Keep Those Trannys Rolling - August - 2019

8HP70 Lack of Power and No Upshift

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In this issue of Keep Those Tranny’s Rolling, we’re looking at a 2017 Jeep Grand Cherokee that’s experiencing a lack of power and no upshift concern at times.

Our 2017 Jeep Grand Cherokee is All Wheel Drive and is equipped with a 5.7L engine and a ZF8HP70 transmission. We first encountered this vehicle when a customer arrived at our shop one morning, complaining that his Grand Cherokee was having a lack of power concern and would not upshift at times. While talking to the customer, he had indicated that he had previously been to several local automotive repair shops to have his vehicle diagnosed. Each of the shops had indicated that the issue we were facing was transmission related and recommended that he take his vehicle to a transmission shop for diagnosis and repair.

VEHICLE ARRIVES AT SHOP

The next morning, the customer arrived at the shop for his appointment. During the initial interview (repair order write-up), the customer indicated that he was not experiencing any problems while driving the vehicle under normal driving conditions. The customer said that he was only experiencing the lack of power and no upshift concerns during hard acceleration.

DUPLICATE TO DIAGNOSE

Before we headed out for our initial test drive, we visually inspected the vehicle for any wiring, connections or obvious fuel system concerns. None were found. We connected our scan tool to the vehicle Data Link Connector (DLC) and scanned the vehicle for codes. The only code found in the system was a P0174 (fuel system lean) stored in memory. We cleared the code and headed out for our initial test drive with our scanner set to monitor the transmission PIDs. During the first part of the test drive, we operated the vehicle under normal driving conditions and the vehicle seemed to drive perfectly. During the second part of the test drive, we operated the vehicle a little bit more aggressively. During hard acceleration, we found that the vehicle seemed to have a lack of power and the transmission would not upshift until you lifted your foot off the accelerator pedal. During the lack of power and no upshift condition, we noticed the transmission was not being commanded to upshift, indicating that we were having a computer control issue. After duplicating the lack of power and no upshift condition, it was time to head back to the shop and start our diagnostic process to determine what was going on.

BACK AT THE SHOP

After our test drive had indicated that we might be having a computer control system problem, it was time to take a look at the vehicle’s computer control system. With the scanner, we rechecked the computer system to see if any codes had reset during the test drive. No codes had reset. We monitored the inputs going to the PCM. We monitored the mass air flow sensor (MAF), throttle position sensor (TPS), manifold absolute pressure sensor (MAP), vehicle speed sensor (VSS), transmission range switch (TRS) and oxygen sensors (O2). We ran the engine at low RPM and high RPM while monitoring the inputs. All the inputs seemed to be working properly. We checked the PCM power and grounds. With all the inputs working properly and the PCM power and grounds intact, it looked like we were heading for an internal PCM problem.

We checked for any technical service bulletins related to our concerns and found none. We then checked the exhaust system for excessive back pressure during hard acceleration. Exhaust back pressure was within specifications. We performed a fuel system pressure test and found that the fuel pressure was within specification as well, indicating no signs of a clogged or restricted fuel filter. With no problems being found, we decided to head out for another test drive with the scanner, fuel pressure gauge and exhaust back pressure gauge connected to the vehicle, so we could monitor everything while the vehicle was acting up.

DRIVE AND MONITOR

During the first part of the test drive, we monitored our PCM inputs, fuel pressure and exhaust back pressure. Exhaust back pressure was less than 3 psi and fuel pressure was steady at 60 psi. Everything seemed to be working okay. As we started to drive a little bit more aggressively, we noticed that the fuel pressure would drop 20-30 psi during hard acceleration, and the vehicle would experience a lack of power concern. When this happened, you would have to press the accelerator pedal to the floor to maintain acceleration. With the accelerator on the floor, the computer system would respond to the high load input (over 95%) and not command the upshift. Once you released the accelerator pedal, the PCM would see the load percentage drop and allow the transmission to upshift. With the drop in fuel pressure during hard acceleration, it was pretty clear we were looking at a fuel delivery problem causing our lack of power and no upshift problem. It was time to take a look at the fuel delivery system.

FUEL DELIVERY SYSTEM

The fuel delivery system consists of:

  • A saddle type design fuel tank
  • A main fuel pump module
  • An auxiliary side fuel delivery module
  • 2 fuel filters (internal to fuel modules)
  • Lines and connections
  • A fuel rail
  • A fuel rail pressure regulator
  • 8 Injectors

HOW DOES THE FUEL DELIVERY SYSTEM WORK?

The purpose of the fuel delivery system is to deliver regulated fuel pressure to the injectors so the engine can run. The fuel delivery system uses an auxiliary side fuel pump module (located on right side of saddle tank) to deliver fuel to the main fuel pump module (located in the left side of the saddle tank). Both fuel pump modules contain fuel filters and fuel level sending units. The main fuel pump module contains the pump pressure regulator, which allows the main fuel pump module to maintain a constant pump pressure of 58 psi +- 2 psi. The main fuel pump module delivers the pressurized fuel to the fuel rail (located on the intake manifold) by way of the fuel lines. The fuel rail holds the fuel injectors in place and distributes fuel to each injector. The fuel rail pressure is maintained by fuel rail pressure regulator.

CHECKING FUEL DELIVERY SYSTEM OPERATION

To check the fuel delivery system operation, it’s necessary to connect a pressure gauge to the output side of the main fuel pump module. To accomplish this, you need to use fuel line adapters and fittings (figure 1). Once you have your pressure gauge connected to the main fuel pump module output or to the fuel rail, all you have to do is turn the ignition key on and watch the pressure on the gauge (figure 2). The main fuel pump module output pressure should be 58 psi +- 2 psi. If the fuel pressure drops below 58 psi at any time during vehicle operation, the main fuel pump module will have to be replaced as an assembly. If main fuel pump module pressure stays constant at 58 psi, it would be recommended to replace the fuel rail pressure regulator to correct this concern.

REPAIRING FUEL DELIVERY SYSTEM

A drop in main fuel pump module output pressure during hard acceleration was an indication of a faulty main fuel pump module. Replacing the main fuel pump module repaired the lack of power and no upshift concern (figure 3). The vehicle was test driven several times with no further problems and was delivered back to the customer.

Well, we’ve done it again. We’ve taken a brief look at the fuel delivery system for the 2017 Jeep Grand Cherokee and repaired a lack of power and no upshift concern. With a better understanding of how fuel delivery systems can interfere with transmission operation, you should have no problem Keeping Those Trannys Rolling.