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Identifying and Converting

4th Gen Fbody Rear End Guide

The purpose of this guide is to clear up some of the differences, particularly on the brake configurations in 93-02 4th Generation Fbody rear ends.

93-97 J41 Drum Rear

Here is a typical 93-97 drum rear, GM RPO code J41. This was the standard rear on all V6 cars. 96-97 V6 cars with the Y87 performance option RPO got an upgrade to rear discs. J41 was not used after 97, starting in 98 all cars got standard 4-wheel disc brakes.

There is a single ABS sensor at the top of the differential and a corresponding sensor ring on the carrier inside. Both brake lines meet just to the left of center, where they join to a T fitting. A single rubber hose makes the connection from the rear brake line on the car to this fitting.

93-97 J65 Disc Rear

The standard rear for all V8 cars and optional Y87 V6 cars from 93 to 97 is the J65 disc rear. From 98 up through 02 the J65 became the standard for both V6 and V8 cars, although the calipers and rotors were upgraded from previous years. The rest of the braking and electrical components however are the same on J65 rears from 93-02.

As can be seen above, the J65 utilizes a single rear brake hose and T fitting just as the 93-97 drum rear. There is also a single ABS sensor mounted in the top of the differential and a sensor ring attached to the carrier inside. The sensor uses a 2-wire harness that connects to the car just to left of center of the driveshaft tunnel. Thus the J41 and J65 rears are very similar, and can be easily swapped by simply disconnecting the hose and harness from one and connecting the same from the other.

93-97 J65 Disc Rear

Continuing with the J65, here we note the absence of any sensor or ring on the caliper mounting bracket and axle shaft. The axle shafts used in both J41 drum and J65 disc rears are interchangeable.

95-97 NW9 Traction Control Disc Rear

In 95 GM began offering ASR or Acceleration Slip Reduction (traction control), RPO code NW9 as an option on certain Fbodies. According to the GM Parts Manual, it was possible to have traction control with drum brakes from 95-97, although I have yet to see that configuration. The NW9 option continued up through 02 however drum brakes were no longer used after 97. The purpose of traction control is to apply brake pressure to a rear wheel when slip is detected on acceleration. Although it uses components of the ABS system to detect wheel slip, the traction control is not meant to control slip during braking, that is the job of the ABS, which was standard on all 4th Gen Fbody cars. ASR should also not be confused with SGS or Second Gear Start, a feature that could be enabled on some automatic transmission cars to deter wheel slip on take off. SGS is a passive slip deterrent, whereas ASR is an active slip detection and prevention system.

Pictured is a 96 rear from a car that had traction control. There is no ABS sensor in the top of the differential, there is not even a hole for it. There is clip a for the ABS wiring harness bolted over where the sensor would be on a non-NW9 car. The brake lines meet in the same location, however there is a bracket where the hoses thread on. Yes there are 2 of them, since the traction control system has to modulate the pressure to the left and right brakes individually in order to control wheel slip. To prevent getting the left and right lines crossed each side uses a different size and thread connector for the coupling at both ends of the rubber hose. Neither of these connector sizes/threads is compatible with the standard rear brake connectors.

95-97 NW9 Traction Control Disc Rear

Continuing with the NW9 disc rear, here we have pictured the individual wheel speed sensors used by both ABS and ASR systems. The sensor itself is mounted through the caliper bracket, and the sensor ring is attached just behind the axle flange. The wiring harness for this system consists of 4 wires, 2 going to each sensor.

93-02 ABS Harness Connector

All 4th Generation Fbodies share the same ABS wiring harness connector. This is the end of the harness that plugs into the socket on the car, just to the left of the driveshaft tunnel. The connector pictured is from a J41/J65 rear, showing that only 2 of the 4 pin positions are used. These 2 wires go to the sensor mounted on top of the differential. On an NW9 wiring harness, all 4 pin positions would be used on this connector, one pair going to the right wheel sensor and one to the left, with the left pair using the pin position pictured above.

93-97 ABS Sensor Ring

This picture shows the ABS reluctor or sensor ring mounted on the differential carrier flange, in this example a 93-97 Auburn limited slip type. The sensor ring is pressed on and held in place by the ring gear, and works with the sensor mounted through the top of the differential housing to determine rear axle speed.

The carrier type can be determined by several visual clues, in this case the spring locking mechanism inside, the tapered end of the carrier and the cylindrical pinion shaft.

98-02 ABS Sensor Ring

GM switched to a Zexel Torsen type limited slip differential in 98, which most consider superior to the Auburn units due to the design, which does not incorporate wearable cone clutches but instead uses element gears. Whereas the Auburn differential is designed to deliver equal torque to both drive wheels, the Torsen uses torque bias to deliver more torque to the wheel with the most traction. More information about this type of differential can be found at the manufacturer's website, www.torsen.com

The differences from the Auburn can be seen in the helical gears inside, the squared-off housing and rectangular shaped pinion shaft. (not shown)

Rear End Swaps

When planning to replace the rear end in your 4th Gen Fbody, whether to change gear ratios, switch from drum to disc brakes, or to replace a damaged rear, it is important to obtain the correct rear for your car. Most importantly, if your car has NW9 (traction control) you should replace it with an NW9 rear, and vice-versa. Although it is possible to convert an NW9 rear to use standard J41/J65 connections, it does involve a good bit more work, and the ABS system may be compromised.

By plugging an NW9 harness into a car that originally had a J41/J65 rear, the single rear channel of the car's ABS system receives signal only from the left wheel sensor. It is possible for a 3-channel ABS system to function with an NW9 rear sensor, however it does not reliably work 100% of the time. The other option would involve drilling roughly a 3/4" hole through the top of the differential housing to insert a center ABS sensor, and installing the corresponding sensor ring on the carrier inside.

The highest (numerical) gear ratio available from the factory is 3.42 which was standard on all 6-speed V8 cars. Therefore it's a commonly sought rear to use to replace the 3.08 or 3.23 rears that came in the V6 cars. An added bonus for V6 owners is that the V8 rears also came standard with a limited slip differential, an Auburn unit from 93-97 and a Zexel Torsen carrier from 98 up. 93-97 V6 owners can also take advantage of the swap from drum to disc brakes. As a complete used rear can usually be found and installed for much less then sourcing the individual components and paying for installation, a rear end swap is an extremely popular modification, especially for the early 4th Gen V6 owners.

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