Circuit Description
There are 2 throttle position sensors located within the throttle which use variable resistors to determine signal voltage based on throttle plate position. TPS1 will read low voltage when closed and FPP will read high voltage when closed. The TPS1 and FPP percentages are calculated from these voltages. Although the voltages are different, the calculated values for the throttle position percentages should be very close to the same. The TPS values are used by the ECM to determine if the throttle is opening as commanded.
Diagnostic Aids
It is possible that a faulty TPS may set this code without setting a TPS high or TPS low code. This is possible if the TPS fails between the high/low limits for the TPS diagnostic.
Known Fix
The gear in the throttle body strips and the throttle plate gets stuck. You are able to disassemble the throttle body and remove the gear and spin it to another position so the throttle motor is using a different side of the gear and it will operate correctly.
Factory Update
You must have access to a dealer laptop to complete the following update. If you are an independent technician, you can contact your local dealer for assistance.
Code 638 – Throttle Unable to Close – Reprogramming an ECM or Loading a MOT (Calibration) File
MODELS AFFECTED: (D187) S40-65XM, (F004) S70-120XM, (C024) S135-155XL2, (H177)H40-65XM, (L005) H70-120XM, (G006) H135-155XL2
Description: Hyster Company has a software calibration upgrade to all fuel systems utilizing a drive-by cable throttle body. Hyster Company suggests that when a service technician is connected, via the diagnostic display tool, that the technician install the latest released software for that specific fuel system. When responding to a Code 638 complaint, you may be required to reprogram the ECM and/ load in a new MOT (Calibration) file to resolve this issue.
NOTE: If your truck is equipped with a traction speed control option you will need to order the upgraded ECM from the Parts Distribution Center in Danville, IL. Installing this upgrade will disable the speed control option. In addition, Hyster Company requires that the software upgrades be installed whenever a throttle body assembly is replaced.
NOTE: Once you have begun to program the ECM, do not turn the key “off”. if you are using a lap top which is not connected to an external electrical supply, make sure your battery is sufficiently charged. Aborting during this process may render the ECM unserviceable.