4WD not engaged when indicated

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Splorg

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Sorry for almost certainly duplicating another post. I tried to search for this specific problem, there are so many permutations that I can't seem to find my exact symptoms.

Last Friday night my water pump quit (2001 Yukon SLT 4WD 5.3L) Being a cheapskate, a friend brought out his truck and trailer and we trailered it the 19 miles to my mechanic and saved $300 on a tow (!!!). It was a wooden decked trailer with metal ramps, and since it was raining (it's never dry when you have to fix something in my personal experience), the ramps and the deck on the trailer were *very* slippery. No big deal, I have four wheel drive and good tires.

Well, apparently I don't.

When shifting from 2HI to AUTO or 4HI at traveling speeds, the LED blinks a few times, there is a mild but still audible noise as the front end of the drivetrain (I assume the hubs) lock and spin up, the LED goes solid, and away you go. However in attempting to get my Yuke up onto the trailer it's obvious the front is pulling nothing at all, either in AUTO or 4HI, even though the switch panel indicates a successful shift and there is no "SERVICE 4WD" warning from the GM Omni-Idiot-Light. First tried auto. Rear broke traction some, no action from front. Backed off the ramp, shifted to 4HI. Same deal, rear spins, front doesn't do a thing.

That being said, there were some extenuating circumstances. The battery charge was awefully weak at this point - I don't know if low current could affect the trucks ability to shift the transfer case. Also, it wasn't as if I could take it for a short drive to give it a chance to "lock in", I had a few feet, no accessory drive, and a few minutes at most to run the thing into the trailer before it overheated or the battery no longer had the charge to drive the ignition.

I don't have any idea with only one shop jack and very little space at home how to test the thing and see if it's actually working now. I tried a steep, grassy frozen hill at my mechanics at his instruction and the old girl went up, but the rear end did break traction briefly before starting to move uphill again. Not sure if this was my magic G80 rear end doing it's thing or what.

So, what do I do to test this, given the roads are cold but bare and dry; or is this a cut and dried case of needing a new transfer case or something given the fact that it *says* it's shifting and engaged but clearly isn't?

Thanks in advance. The weather forecast is making me nervous without knowing if the stupid thing works.

Hope everyone had a great holiday!

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You may not get any messages on the DIC for this relatively common problem. From the sound of it the front diff actuator isn't, well, actuating; the hubs are already locked to the halfshafts, so the diff does all of that work. I'll edit this post with a link but there are several causes for that if you are able to verify that it's not engaging the front diff.

Typical cause is a a lazy or malfunctioning front diff actuator. That could be a bad actuator or the voltages on the actuator pins may not be good (think bad ground, or bad dash switch).

EDIT: https://www.tahoeyukonforum.com/threads/service-4wd-and-no-4wd.110018/#post-1313805 posts #9 & #9, but the full thread might give you some help.

EDIT#2: (gotta collect my thoughts better....) An issue I had with mine was that the transfer case encoder motor was indicating 4x4 by the dash switch light, but was actually in AUTO mode. Mine worked properly in AUTO, so I don't think your issue is that. I was able to see the problem using the Tech 2, and had to change out the TC encoder motor.
 
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Splorg

Splorg

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Thank you!

Here's what I've done, so far:

Checked the fuses. All have good continuity.

Pulled the switch cluster and tested it. Works as expected.

Got under the truck, had my hapless assistant switch it from 2HI to 4HI. Stuff makes noise, linkage....well it kinda moves but not really.

Then I saw it. That gawd awful ground stud. Singular. I assume having both ground points run to one bolt means this truck has had this issue before, all the illustrations I've happened upon at least lead me to believe there are two. Man was that thing a mess. Dremeled the rust off the frame attachment point, replaced the washers, crimped and sealed new ring terminals on the wires. Checked for ground, ground is good. Dielectric grease.

Lo and behold, that at least seems to have solved the problem. As I've mentioned before, I love on a pretty steep hill, so I backed the old gal out onto the wet pavement pointed uphill, put it in AUTO, and mashed the gas. Back end spun, felt...something I can't quite describe and before the rear end locked in I was already moving at a significantly higher rate of speed than I had anticipated. Fortunately there wasn't anyone observing my highly scientific test.

