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Old 09-25-2018, 02:20 PM   #1
Bryan59EC
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99 Suburban--Axle ratio?

I have a 98 K1500 with a front differential that will not engage.
Electrically fine, and the motor is working.

A friend is gearing up to dismantle the 99 Suburban and it is also a
4x4.

I have just gone thru every RPO code in that particular car and did not come up with an axle ratio---so I am guessing the ratio would be standard equipment.

My Pickup is utilizing 3.73 differentials and I was hoping to appropriate the front differential from the suburban is the gearing was right.
The Suburban has an M-30 trans and a 5.7.

Would anyone out here know what the standard axle ratios would be for a 99 Suburban with an AT and 5,7? The Car is pretty well loaded with options and apparently delivered to a Salt Lake dealer new.

Just don't want to pull the diff and find out it will not work.

Or perhaps someone could tell me where to find an axle code and what it would be for a 3.73 (what my 98 K1500 is sporting)
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Old 09-25-2018, 03:04 PM   #2
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Re: 99 Suburban--Axle ratio?

GT4 is the RPO code for 3.73.

It should be on the sticker in the glovebox.
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Old 09-25-2018, 05:59 PM   #3
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Re: 99 Suburban--Axle ratio?

There is no axle code in the glovebox of the suburban at all.

GT4 is what I was looking for and it flat is not there.

It is on my 98

That's why I was asking.

I also do not know where to look for the code on the differential either.

If I can get an idea where to look, and what to look for, that would help.

The engine is getting replaced in my pickup this weekend, so it would be easy
to find it on that, but the suburban is kinda sitting in the weeds.

My friend is getting ready to dismantle it soon, but I am not sure when.
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Old 09-25-2018, 07:39 PM   #4
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Re: 99 Suburban--Axle ratio?

The most popular RPO codes for differentials:


GU6, 3.42
GT4, 3.73
GT5, 4.10


If you can’t find any of those codes, post back and tell us all of the codes that start with G.
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Old 09-25-2018, 08:18 PM   #5
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Re: 99 Suburban--Axle ratio?

My bad----axle code is there
Took a pic and went thru every one and checked---or so I thought (dumass me).
It's a 3:42---GU6
Well then---I guess I will keep looking for a 3:73.

Had a transfer case issue last year and for some strange reason the truck would engage the front end at a whim.

Turns out the oil pump wore a hole in the case and it just ate itself up.

I THOUGHT it was a wheel bearing failing when I happened to look down when the truck started shaking and low & behold---shift lever in in 2hi and the 4hi light was on.

Changed transfer case---took some time to find one suitable for the 5-spd---and the 4x4 worked ONE time and no more.


No noise now, and the differential motor engages, so I am suspecting something amiss in the differential----not sure.

Since I'm taking it in for an engine replacement, we will put it on the hoist to see if it is the transfer case again---or the differential.


Sorry to bug you ----- I am blond, blue eyed & polish----trifecta of idiocy
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Old 09-26-2018, 07:15 AM   #6
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Re: 99 Suburban--Axle ratio?

Out of curiosity, do you have a 3 button 4wd switch(NP243) or a 4 button 4wd switch(NP246)?

The NP246 is more problematic. When you shift into 4wd, do the yellow lights on the switch light up solid, or do they flash?


You can check to see if the problem is in the front diff or the transfer case

Chock the rear wheels
Jack up the front of the truck and put it on jack stands.
Turn the ignition on but do not start it.
Leave it in park, and shift to 4wd High
Can you spin the front drive shaft? if you can't, your transfer case/shift motor is working.

If you can, you have a problem in your transfer case/shift motor


While its up in the air and in 4wd, try and spin the front wheels at the same time. If they don't move(or they move only when the driveshaft turns) the axle is engaging.

If it doesn't engage, you may have an actuator problem(pretty common) or the shift fork/bushings are worn out in the differential(not super common, but it still happens)

The actuator can be replaced without removing the differential.

The shift fork/bushings require the differential to be removed.



***The Np246 transfer case has clutches in it, If they are partially worn, everything will test ok when trying to turn stuff by hand, but it will slip when trying to push a big suburban.***
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Old 09-26-2018, 09:41 PM   #7
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Re: 99 Suburban--Axle ratio?

NP 241 C

Manual trans truck with the TC shifter on the floor.

When put into 4hi, the floor indicator comes on showing all 4 wheels

In 4low it shows all four wheels

I have no front wheel action in either position, but the low range is working.

I live in a sandy area and my burn pile is out back----cant take the truck out there anymore, 'cause I might not make it back to the house.

Did you know, that you only find out the 4x4 ain't working----when it is too late!
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Old 09-26-2018, 09:55 PM   #8
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Re: 99 Suburban--Axle ratio?

If it doesn't engage, you may have an actuator problem(pretty common) or the shift fork/bushings are worn out in the differential(not super common, but it still happens)


The actuator works just fine---even the old one is working
I went so far to buy a new switch and installed it---to no avail

disconnected it and connected the harness to the old switch and let it hang.

Hanging would engage the actuator, and depressing the ball at the end of the switch would retract it.


I'm thinking that the issue I had with the old TC and with it engaging and retracting at will might have damaged something in the differential.


This vehicle sees 80mph on a regular basis and those are the speeds where the chaos reared it's ugly head.

When I finally figured out it wasn't a bearing, I pulled over and for sport and amusement put it in 4x4-----truck would barely move and was shaking pretty hard.
Not to mention the loud chattering coming from the bottom.


Changed TC (finding one was a bigger problem than changing it) and it all worked fine for a while.

When the 4x4 quit again, I changed the actuator

It worked one time after that and fooled me into a sense of security----till I took the truck out back. HAH HAH

Got back to the house in reverse in 4low (front wheels just going along for the ride) just chugging away at the sand. Took awhile, but I made it----have not taken it out back since.
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Old 09-27-2018, 08:00 AM   #9
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Re: 99 Suburban--Axle ratio?

I misunderstood your post.

I thought you had a 98 K1500 suburban, and your buddy had a 99 K1500 Suburban.

So you Have a 98 K1500 pickup, and you buddy has a 99 K1500 suburban.

How did you verify that the actuator is working? did you unscrew it from the axle and put it in 4wd?

If the actuator moves, then we know electrically everything is good.

The fork isn't too bad to change in the differential. You will have to remove the differential, take off the tube on the right side, and then its obvious. Just keep track of the order of all the parts.

BTW, the NP241C was the most popular transfer case on these trucks. I have 2 here and I don't collect parts.
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Old 09-27-2018, 11:22 PM   #10
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Re: 99 Suburban--Axle ratio?

yep--I have the 98 Pickup

I have unscrewed it and put in 4-wheel
Actuator extends

I can hear the actuator motor running when I shift into 4-wheel

Actuator out and operated with a second switch by hand works.

Everything is electrically fine in the truck (except for backup lights that wont come on and there is power at the switch) and none of the wiring has been altered.

Will do some more investigating over the weekend----motor comes out on Friday morning to get replaced due to no oil pressure at idle.

Gonna rebuild that motor and put it into my 66 Elky----should be a nice improvement over the very old and wheezy 307 in there now.

I figure if the stock 98 Pickup engine will lug around that truck and anything I want to drag with it----should make that Elky a bit more spirited without a big loss in fuel mileage. that 3oh7 is giving me 25mpg and with the tall rear in that, cruises at 80 @ 1900rpm in 5th gear
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