C0374 code and crowhopping in A4wd?

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Okay, I am stuck here. Replaced most common failure items, but still get a C0374 code. All is good in 2WD. If I switch to 4WD Auto, when I corner next, "Service 4WD" lights with code C0374. If I turn sharply, it starts crazy crowhopping as if it was in 4WD and not A4WD. It then stays stuck in A4WD until I stop and restart and the service light goes away, until the very next corner. To get back to 2WD, I have to stop, shut down, restart and shift immediately. I don't even know if it is actually in 4WD or not.

2004 Trailblazer, so TCCM should be fine. (Pulled fuse #8 anyway just for fun.) Transfer case fluid was replaced 20,000 KM ago.

4WD switch was replaced last year when it began to get a bit lazy switching to A4WD. I just replaced the transfer case encoder motor and the 4WD actuator. No change. (I used Genuine GM parts for all.)

When I was replacing the actuator, I tested the shift fork and it seemed to move so it was not broken completely. I rattled the front passenger driveshaft at the disconnect, and there is about 1 to 2mm play; enough to cause a clacking sound when I shake it.

I read the TSB on the C0374 but haven't had a chance to get back under and check the wiring. I am now confronted by two alternatives:
- pick up a transfer case speed sensor and a new harness connector, or;
- just pick up a new 4WD disconnect, and use my new GM actuator instead of the cheap junk actuator these Chinese parts usually come with.

Has anyone solved a C0374 code with a new disconnect?

Do the bearings in the disconnect sound really bad or is a bit of play and some clacking noise when you shake the driveshaft normal?
 
Last edited:

TJBaker57

Guru
Aug 16, 2015
2,000
Colorado
Simplest answer.... Don't use A4WD!!

When the C0374 sets shifts are disabled until you turn off the ignition. Normal behaviour there.

I would be looking at the front propshaft speed sensor wiring and/or the sensor itself. But the wiring first.
 

linneje

Active Member
Apr 26, 2012
399
The disconnect doesn't have any sort of sensors inside. The electronics do not have any way to tell if anything is broken in the disconnect. The actuator is connected electrically, but it basically just extends and retracts - it doesn't sense if if the disconnect is broken or has failed to engage.

Since you have experienced crow hopping, the disconnect must be working/intact. Really doubt the actuator is at fault here - it just pushes on the collar to connect the two half shafts in the front, or retracts to disconnect the two half shafts. Nothing sophisticated there, and I doubt that there is a problem with the actuator. I agree to check on the wiring since you already replaced the transfer case encoder motor.
 

TJBaker57

Guru
Aug 16, 2015
2,000
Colorado
Some would say the front propshaft speed sensor would set a code if it was bad. And yes, there is a code for that too but it at least looks like that code has a more stringent set of circumstances that take 30 seconds to set. The code you have sets in 10 seconds.

The service information for the front propshaft sensor would tell you to jack up the truck and disconnect the sensor to test for resistance and if that passes then jack up the front end, hook a DMM to the sensor and spin one wheel while holding the other wheel still and look for at least 0.5 volts AC. Maybe it's just me but that just seems dumb. I would first pop open the left end dash access panel, pull the TCCM connector 3 and test from there. That's where the TCCM is working from. If the resistance test fails there then sure, you then have to pursue the issue down at the sensor. The purple and the yellow wires lead to the front propshaft sensor. And as for jacking up and supporting the front end.... I would jack up one front wheel only.
 
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budwich

Guru
Jun 16, 2013
1,719
kanata
Simplest answer.... Don't use A4WD!!

When the C0374 sets shifts are disabled until you turn off the ignition. Normal behaviour there.

I would be looking at the front propshaft speed sensor wiring and/or the sensor itself. But the wiring first.
I agree with "don't use a4wd" ... at this point. It is possible that you have a transfer case clutch problem since you have replaced the encoder motor and it didn't improve the situation. Of course, doing some electrical checks of the wiring to confirm that integrity therein will help with repair guidance.

Depending on the transfer case, there is a "physical check" where you can remove the encoder motor, set the clutch pack and attempt to turn front propeller shaft (at the u joint). If it turns, you have transfer case issues.
 
OP
Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Well, the more I research this, the more I understand that if the transfer case is in good shape and the encoder motor is brand new, the crowhopping can't be caused by the disconnect. I think you are all correct. Signs are really pointing to the front propshaft speed sensor or the wiring on the transfer case. I will start there.

But the one question remains - is that too much play at the front passenger driveshaft at the disconnect? It moves 1 to 2mm, and I can hear a clacking sound when I shake it back and forth. (No rattle sounds when driving though.)

As for never using A4WD, I can understand that, but in the great white north, we get a lot of mixed snow/dry pavement conditions that would severely strain 4WD, so A4WD works good for me. But I am also gentle on the throttle, and have NEVER let it engage with a slam. I also shift at slow speeds, and always with foot off the gas. I understand the limitations, but I would miss the advantages.
 
