SHOP TALK - our Diesel Tech Blog

What is an accelerator pedal position sensor (APPS) and is it not simply a throttle position sensor (TPS)? Good questions. The ’94-’98 models equipped with a 12-valve engine and an automatic transmission utilized a TPS. This sensor controlled shift timing and overdrive lockup. The 1998.5-to-present turbo diesels utilize an APPS for engine control and it provides input for automatic transmission (if so equipped). The 2005 to present Turbo Diesels continue to utilize the APPS. However, the sensor has been moved inside the truck and is part of the throttle pedal assembly.

Let’s go back to the easy-to-understand ’94-’98 12-valve engine. The TPS is actually a rheostat or potentiometer, with three electrical terminals [reference voltage, ground (called a return in most Mopar service info) and signal]. The signal has a varying voltage relative to throttle position, less at idle and more at wide open throttle (WOT). Example: at idle 0.6 volts

What has caught the attention of the Turbo Diesel Register and Geno’s Garage staff from this past week’s automotive industry news?

A VOLUNTARY DO NOT DRIVE? I DON’T HAVE AN ANSWER

We all see much discussion on the TDR web forums and in the TDR publications regarding performance/drivability issues, which are often traced to a low pressure fuel delivery problem. Although the narrative that follows is specific to the 12-valve Turbo Diesel trucks, model year ’94 to early ‘98 (pre 24-valve engine), the discussion on the components from the fuel tank (float and sending unit) to the fuel transfer pump apply to all Second Generation trucks.

The term low pressure fuel delivery system covers the components from the fuel tank up to the truck’s fuel injection pump and from the injection pump back to the fuel tank. Common problems that will be discussed:

• Fuel level sending unit
• Fuel heating element
• Fuel transfer pump
• Overflow valve
• Fuel return line

Let’s start at the

Are the summer months in your truck spent with the old-school “2-60” air conditioning (two windows down at 60mph)? A properly operating HVAC system provides comfort and safety. The pages of many prior issues of the Turbo Diesel Register magazines have been devoted to the removal of the instrument panel to replace a leaky air conditioning evaporator core (‘98-‘02), leaking heater core, flimsy blend air doors (‘03-current), etc. And you'll find some of the A/C parts for the DIY-er at Geno's Garage.

Instead of covering “how to change a part,” let’s discuss the theory of the system’s operation and some real world technical tips not found in your factory service manual.