|Trouble Code||Fault Location||Probable Cause|
|P0891||Transmission control module (TCM) power relay sense circuit high||Wiring, short to positive, TCM power relay, TCM|
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What Does Code P0891 Mean?
OBD II fault code P0891 is a generic code that is defined as “Transmission control module (TCM) power relay sense circuit high”, and is set when the PCM (Powertrain Control Module) or in some cases, the TCM Transmission Control Module) detects an abnormally high voltage from the relay that supplies power to the shift solenoids.
All automatic transmissions use a relay to supply power to the shift solenoids, However, before the relay is activated, the PCM/TCM checks that there is no voltage at the shift solenoids. In this condition, the transmission is held in a fail-safe or “limp mode”, and gear shifts cannot be initiated. When this condition is verified, the PCM/TCM activates the relay, and power can be supplied to the shift solenoids.
When all required conditions are met, the PCM/TCM supplies power to the relay and the output from the relay is then fed back to the control unit via a dedicated sensing circuit. In a fully functional system this feedback voltage should be equal to (or very close to) battery voltage, but should the control unit detect a feedback voltage that is higher than expected, or is higher than battery voltage by a predetermined value for a predetermined period of time it will set code P0891, and may illuminate a warning light.
The image below shows the typical appearance and location of a transmission control module (TCM) power relay, such as might be found on most modern applications.
What are the common causes of code P0891 ?
Common causes of code P0890 could include the following-
- Damaged, shorted, disconnected, or corroded wiring that cause open circuits
- Low battery voltage
- Defective shift solenoid control relay
- Failed or failing PCM/TCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced.
What are the symptoms of code P0891 ?
Some common symptoms of P0891 could include the following-
- Stored trouble code, and illuminated warning light
- Additional codes relating to system voltages may be present
- Transmission may be held in a failsafe or “limp mode” until the problem is corrected
How do you troubleshoot code P0891 ?
NOTE: Take note that on some Korean applications, or any application on which the brakes have to be applied before the engine can be started, short or open circuits in the brake light circuit can also cause this code, since the brakes have to be applied before power can be supplied to the shift solenoid control relay.
Record all fault codes present, as well as all available freeze frame data. This information could be of use should an intermittent fault be diagnosed later on.
If codes relating to system voltages are present, check that the battery is fully charged, and that it is in good condition. Inspect the battery terminals for corrosion, tightness, and make repairs or clean and retighten the terminals. Be sure to check all battery grounds, and check that full battery voltage flows between the positive terminal and all system grounds. Make repairs as required.
If the battery voltage checks out and there are no other codes present, refer to the manual to locate the shift solenoid control solenoid as well as all relevant wiring. Also determine the function and color coding of all associated wiring to prevent a misdiagnosis.
Bear in mind that while the most common cause of code P0891 is open circuits, the actual testing methods to find open circuits vary between manufacturers. Thus, refer to the manual to identify the feedback signal wire, and check its continuity between the relay and the PCM/TCM connector. If there is continuity, check the resistance of the wire; compare the obtained reading with the value stated in the manual, and make repairs or replace wiring if a deviation from the stated value is found.
If the feedback circuit checks out, perform resistance and continuity checks on the relay’s input circuit. Locate any fuses or fusible links in this circuit, but do NOT replace any blown fuse or fusible link(s) until the cause of the short circuit that caused them to blow has been found and repaired.
Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
NOTE: Bear in mind that fuses or fusible links do not always need a reason to fail. Long use can cause fuses to fail at any time, but do NOT assume that this is always the case. Never replace fuses without making absolutely sure that there are no unresolved short circuits in the circuit.
If all wiring checks out, remove the shift solenoid from the vehicle and perform resistance tests between all pins as per the instructions in the manual. Note that “close enough” is not good enough: the resistance values on these relays vary greatly between applications, as do the maximum allowable deviations from stated values.
Compare all obtained readings with those stated in the manual, and replace the relay with an OEM replacement if ANY value does not agree with a stated value very closely.
While steps 1 through 5 will resolve code P0891 in nine out of every ten instances an intermittent fault may be present, which can sometimes be extremely challenging and time consuming to find and repair. Thus, if an intermittent fault is suspected, repeat all steps/tests/checks until the fault is found and repaired, or refer the vehicle to the dealer or other competent repair shop for professional assistance.