Isuzu NPR/GMC Tiltmaster - Transmission Digest

Isuzu NPR/GMC Tiltmaster

The Isuzu NPR Commercial Truck and GMC Forward Tiltmasters use an exhaust-brake system that assists in engine braking during downhill coasting while the trucks are carrying heavy loads. This system is used instead of the familiar Jacobs Vehicle System known as the “Jake Brake.” When the exhaust brake is activated, it restricts the flow of exhaust with a flapper valve in the exhaust pipe similar to that of a heat-riser assembly (see Figure 1). When the flapper valve closes, exhaust pressure increases, causing air intake to decrease. This increased backpressure and slower air intake reduce torque transfer to the geartrain, producing the desired on-demand engine braking needed during downhill coasting with a payload.

Isuzu NPR/GMC Tiltmaster

Technically Speaking

Author: Wayne Colonna, Technical Editor

Exhaust-Brake Operation

Technically Speaking

  • Author: Wayne Colonna, Technical Editor

Exhaust-Brake Operation

The Isuzu NPR Commercial Truck and GMC Forward Tiltmasters use an exhaust-brake system that assists in engine braking during downhill coasting while the trucks are carrying heavy loads. This system is used instead of the familiar Jacobs Vehicle System known as the “Jake Brake.” When the exhaust brake is activated, it restricts the flow of exhaust with a flapper valve in the exhaust pipe similar to that of a heat-riser assembly (see Figure 1). When the flapper valve closes, exhaust pressure increases, causing air intake to decrease. This increased backpressure and slower air intake reduce torque transfer to the geartrain, producing the desired on-demand engine braking needed during downhill coasting with a payload.

The overall system’s operation is similar in trucks using either a manual or automatic transmission. The difference is that the clutch switch is used in trucks with a manual transmission, and with automatics a signal is sent to the PCM (powertrain control module), as seen in the system wiring diagram in Figure 2. This signal informs the PCM that the driver has requested exhaust-brake operation, and when certain driving conditions are met, the PCM elevates the transmission’s main line pressure. This elevation in line pressure decreases the possibility of the band and various clutches slipping – especially so for the overrun clutches, since they are being used to provide engine braking also.

As designed, this exhaust-brake operation aids in extending the life of not only the brakes but also the transmission. But when this system malfunctions, it can produce various complaints, some of which will affect transmission operation – premature transmission failure, poor engine performance or no engine braking, to name a few.

The exhaust-brake switch is incorporated into the smart arm (see Figure 3). Pulling the arm forward opens and closes the switch. When the vehicle is at operating temperature and the exhaust-brake switch is activated, the same symbol on the smart arm appears in the dash (see Figure 4). This indicates that the exhaust-brake relay, exhaust-brake switch, engine-warm-up switch, engine-warming-cut relay and thermo switch are functioning correctly.

To verify that the signal is being sent to the PCM, probe into the light-green wire in the 20-pin connector (see Figure 5). When the switch is off, you should see no voltage (see Figure 6). When the switch is on, you should see system voltage (see Figure 7). Without this signal, the PCM will not elevate line pressure when needed, causing premature clutch and band failure.

The same light-green wire goes to the exhaust-brake magnetic valve in the frame rail close to the exhaust-brake assembly in the exhaust pipe (see Figure 8). This magnetic valve opens and closes a vacuum supply to the exhaust-brake assembly that opens and closes the flapper valve in the pipe. You should see system voltage here in the same manner as at the PCM.

If you see system voltage on the light-green wire at both the PCM and the exhaust-brake magnetic valve when the exhaust-brake switch is turned on, and no voltage when it is turned off, the entire electrical system has been verified to be functioning properly. If engine braking is still not functioning, the possible causes may be:

  1. A loss of vacuum supply to the magnetic valve.
  2. A mechanically failed magnetic valve.
  3. The flapper valve in the pipe may be seized in the open position.