Right offhand I'm guessing that the shift actuator and the transfer case and transfer case module are okay. Seems like my encoder motor may be lazy or weak. A GM inclined friend of mine is going to put it on his lift tomorrow and we'll give it a real test, for the low low price of a case a beer. That's a bargain in my book.

Thanks again for pointing me to what I needed to get started!

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wjburken

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Thanks for sharing your results.

Once more a problem is resolved by fixing a bad ground.
 

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That's great news.

On mine I had to put a bolt and nut in one of the grounds underneath the driver's seat (behind the LF wheel), since the threads were gone. I have two grounds on one bolt, and one on the other.

For a short test you could put it in 4x4 on dry ground and make a somewhat slow sharp turn. If it crow hops, the fronts diff is engaged. It shouldn't crow hop in AUTO.

Let us know what you find.
 
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Splorg

Splorg

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Results are in, even though it is raining here in beautiful rural central Pennsylvania!

It skips it 4HI.

But then it skipped in 2HI.

I immediately got out and can turn the driveshaft to the transfer case by hand. Still skips.

Put back in 4HI. Did a wicked donut in which the front end did not pull at all. Not a bit.

2HI, same result.

4HI, now it *won't* spin around. Pulls hard, feels like the front engages and gets power, steering hop, the works.

So, it engages. Sometimes. Sometimes it doesn't. To make matters worse, now I'm not sure of the hubs are disengaged on 2HI and that's what is causing it to skip, or if I have something else wrong. Oh, and now I have a pronounced squeak from the rear of the vehicle when it is in motion that is not in sync with the rear wheels turning. Only hear it when my foot is on the accelerator. Not there when I coast. That sounds like a bearing someplace in the driveshaft? I checked the driveshaft and there's no play, so I think the universal joints are alright, but that's hardly an exhaustive test.

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Time to check the voltages and the action of the front diff actuator. It may be intermittent or lazy. If it's not that, it may well be the transfer case, to be properly diagnosed.

EDIT: a Tech 2 diag session might be helpful in indicating what's (not) happening. I recall my indicators were all positive until I found out that the 4hi was acting as if it were in AUTO.
 
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Splorg

Splorg

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Okay, finally had the time and spare cash to work at this (I don't have a garage and damn it finally feels like winter here in Pennsylvania!). Here's what I've found so far.

Video on replacing the linear actuator said "a lift is not required, we used it for ease of filming". My left arm determined that was a lie. Skid plate removed via my left wrist. :/

I pulled the actuator. Connector is beautiful inside, watertight integrity seems to be good. Meter gives back the expected voltage changes. Before just throwing the new one in, I decided to plug it back in and hit the 4HI switch. Nothing. Nothing. Then it extended. Pressed 2HI, it retracted. Plugged in the new one and it has the same behavior, so I assume the delay is normal, and it extends the same distance, etc.

Where the old one is inside the box is absolutely covered in some sort of greyish lube. Is that normal and expected?

I just put the old part back in and returned the new actuator. Still, no warning on the dash, truck acts like it shifts, hubs engage, but no power is sent to the front wheels in any mode.

Trying to save the expense of taking it to my mechanic. Not sure if that's avoidable now. I don't have access to a tech 2, unfortunately, and my little Bluetooth scanner says nothing about the powertrain. Open to any suggestions before I start the negotiations with the wife about the cost of repair. Being Pennsylvania and it actually acting like winter, 4WD would come in *very* handy.

For those curious, I didn't replace the skid plate yet :D

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I'm not sure if there's a spec for the time to actuate, but if it's more than about 2 seconds something is not right. Have you powered it off the truck with 12v to see if it works faster when wired directly?
 
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Splorg

Splorg

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I didn't directly power it, no; but without checking it again it wasn't long, perhaps a second and a half. I just got kind of excited when it didn't do anything - all right! I found the culprit and this isn't going to be ha.....oh, never mind. :D

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