Copyright © 1998 General Motors Corp. 6–113
WTEC II ELECTRONIC CONTROLS TROUBLESHOOTING MANUAL
Main code 64 indicates the ECU has detected a voltage signal from the retarder request modulation sensor
(consisting of a module and a retarder control device) in either the high or low error zone. These codes can be
caused by faulty wiring, faulty connections to the resistance module or retarder control device, a faulty resistance
module, a faulty retarder control device, or a faulty ECU. Power wire 133 and ground wire 135 for the retarder
modulation request sensor share a common power and ground with the TPS and OLS devices. A short-to-ground
on the common power wire causes a “sensor failed low” code for the other devices (Codes 21 12, and 14 12). An
open or a short-to-ground on retarder modulation request sensor signal wire 164 results in a Code 64 12 only.
Retarder modulation request sensor diagnostic codes may not reflect retarder response problems. If this occurs, test
the retarder control devices for proper voltage signals at each of the percentage of retarder application settings.
Table 6–8 contains the voltage measurements for each device’s application percentage and resistances measured
across terminals A and C of the retarder request sensor. Use test wiring harness J 41339 when conducting
voltage tests. Table 6–9 shows connector and wire numbers to assist in making these checks. A TPS failure
changes the status of the output retarder. The retarder is enabled by the Service Brake Status (wire 137) when a
TPS code is active (21 XX). If a Code 63 40 is also active, the Service Brake Status (wire 137) is ignored and the
retarder will not work.
Active Indicator Clearing Procedure:
• Power down
NOTE: Before troubleshooting, read Pages 6–17 and 6–18 of Section 6–5. Also, check battery and ECU input
voltages.
NOTE: Intermittent connections or lack of battery-direct power and ground connections can cause this and
other electronic control codes.
Troubleshooting:
1. Plug in the DDR and set to read retarder counts and percent (0 percent will be between 15 and
60 counts and 100 percent will be between 150 and 233 counts). A retarder request sensor failed
high code can be caused by a short-to-battery of either signal wire 164 or power wire 133 or an
open on ground wire 135. An open in the portion of the ground circuit common to the TPS and
OLS devices will also result in a Code 21 23 and a high fluid level reading. A retarder request
sensor failed low code can be caused by an open or short-to-ground on either signal wire 164 or
power wire 133.
2. Isolate and repair any wiring problems found. See Appendix E for connector service information.
3. If no wiring or connector problems are found, check the retarder request sensor voltages for each
position on each of the retarder request sensors used on the vehicle. If two resistance modules are
used, disconnect one of them when measuring voltage signals from the other. If problems are
found, replace the resistance modules or retarder control devices.
4. If the problem persists, replace the ECU. If replacing the ECU corrects the problem, reinstall the
original (bad) ECU to confirm that the problem is in the ECU. If the original ECU now works,
inspect the ECU connectors for any corrosion or damage which may cause an intermittent
condition. If the original problem recurs, reinstall the replacement ECU.
Main
Code
Subcode Meaning
64 12 Retarder Modulation request sensor failed Low (14 counts and below)
64 23 Retarder Modulation request sensor failed High (232 counts and above)
CODE 64 XX — RETARDER MODULATION REQUEST DEVICE FAULT
(Figures 6–38, 6–39)
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