Last edited:

gmcman

Guru
Dec 12, 2011
4,526
Keep in mind A4WD is not 2HI in a standby mode, A4WD has 4HI partially engaged and just awaits slip to engage

If in fact you replaced the electric actuator from the front splined disconnect, you could have solidified grease or another issue inside the disconnect keeping the front wheels engaged.
 

MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,714
Tampa Bay Area
As suggested by @TJbaker ...With the presence of the C0374 Code... If you cannot communicate with the TCCM (Transfer Case Control Module) then it will likely be the cause of the problem and definitely require an R&R. You would NOT be hurt too bad 'in the pocket' by taking a Risk on some of the Used TCCM Offers over on eBay that start around $30.00 and go up from there:

ebaytccm2004trailblazer.jpg

 
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TJBaker57

Guru
Aug 16, 2015
2,000
Colorado
Here is the thing that bugs me about this. If it were the front propshaft speed sensor.... Why would it wait for a turn to set this code?

If I understand the code description correctly it sets when the mode is A4WD, and the TCCM has detected a rear propshaft speed higher than the front propshaft speed and has been increasing the 4WD clutch apply pressure for 10 seconds and not seeing the front propshaft speed come up to match the rear.

I would expect after 10 seconds of increasing apply pressure that it would then have reached full 4WD apply position and that would explain the crowhopping, since after that code sets any apply pressure adjustments are locked out for that ignition cycle leaving you likely in full apply.

But if it were just a speed sensor why would this not happen going straight?
 

gmcman

Guru
Dec 12, 2011
4,526
But if it were just a speed sensor why would this not happen going straight?

Maybe when in a turn, the ABS/wheel speed sensor on the front will show different readings. Couple that with the rear speed sensor and the PCM finally throws a code since it thinks the wheels are slipping with no 4WD engagement despite being commanded?

When in a turn, does the A4WD engage with some perceivable feedback or does it act like it's stuck in 4HI?

If the front disconnect was sticking , would that cause this scensrio? How easy or difficult is it to press on the plunger portion of the disconnect when tbe actuator is removed?
 
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Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Good question, and all I can answer on that one is that perhaps it is fully locked and I just don't notice it straight ahead. Or the other answer is that once it is fully locked, perhaps it will unlock again after a few seconds, looking for another speed difference. (The Service Light would prevent that from happening, which may explain why it crowhops so bad only in the first few feet.)

As for gmcman's comment about a sticking disconnect, that is a good point. I am not sure how to test it if the shafts are locked while in 2WD. I really don't think so, because it was working just fine in the spring. The fork seemed to push in easy and come back out against the spring pressure but I was upside down with very little clearance, so it was hard to tell. Even just backing up a few feet in A4WD, it is obvious that it is FIRMLY locked in 4WD.

I think you are all correct. All signs point to a broken wire or a bad sensor.

All I did was wash the truck recently. It may be a 2004 but I only have 200,000 KM on it. (About 125,000 miles.)

And one of the things I like about A4WD in mixed conditions is that, as gmcman says, it is not just waiting for slip to lock up the clutches; it actually provides a certain degree (5% according to GM) of partial engagement. This helps stabilize the truck on conditions such as cold pavement or gravel roads.
 

gmcman

Guru
Dec 12, 2011
4,526
This helps stabilize the truck on conditions such as cold pavement or gravel roads.

I may be wrong, but the way I understand it is when in automatic 4 wheel drive (A4WD) the transfer case is engaged and it only waits for slip to activate the front disconnect reducing the amount of time takes to engage 4HI. This is useful in parking lots when you need to make sharp turns and you won't bind the front wheels.

If you want to test that it stuck in 4HI, go to an open parking lot and turn the wheels slightly to the right or left and let it creep forward under idle, it will eventually stop because it's binding.
 

TJBaker57

Guru
Aug 16, 2015
2,000
Colorado
Maybe when in a turn, the ABS/wheel speed sensor on the front will show different readings. Couple that with the rear speed sensor and the PCM finally throws a code since it thinks the wheels are slipping with no 4WD engagement despite being commanded?

When in a turn, does the A4WD engage with some perceivable feedback or does it act like it's stuck in 4HI?

If the front disconnect was sticking , would that cause this scensrio? How easy or difficult is it to press on the plunger portion of the disconnect when tbe actuator is removed?



I am of the notion that the individual front wheel speed sensors don't come into play here. They are not connected to the TCCM and as far as I know the TCCM has no access to their data. And this code comes from the TCCM, the PCM has no knowledge of it.

The GM SI information for the code speaks only of the the two propshaft speed sensors , front and rear.

Something just doesn't add up.
 

gmcman

Guru
Dec 12, 2011
4,526
If the front splined disconnect was seized up and kept the front axles engaged, when you place it into 2HI, could that be the code that's being generated because the front prop shaft is rotating at the same speed as the rear, signaling 4WD engagement in 2HI?
 

TJBaker57

Guru
Aug 16, 2015
2,000
Colorado
I may be wrong, but the way I understand it is when in automatic 4 wheel drive (A4WD) the transfer case is engaged and it only waits for slip to activate the front disconnect reducing the amount of time takes to engage 4HI. This is useful in parking lots when you need to make sharp turns and you won't bind the front wheels.

