Transmission Control Module (TCM) Power Supply Voltage
TCM is not receiving the correct voltage for operation.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The Transmission Control Module (TCM) needs a stable power supply to operate. If it doesn't get the right voltage, it can't communicate properly with the rest of the vehicle, causing shifting issues or complete failure to start.
What the computer is actually seeing
The TCM monitors its own power supply voltage. If this voltage drops below a specific threshold (usually around 8V), it sets SPN 1790.
What a healthy reading looks like
TCM should see a voltage between 9V and 16V while the vehicle is running.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check the vehicle battery voltage. It should be between 12.6V and 14.7V. If low, charge or replace the battery.
- 22. Inspect the wiring and connectors from the battery to the TCM. Look for any signs of damage or corrosion.
- 33. Verify the TCM connector. Make sure it’s tight and free of dirt or moisture.
- 44. Test the voltage at the TCM power supply terminal with the key on. It should be between 9V and 16V. If not, trace back to find the break in power supply.
- 55. If all connections and voltage tests are good, suspect the TCM itself may be faulty.
Common causes
- Bad battery connections
- Faulty wiring or connectors to the TCM
- Defective TCM
- Low battery voltage
- Fusible links blown or damaged
Typical repair cost
$$$
Related codes
Frequently asked questions
Can I drive the vehicle with this code?
No, it's not safe. The transmission may not operate correctly, and you could lose shifting ability.
Will this code come back after I clear it?
If the underlying cause hasn't been fixed, it likely will return.
Why does this code sometimes pair with SPN 1791?
SPN 1791 indicates a similar issue, often related to the vehicle's power supply circuits being affected in tandem.
Working a real vehicle right now?
Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
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