Chevrolet full-size truck/suburban 1999-2006

[Jammer]

A thread to post:
- upgrades
- service difficulties
- experiences
..specific to the GMT800 truck platform used on the 1999-2006 full-size GMC and Chevrolet pickups, Suburban, Yukon XL, and Escalade ESV in both 1/2 ton and 3/4 ton chassis types.

Much of the chassis and power train are also shared with the shorter-wheelbase SUVs: Tahoe, Escalade, Yukon and also the Avalanche.

Some resources:
GMT800 - Wikipedia, the free encyclopedia
Chevrolet Suburban - Wikipedia, the free encyclopedia
Chevrolet Silverado - Wikipedia, the free encyclopedia

Quick reference:

Six engine choices, V8 except as noted:
4.3 gasser v6 (half ton only) .. 195 hp, 260 lb ft
4.8 gasser (half ton only) .. 270 hp, 315 lb ft
5.3 gasser (half ton only) .. 285 hp, 325 lb ft
6.0 gasser .. 335 hp, 375 lb ft
8.1 gasser (3/4 ton only) .. 340 hp .. 450 lb ft
6.6 diesel (3/4 ton only) .. 300 hp .. 550 lb ft

Performance figures are typical and vary from year to year and among models.

The half ton and SUV models generally use the 4L60e automatic transmission. The 3/4 ton gassers use the 4L80e automatic transmission, and the diesels use the Allison 1000 automatic transmission.

Some of the 8.1s are built with a 4L85e transmission which is identical to the 4L80e except for the presence of a fifth planetary gear on the input shaft.

There are a few trucks out there with manual transmissions, usually the NV4500.

Relevant RPO codes

GT4 - 3.73 rear axle ratio
GT5 - 4.10 rear axle ratio
G80 - limited slip rear axle
MN8 - 4L85e transmission
MT1 - 4L80e transmission

4WD versions were equipped with the NP-246 auto-trac transfer case.

A complete list of RPOs is available here.

I believe all models had independent front suspension using torsion bars.

Standard tires were LT265/70R16 for the half ton and LT245/75R16E for the 3/4 ton.

[Jammer]

Maintenance highlights

All engines and years used Dex-cool type coolant.

The NP246 transfer case requires special auto-trac fluid. Aftermarket equivalents are available but specifically identify themselves as being for use with auto-trac. The fluid is marked with a blue dye. Caution should be used when torquing the fill and drain plugs as the case is magnesium and is easily damaged.

Transmission was specified to use Dexron III or VI depending on year. Dexron VI is compatible with all years and can be mixed with older fluid without damage. For the Suburban type vehicles there is a frame crossmember right in the way of dropping the pan but it is possible to raise, carefully, the transmission tailshaft assembly with a jack to get enough clearance to pull it out without removing the crossmember.

GM switched from 80W90 to synthetic 75W90 gear oil for front and rear axles during production due to lubrication failures in extreme cold weather.

Weld failures on the OEM trailer hitch receiver can occur particularly with the use of weight distributing hitches over the course of many miles. Inspect carefully especially if the receiver has had a hard life.

8.1l and diesel are equipped with an engine oil cooler mounted inside the radiator. The lines are prone to leakage after a period of years. Replacements are available through Chevrolet dealers and from rock auto but most other aftermarket parts dealers don't have them. Front axle is right in the way of replacing them on 4wd models but it can be done without dropping it (at least on the 8.1).

[Jammer]

Mods

There are a variety of aftermarket trailer hitch receivers available with capacities to 15,000 pounds. In most cases the receiver height is the same as the OEM hitch. The aftermarket designs I have seen do not rely on the bumper for structural support.

A front mount hitch is also available that bolts to the frame in the same location as the factory tow hooks. The factory tow hooks are retained in the same location with the hitch frame attaching to the opposite side of the frame rail.

The factory roof rack crossmembers are compatible with some rooftop accessories from Thule and Yakima. You can get a large basket, cargo containers, ski and snowboard racks, watercraft holders, and bicycle racks.