If you want to test that it stuck in 4HI, go to an open parking lot and turn the wheels slightly to the right or left and let it creep forward under idle, it will eventually stop because it's binding.

In A4WD the front disconnect is engaged at all times. It is the transfer case shift motor that is in a "ready" position, applying a minimal amount of clutch apply. When the TCCM senses the rear propshaft turning faster than the front propshaft it engages the shift motor (some call it an encoder motor) to apply more clutch pressure. It is when the TCCM applies more and more clutch pressure and does not see the front propshaft speed come up equal to the rear that it sets this code.
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
21,832
Ottawa, ON
I'm thinking it's your encoder motor. Here's how I understand what's happening. When in A4wd, because the disconnect is locking your front axle and you're crow hopping, the transfer case is stuck in 4hi. When you get it back to 2hi, the front disco unlocks your axle and no crow hop but your driveshaft is still spinning at speed because the TC stuck in 4hi thus you get the code.

Take the encoder off and see if it moves or not with commanded modes.
 
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OP
Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Moose, that was my first thought. That was why I changed out the encoder motor for a new GM one. It seems to cycle through all the modes normally. But it wouldn't be the first new part that failed out of the box ... but I highly doubt it with a Genuine GM part. I am also wondering if the encoder doesn't change out of 4WD, would I get a service light when I commanded a return to 2WD and it didn't move?

Right now, the only codes are C0374, and a C0305 on one occasion too.
 

budwich

Guru
Jun 16, 2013
1,719
kanata
depending on what easier, electrical troubleshooting or mechanical, as suggested pull the encoder motor and set the clutch pack and see if the front prop shaft rotates. Go from the result.... meaning if it doesn't, then it is more likely an electrical problem.

Electrically, basic test is to measure the resistance of the sensor... should return 1.3k-2.7k ohms.

Since you are seeing a c0305, it is more likely that you have an electrical problem (ie. sensor or wiring).
 
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Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Hmm, thanks. I just may pull the cover off the TCCM and pull connector three. As TJ Baker pointed out, the purple and yellow wires in connector three lead to the front propshaft. So now I know I am looking for 1.3k to 1.7k ohms. That might be easier than borrowing some ramps and crawling underneath.

On the other hand, I may encounter a broken wire or a burned connector that is immediately obvious.

New GM sensor and connector are on their way to me just in case, so I think I will try the TCCM probe first, then get dirty.
 

budwich

Guru
Jun 16, 2013
1,719
kanata
certainly do your test from the easiest access (for your work space). The test from the tccm is indeed the best test as it covers the "entire path". Go from the result... ie. if it is bad, crawl towards access at the front shaft sensor and do the same test compare... that will quickly tell you which direction the issue might be. Hopefully, you are thin enough to get under to get access. I use the street curb to quickly lift one side / wheel to help with my fat body without ramps.
 
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Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
Okay, solved. It was, as many of us suspected, a broken wire to the front propshaft speed sensor. It was not in the place identified in the TSB; it was under the fuse block, near the engine. It took a LOT of tracing wires to find it, and wrapping and unwrapping harnesses, but at least we found it. It will take many more hours to fix, but the strange part is that it is no where near where there would be debris, rubbing or chaffing. My mechanic is convinced it was crushed at the factory, and it finally gave out.

What happens is exactly what I diagnosed (after I replaced the 4WD actuator and the transfer case encoder motor.) The speed sensors on the transfer case all need to agree on the speed or it locks into the existing 4WD mode and sets a trouble light. What was happening is that the front shaft was not getting a speed signal, but the back shaft was. The TCCM initially thought the back tires were slipping, and when I put it into A4WD, it locked up the axles solid. (Exactly like it is supposed to do.) But when the axles are fully locked by the transfer case and it still gets two different speed readings, it locks it into that mode and lights the service light.

So now we know what to focus on if A4WD selection results in a locking into 4WD of the axles, followed by a service light. It's also nice to know my transfer case works perfectly.

It will not be cheap because we found a couple of other problems, only mildly connected. The 4WD disconnect had a lot of play at the bearing, so we replaced it. (I used an SKP one from Rock Auto, which seems virtually identical to the Dorman one, at half the price. I like the hex cap screws instead of bolts.) We also found a left wheel ABS sensor dropping to zero while driving. We tried cleaning the sensor and wheel, but it broke off from the rust, so it means a new wheel hub.

The very interesting part is that his supplier highly recommended the brand-new premium line from Mevotech, and their wheel hub looks REALLY nice. I am an SKF guy, but we are going to throw the Mevotech in there and see how long it lasts. It comes with a FIVE YEAR warranty! (Two years longer than the SKF ones!)
 
OP
Chickenhawk

Chickenhawk

Hobbyist
Original poster
Dec 6, 2011
715
This is the new Mevotech premium line.
TXF513188_ANG__ra_p.jpg

Warranty is limited lifetime in the U.S., and 5 years in Canada. (Not sure why.) Very impressive, and looks like it is built to last. Will let you folks know in a year.
 

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