GM IFS Front Drive Axles - Transmission Digest

GM IFS Front Drive Axles

In 1983 General Motors redesigned the front drive axles on its four-wheel-drive S and T trucks. The new design was an independent front suspension (IFS) for better handling and ease of manufacturing and repair. In 1988 GM added IFS front drive axles to its full-size-truck lines (K trucks).

GM IFS Front Drive Axles

Up to Standards

Subject: Redesign of front drive axles
Vehicle Applications: GM S, T & K trucks with four-wheel drive and independent front suspension
Essential Reading: Rebuilder, Diagnostician
Author: Mike Weinberg, Rockland Standard Gear Contributing Editor

Up to Standards

  • Subject: Redesign of front drive axles
  • Vehicle Applications: GM S, T & K trucks with four-wheel drive and independent front suspension
  • Essential Reading: Rebuilder, Diagnostician
  • Author: Mike Weinberg, Rockland Standard Gear Contributing Editor

In 1983 General Motors redesigned the front drive axles on its four-wheel-drive S and T trucks. The new design was an independent front suspension (IFS) for better handling and ease of manufacturing and repair. In 1988 GM added IFS front drive axles to its full-size-truck lines (K trucks).

We will call these units “front drive axles,” as terms like the “front rear” seem to confuse issues. When you are ordering parts it is easier to ask for a ring-and-pinion set for a front drive axle than for a front rear, which can sometimes result in your getting a gear set for a rear differential.

The front drive axle uses a ring and pinion that are cut to reverse rotation because of the forward placement. The IFS units use a ring-and-pinion/differential assembly housed in an aluminum case that is mounted to the truck frame and uses halfshafts with CV joints to get power to the front wheels. These units use an electrical disconnect for the front axle to ensure that the front wheels freewheel in 2WD for better fuel economy and less wear on the moving parts.

The IFS front drive axles are manufactured in four sizes that correspond to the widest part of the ring-gear diameter. 1983-2001 S and T trucks use a 7.25-inch ring gear. 2002-and-up S and T trucks and Trailblazer, Envoy and Bravada use the same-size ring gear but have a “pan mount” front drive axle, an arrangement whereby one of the axle shafts passes through the engine oil pan. The Hummer H3 uses a 7.6-inch ring gear in its front drive axle. The 1/2- and 3/4-ton K trucks have an 8.25-inch ring gear from 1988 to 2008, and the 1-ton HD-option vehicles use a 9.25-inch ring gear from 1988 to 2008 in vehicles equipped with 8.2-liter gasoline engines or diesel engines.

There is a problem with the front-drive-axle actuator, which is electronically controlled to engage 4WD when the transfer case is shifted into one of the 4WD ranges. From 1988 to 1990, these actuators had a silver metal body and two wires. These were replaced in 1991 and up with a black plastic-cased assembly having three wires. All the boxes for the parts I have seen had no wiring instructions or schematics on the three-wire system.

If you have to replace an early two-wire actuator with a late three-wire unit, here is the way to make it work. The three-wire assembly has two brown wires, one long and one short, and a black or blue wire. The short brown wire goes on the original positive side of the transfer-case switch. The long brown wire goes to the ignition on the 25-amp AC/heater fuse. The black or blue wire goes to ground. On 1998-and-up models, if the 4WD light continues to blink in 4WD, it’s usually an indication that the front axle did not connect.

A word of caution when you’re handling these components. The reason the earlier actuator was replaced was that the time it took to get into 4WD was too long. The black-bodied plastic replacement unit is a gas-actuated unit that engages very quickly. If you wish to put 12 volts to one of these units to test it, make sure it is installed on the axle and bolted in place. If you are holding one in your hand and actuate it with 12 volts, it will become a deadly weapon.

Figure 1 and Figure 2 above provide electrical schematics for both early and late actuation systems.

All the original-equipment GM ring-and-pinion and differential parts are manufactured by American Axle and Manufacturing (AAM). AAM uses the latest state-of-the-art gear-cutting processes, which I wrote about in an article in the May 2007 issue. The tolerances the company can hold are much improved over earlier methods of manufacture, and that has a big influence on your backlash setup when you’re replacing one of these ring-and-pinion sets.

These units all use screw-type adjusters to set ring-and-pinion backlash to specifications. If you’re using OE parts from AAM, they should be set to 0.004-0.006 inch of backlash. You can unscrew the vent-plug cover from the case to observe and measure the backlash. This is a much tighter spec than is usually found in other manufacturers’ product, so be careful if you are substituting aftermarket gears to follow the other manufacturer’s instructions on setup.

A second note here is to save the original pinion shim and use that to start your setup. Modern manufacture by AAM is so good that you will very rarely have to change from the original pinion shim, and by reusing it you will save quite a bit of time.

