinning...
But it doesn't.

I would ideally like for the rear differential to act like the 12 bolt posi in my Firebird - I never once had just one tire spinning - EVER.
When I drove that car year round, it NEVER got stuck.

I would have to think that something is wrong with the front axle actuator, and I intend to get on this as soon as the snow goes away.

BUT

Can anyone offer advice on how to determine if the differential (some mechanically actuated posi) is kaput?

I have been paying close attention to the truck when my wife is pulling away recently, and on occasion the rear passenger tire will do a slow turn, or even more seldom, it will spin - but generally it just sits there while the drivers side spins and the truck goes no where...

If the posi setup is kaput, does someone have a recommendation of a better posi that I can replace it with (in the existing housing of course)?

[PunchT37]

It`s prolly a governor lock diff. Not old school like our classics. One tire has to loose traction for a bit to cause the diff to lock. You can actually hear when it locks.

Either that, or it`s busted. A common problem with those gov lock rears.

[Bills Auto Works]

I don not have much info to pass along as far as the front axle & actuator go as my stuff has mostly always been 2 WD. However I can highly recommend a Detroit Tru-Trac limited slip carrier for the rear!

My axle/driveshaft guy ( Terry @Henderson Driveline in Grafton, Oh a friend for 30+ years) suggested this carrier when I had him rebuild the rear axle in my 97 GMC Sierra 1500. I trust him 100% or I would not use him. This carrier now has well over 250,000 miles on it & is still tight. The 1st 200,000+ miles were pulling my Featherlite open trailer all over the country. This thing still sqauks th etires going into 2nd gear & you can feel how tight it still is when slowly pulling out of my gravel drive! The truck has 648,000 miles on it & has been relegated to winter/WalMart/Bar Parking Lot duty for the last half dozen years or so. Ironically I just pulled the diff cover a couple of days ago to change the lube in it & even after all those miles everything looks great!

God bless
Bill
https://www.jalopyjournal.com/forum/...closed.614419/

[Sirrotica]

I own 2 K3500 dualies, a 93, and a 97, I've had trouble with both of the 4 wd activators on the front ends.

The front axle is heat activated, the unit has a delay from the time it is activated, until the unit gets hot enough to engage the front differential fully into the side gears in the differential. I believe there is fluid and a heating element that makes the unit shift when the fluid expands.

These activators are known to fail, due to old age, or when you may be submerged in cold water (such as fording a creek) the cold water will cause them to deactivate, of course when you really need them.

There are aftermarket replacement units that are cable activated to eliminate the failure prone heat activated OEM units. The OEM activators were upgraded after a few years with a better design (I believe mid 90s), but the updated ones can fail too. The cable type makes the engagement fully mechanical, eliminating the heat activated OEM style.

The wiring is also subject to deterioration, there is a weatherpack plug where the activator plugs into the wiring harness on the front axle. I've seen these connectors fail due to road salt corrosion, and also the wiring under the truck, either rot from salt, or get snagged, and break the circuit. Ideally you need to probe the wiring at the plug to make sure you have current, and that the plug connector hasn't failed internally. The unit may also be fused, I can't recall if there is a fuse for the activator for sure. Hopefully some of this info is helpful.

I've never owned one with posi, so I've got no experience with the rear carriers, but Bill gave you a pretty good reference with his experience, and durability with high mileage.

['ol Pinion head]

Jon, To address your question more clearly, the pickup being discussed is a 1/2 ton model ???

[maxpowerta]

Those Thermal (paraffin filled) actuators are prone to failure and can take awhile to engage, particularly when its -37 like last week. I upgraded mine to use a motorized actuator and never had another issue with it.

[unruhjonny]

Oh sorry;
Yes it’s a half ton.

I was wondering if the brutal cold was a factor, but have been paying attention to this all winter;
as soon as the snow fell, I could tell things were not the way they should be.

I watched the truck today;
my wife had to reposition the truck three times to leave the house this afternoon - all because just the one wheel would spin.

[Sirrotica]

Good read about the poorly designed acctuator, and GM TSB bulletins referencing recommendations for retrofitting the motorized acctuator that was a redesigned fix.

Link: https://www.dieselplace.com/threads/...6-k2500.18840/

The pricing on the cable units has increased quite a lot since I checked on them about 10 years ago, $175 now, previously were about $110.

Link to Summit: https://www.summitracing.com/search/...systems-manual

[unruhjonny]

Thanks for the link to that TSB, I did some more digging and created jpgs of the FULL TSB and saved them to my computer.

That TSB prompted me to decide to get the LMC electric (replacement for a thermostatic) actuator & corresponding actuator harness upgrade.

Between the front diff, and the rear, what is the logic for these actuators?

Is it because of the theoretical(?) increase in mileage from a more conventional posi - based on the merits of both wheels receiving power full time equating to lower mileage??

[Sirrotica]

Originally Posted by unruhjonny

Thanks for the link to that TSB, I did some more digging and created jpgs of the FULL TSB and saved them to my computer.

That TSB prompted me to decide to get the LMC electric (replacement for a thermostatic) actuator & corresponding actuator harness upgrade.

Between the front diff, and the rear, what is the logic for these actuators?

Is it because of the theoretical(?) increase in mileage from a more conventional posi - based on the merits of both wheels receiving power full time equating to lower mileage??

It's for lazy people, that don't want to exit the vehicle to turn in manual hubs. Also, all the driveline stops turning when the differential side gear disengages, helping fuel mileage as well as slowing wear in the front propeller shaft, transfer case bearings, and U joints. Both axle shafts and CV joints continue to turn, even in 2 wheel drive.

Manual hubs would stop the CVs, and axles from turning, further decreasing wear, and raising the MPG. Evidently GM thought the buyers would be resistant to having to manually engage/disengage hubs, so they chose to just disengage the side gear from the spider gears in the front differential.

Warn used to make hubs called selectamatic that would engage when the transfer case was shifted into 4 wheel drive, letting the occupants stay in the vehicle. I haven't seen any of these hubs for decades though.

Back in the 70s when all the manufacturers made full time 4 wheel drive in all the SUVs, and light duty trucks. It only took a few years for the aftermarket to make a kit to change over the full time, to part time. This helped fuel mileage, as well as save a lot of wear and tear on the double cardan U joints on the front shaft, and the exposed U joints in the axle shafts. Gas shortages also played a big role in trying to attain better fuel mileage by converting the full time 4 wheel drive to part time.

Detroit stayed away from full time 4 wheel drive after people that had owned a full time rig didn't want to buy a new replacement with full time 4 wheel drive.

At the time i owned a Jeep with full time Quadra trac, and when the chain broke in it I opted for an early transfer case, and manual hub setup to get rid of the problematic Quadrea trac. Jeep was the only full time system that the aftermarket didn't make a part time kit to eliminate it.