|Trouble Code||Fault Location||Probable Cause|
|P0885||Transmission control module (TCM) power relay control circuit open||Wiring, poor connection, TCM power relay, TCM|
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What Does Code P0885 Mean?
A stored code P0885 simply means that the PCM has detected an open condition in the power relay control circuit for the transmission control module (TCM). Open could be translated as disconnected or cut for diagnostic purposes.
Usually the TCM is integrated into a single housing with the engine control module (ECM). This integrated assembly is known as the powertrain control module (PCM). Even though they are integrated, the portion of the module which controls (automatic) transmission function may be powered up separately from the ECM. The TCM is typically powered using switched (only present with the ignition switch in the ON, start, or run position) voltage.
The TCM power relay control circuit is generally constructed with a fuse and/or a fusible link. A relay is used to initiate a smooth transference of voltage to the respective component without danger of a power surge. The PCM and TCM are most often powered using the same relay, despite using separate circuits. Gain access to a reliable vehicle information source for details of the TCM power relay control circuit as it applies to the vehicle in question.
The PCM performs a self-test (of all controllers) each time the ignition switch is turned on. If there is not an acceptable TCM power relay control voltage signal (battery voltage), a code P0885 will be stored and a malfunction indicator lamp may be illuminated, The TCM will also enter limp-in mode, on some models, if a code of this type is stored.
What are the common causes of code P0885 ?
- Defective TCM power relay
- Blown or corroded fuse
- Burnt fusible link
- Shorted or open TCM power relay circuit
- Bad TCM/PCM or programming error
What are the symptoms of code P0885 ?
- Delayed transmission engagement
- Harsh or erratic shifting
- Engine may stall at idle
- Shift interlock function may be disabled
- Intermittent or total speedometer/odometer failure
- Other transmission control codes are likely
How do you troubleshoot code P0885 ?
As always, inspection of all system wiring and connectors, along with checking all system fuses and fusible links, is a good starting point in your diagnosis. Utilize the DVOM (on the voltage setting) to accomplish the former task. Since some fuses will appear normal until a load is placed on their respective circuit (at which point they will fail) you will want to test them with circuits loaded. You may consult your vehicle information source for fuse and fusible link locations. If all fuses and fusible links are in order, and there is not battery voltage at the TCM power relay connector, suspect a broken (or otherwise open) circuit between the respective fuse/fusible link and the TCM power relay.
After making sure that the TCM power relay has voltage at the appropriate pin/s, here’s a quick diagnostic tip that you won’t find in any service manual: If all fuses are in working order, you can check TCM relay operation by swapping identical relays. Usually the horn relay, starter relay, fuel pump relay, and air conditioning clutch relay are similar enough in design to swap (temporarily) for diagnostic purposes. While this is a quick and easy method to discover relay failure; make sure to insert relays back in their original position after you have arrived at a diagnosis. Naturally, you will want to replace failed fuses and relays as required.
Now you will want to connect the scanner to the vehicle diagnostic port and retrieve all stored codes and freeze frame data. Having this information written down has proven helpful to me in the past; particularly as the diagnosis unfolds. Next clear the codes and test-drive the vehicle to see if the P0885 is reset.
Assuming that the fuses and relays are good, you will need to locate the PCM/TCM and the power relay control signal circuit using your vehicle information source. Place the ignition switch in the ON position, and probe the appropriate circuit with the positive test lead of the DVOM while grounding the other test lead.
- If there is battery voltage present, suspect a defective PCM or a programming error
- If there is no voltage present, suspect an open circuit between the TCM power relay and the PCM.
Codes Related to P0885
- P0700 Transmission Control System Malfunction
- P0701 Transmission Control System Range/Performance
- P0702 Transmission Control System Electrical
- P0800 Transmission Control Module (TCM) – Power Input Signal Malfunction