Getting back to the screw-type adjusters, they are typical spanner-type adjusters that are secured in position by small locking tabs. In the exploded view of the assembly in Figure 3, the locking tab is # 40, and two are required. Do not reuse the old locking tabs during rebuild. They typically will be bent, and you risk fatigue cracks if you try to straighten them. If they should crack after rebuild, the adjuster will move off its setting and will quickly cause the ring and pinion to fail. The lock tabs cost $5 and are a good safeguard for your reputation and your wallet.

With the current state of the economy and fuel prices you will have many customers who would like to change ratios in their trucks to achieve better mileage. The IFS front drive axle is no more difficult to set up than any other unit. Make sure that the ratios on the front drive axle and the rear axle match and add to your bottom line.

You May Also Like

MP3023 T-Case: Simple Mechanics, Complex Electronics

The MP3023 is an active automatic transfer case that is found in a wide variety of vehicles. This unit will be found in GM trucks 2007-13, Jeep Grand Cherokees 2011-19, and in Dodge Durangos 2010-up. We will be discussing the Jeep version here, which has very sophisticated control electronics. The transfer cases are basically all the same across the product line, but there are considerable variations in the electronics, which will make diagnostics outside of the transfer case a learning experience.

Up To Standards

Author: Mike Weinberg

Subject Matter: MP3023 T-case

Issue: Complex electronics

The MP3023 is an active automatic transfer case that is found in a wide variety of vehicles. This unit will be found in GM trucks 2007-13, Jeep Grand Cherokees 2011-19, and in Dodge Durangos 2010-up. We will be discussing the Jeep version here, which has very sophisticated control electronics. The transfer cases are basically all the same across the product line, but there are considerable variations in the electronics, which will make diagnostics outside of the transfer case a learning experience.

Simple Routines Can Leat To Solutions

For whatever reason, the tech lines get an inordinate number of calls regarding a few specific is-sues. That such a high volume of calls is generated by just a few problems leads to the belief that we need to revisit and speak about the lack of understanding by the tech-nician that leads to all this wasted time and phone traffic, as well as failure to get the job right the first time. Let’s start out the year by get-ting to the nitty-gritty of why cer-tain issues seem to confuse so many people.

Lubricants: Understanding the Mysteries

Lubricating oils or lubricants have been around since the invention of the wheel, and every class or type of machinery uses and needs them. But, how much do we really understand about these products and about the amazing amount of engineering that is found in a can?

Tires Vastly Improved, but Check the Specs

The advancement of technology in the automotive field is rapid and unrelenting. Forces that shape the marketplace, state and federal regulations, the need to attract new customers, and the need to be different and at the same time profitable are driving the car makers to develop technology at a pace never seen before.

Are We Speaking the Same Language?

If you are repairing transmissions for a living, you will invariably spend some time on the phone ordering parts and speaking with technical hotlines to assist in your diagnosis of problems. Having been on both ends of a tech line for over half a century and an equal amount of time buying parts, I have learned a whole new language. To be successful communicating with those entities, one must understand the language and be speaking about the same correct topic with whoever is on the other end of the conversation.

Other Posts

Electronics In Dual Clutch Units

This article is the final segment of our exploration of the dual-clutch transmission, or direct shift gear box (VW and Audi) or DualTronic in BorgWarner’s brand. We have included several schematics from a VW Touran model, as VW has the largest amount of these transmissions in use. VW using its VAS 5051 diagnostic system provides for, as they say, “guided fault finding,” which means that a test schedule is available for the unit and provides testing for sensors, actuators and the mechatronic (computerized) valve body.

Hydraulic & Fluid Controls in Dual-Clutch Units

To recap, we have looked at how the dual-clutch transmission functions, essentially two gear boxes in one with the input shafts driven by hydraulically applied clutch packs that drive three concentric shafts that are one inside the other. The innermost shaft drives a gerotor-type of fluid pump that provides pressurized fluid to actuate the dual clutches, lubricate and cool the components, and shift the transmission into the selected gear. The next two shafts are driven by the two clutch packs with one shaft shifting the even-number gears and one shifting the odd-numbered gears.

Controls Make Shifts Happen in Milliseconds

If you have been following the previous chapters of this series of articles, you are starting to understand the function of dual-clutch transmissions. We have used illustrations from the VW Direct Shift Gear Box (DSG) as VW has about 2 million of these units on the road at present.

The Beauty of Having Two Separate Gear Sets

In last month’s article, we began to study the dual-clutch transmission, looking at its potential to eventually replace the common torque converter-planetary automatic transmission. In this chapter, we will look at the mechanical theory of operations on how these gear boxes work. We will be using illustrations from VW and Audi who were the first to mass market this design in 2003, in the Audi TT and VW Golf models.