A bug deflector for the hood is a dealer option. Aftermarket ones are also available. My aftermarket one cracked due to shrinkage in the cold weather here. I replaced it with an OEM one which has slots for the screws so it can slide when it shrinks rather than breaking.

The charge line to the 7-way trailer connector is disabled from the factory by removal of the fuse. The underhood fuse panel will have a place for it, marked "stud 1 auxiliary power."

There is a connection for a brake controller wiring harness on the firewall, above and to the left of the break pedal, quite close to the the side. There is a junction box about 6" square with a black cover. The cover must be removed to access the brake connector. If there is not a factory-installed 7 way connector (rare) then the blue and red wires terminate inside the fuse box on the left side of the engine compartment.

Most vehicles shipped with an AD-230 105 amp alternator. The large frame AD-244 140 amp alternator was a factory option, and is bolt-on compatible in most cases (a 1" longer serpentine belt may be required). Both the AD-230 and AD-244 were made with a 4-pin control connection through mid 2005 and a 2-pin control connection from 2005 up so be sure you get the right one.

Dual alternator kits are available, and one or two airforum members have installed them with good results.

A few vehicles shipped with the optional power-exending towing mirrors, which are also available as an aftermarket upgrade.

Oversize aluminum transmission oil pans and rear axle covers are available from Mag Hytec. The PO of my suburban put them on. They increase fluid capacity and are said to improve cooling.

[slowmover]

Nicely done!!

[wingfoot321]

I had my trans fluid changed at 59,000 miles. Specified syn fluid but, did not verify brand or actual fluid used.

Question: I don't seem to have ever recorded as good mileage since the change. Does anyone thing fluid could make a difference? I am very careful with my driving and just know that something has changed. It is off at least .5 mile per gallon.

[r carl]

Yes better fluids and greases will help with everything.

[drunkensloth]

I had a couple things to add through my experience. Make sure you do not use dex 6 in a manual transmission that specifies dex 3. Not compatable. Also, I wouldn't synthetic in the front diff unless it's all wheel drive. The GM fluid has friction modifier in it that can wear the bearings prematurely

[ROBERT CROSS]

IMO.... The power extendable "tow" mirror not worth the $$$, too small and no convex. Manual tow a much better option for towing AS wide body.

Bob

[Jammer]

Originally Posted by drunkensloth

I had a couple things to add through my experience. Make sure you do not use dex 6 in a manual transmission that specifies dex 3. Not compatable. Also, I wouldn't synthetic in the front diff unless it's all wheel drive. The GM fluid has friction modifier in it that can wear the bearings prematurely

The lubricant specified for the front diff is 75W-90 synthetic gear oil without a friction modifier. This oil is now widely available from aftermarket suppliers and does not have to be obtained from GM. If the rear axle contains a positraction diff then a friction modifier must be added to the 75W-90. I have 75W-90 synthetic in the front and rear axles of both my 2004 Suburban and my 1997 K2500 and have been running it for several years with good results. Both have posi rears and therefore have the additive in the rear axle.

[Jammer]

Originally Posted by ROBERT CROSS

IMO.... The power extendable "tow" mirror not worth the $$$, too small and no convex. Manual tow a much better option for towing AS wide body.

Bob

You might be right. It is true that your mirrors are wider. However it is possible to add a small convex circle to the inside of the mirror portion of the power mirrors. I had this arrangement on the right side at one point and it is now missing since I had to replace the glass after an unfortunate incident involving a concrete pillar in a parking garage intended for subcompacts. I plan to replace it.

[ROBERT CROSS]

Originally Posted by Jammer

You might be right. It is true that your mirrors are wider. However it is possible to add a small convex circle to the inside of the mirror portion of the power mirrors. I had this arrangement on the right side at one point and it is now missing since I had to replace the glass after an unfortunate incident involving a concrete pillar in a parking garage intended for subcompacts. I plan to replace it.

The one PITA with the manual retracting tow mirror..... is the onset of manual retracting tow mirror brain fade when entering the garage. No broken glass yet...just the ohcrap scrapping sound.

Bob

[r carl]

Silverado/Sierra 2500HD & 3500HD - GM-Trucks.com