If the vehicle is experiencing poor engine performance, it is possible that the magnetic valve has failed mechanically and is stuck open or that the flapper valve in the pipe has seized in the closed position.

One other note regarding bad performance: Throttle cables on these units like to stretch beyond adjustment. Sometimes the pedal is halfway to three quarters depressed before the throttle body begins to open. This becomes a problem, as the TPS is on the throttle-pedal bracket. The computer sees medium to full throttle before the vehicle begins to move. This makes for complaints of late and harsh upshifts.

Many thanks to Rolly at Rainbow Transmissions in Hialeah, FL, for letting me into his shop to take these photos and for buying me lunch while I was there!

You May Also Like

Sherlock Holmes Approach to an AB60 No-Move Situation

The effectiveness in diagnosing automatic transmission malfunctions is an art form. Although there are similarities among the wide varieties of transmissions on the road, each transmission has its own peculiarities. Aside from having mechanical, hydraulic, and electrical hardware systems to contend with, software/programming issues and various vehicle platforms make diagnostics much more difficult.  Using scopes provides

ab60

The effectiveness in diagnosing automatic transmission malfunctions is an art form. Although there are similarities among the wide varieties of transmissions on the road, each transmission has its own peculiarities. Aside from having mechanical, hydraulic, and electrical hardware systems to contend with, software/programming issues and various vehicle platforms make diagnostics much more difficult. 

GM 6T40 Pump Identification Guide

The 6T40 was introduced in 2008 for General Motors front-wheel-drive cars in the Chevrolet Malibu and has gone through several changes throughout its three generations, specifically in the pump area. The 6T40 is closely related to the more lightweight 6T30 and the heavier duty 6T45 and 6T50. Generation one started phasing out during the 2012

Seeing the Forest AND the Trees

They say that the proverbial phrase “I couldn’t see the forest for the trees” means that a person or organization cannot see the big picture because it focuses too much on the details. Related Articles – 4L60E Harsh 1-2 Shift – TASC Force Tips: Diagnosing 8L45 & 8L90 Shift Complaints – TASC Force Tips: Hydraulics

The Manifold Pipeway

The Honda six-speed transmission has been on the bench of many specialty shops for one reason or another (figure 1). But, for those of you who have yet to lay your hands on one, mounted on the upper side of the unit is one of the largest, if not the largest solenoid and pressure switch

8L90 Vacuum Testing

Below are the diagrams for vacuum testing GM 8L90 transmissions. Note: OE valves are shown in rest position and should be tested in rest position unless otherwise indicated. Test locations are pointed to with an arrow. Springs are not shown for visual clarity. A low vacuum reading indicates wear. For specific vacuum test information, refer

Other Posts

Sometimes, a diagnostic code is all you need

With ATSG having the opportunity to help shops solve problems, sometimes we get faced with some real doozies. A shop will call and give us a laundry list of DTCs, leaving us to think someone must have a bulkhead connector unplugged. We then go through the arduous task of deciding which codes prompted other codes

10L80 and 10R80 pump gear differences

You may have seen an article in the August 2023 issue of Transmission Digest called “GM 10L80: A new kind of pump noise,” which goes over how the front cover housing in the 10L80 is fitted with a converter drive gear and idler gear. The idler gear drives the pump’s driven gear, and is press

Spotting different 68RFE designs through the years to avoid issues

The Chrysler 68RFE has had several changes through the years. Its four-speed predecessor began with a noisy solenoid pack identified by a black colored pass-through case connector (seen in Figure 1).  Related Articles – A guide to common GM, Ford and Nissan programming issues – Shift of the shaft: Diagnosing Chrysler 48RE manual shaft issues

Sometimes, you should sweat the small stuff

It’s a common phrase: There may have been a time when you worried about something, and someone who knew what you are going through said, “Hey, don’t sweat the small stuff.” Sometimes, this may be good advice. But other times, it may be wise to handle the small stuff before it becomes bigger “stuff.”  Related