The majority of calls we receive on our technical help line are with vehicles 15 years old and newer. With the complexity of newer vehicles, these calls have become increasingly more difficult to do. Suffice it to say that when a call comes in for a vehicle that is from the ’90s or the early 2000s, it usually becomes a break against the brain bumrners we normally receive. That doesn’t mean the older vehicles are immune to being difficult. There are plenty of times when those seemingly “easier” calls come in and end up being a “lunch eater.” These problems may arise due to maintenance not being in the owner’s vocabulary. Or, when previous work was performed with incorrect parts, or a non-sophisticated operational system is not completely understood. Such was the case with a 2001 Isuzu NPR 4.8 Diesel using the Aisin 450-43LE Transmission. After many failed attempts at fixing a TPS code 21 from a variety of shops, it eventually found its way to Lorenzo Ortiz at Phillips Transmissions in Arizona.
Lorenzo questioned the owner of the truck where he learned that both the TPS and the TCM had already been replaced by one of the shops in their attempt to resolve the problem. His initial diagnosis verified that TPS code 21 was stored (Figure 1), yet the TPS was functioning correctly at the TCM. Power and grounds at the TCM were checked and determined good. After clearing the code and test driving the vehicle, it is noticed that the PID for the engine RPM read nothing and the idle switch read “engine is NOT at idle” when in fact it was (Figure 2). By punching the throttle to see if a change would occur, an 800-plus RPM suddenly appeared near wide open throttle at which time code 21 for the TPS set.
Lorenzo referred to a wiring diagram in preparation for inspecting the engine RPM sensor and idle switch. It was then discovered that this vehicle does not have an “actual idle switch” and the tachometer sensor is the signal source for engine RPM. It is the ECM that determines an idle signal, which is presented in the scan tool and an idle switch.