Sep 15, 2021 at 11:03 AM
I changed out the ECT and temperature sending unit and still receiving the same code 51 for ECT. Any other thing I should check on? 88 XLT 5.0
Bronco, Bronco II
code 51 ect
2 posts
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Oct 22, 2026 at 10:14 PM
When a new sensor does not clear code 51, the wiring or connector between the sensor and the computer is the next suspect. Code 51 means the computer sees the coolant temperature signal stuck at the cold, high voltage end of its range, which is exactly what an open circuit looks like.
The engine coolant temperature sensor (the one for the computer, not the gauge sender) has two wires: a signal wire the computer holds at a reference of around 5 volts, and a signal return. Key on, engine off, sensor unplugged:
1. Measure across the two harness terminals, signal to signal return (not signal to chassis ground). You should see about 5 volts. Zero volts means the signal wire is open or not getting reference.
2. Check the signal return wire for continuity back to the computer's signal return pin with the battery disconnected. An open return gives you the same code 51.
3. Jumper the two harness terminals together and run the self test again. If the code changes to 61 (the opposite, shorted low reading), the wiring is good and the sensor or its connector contacts were the problem.
Corrosion inside the connector shell is common on these, so clean the terminals and check that they grip the sensor pins.
The pinout for the computer and its sensors on the 1986 to 1995 speed density V8 trucks is on our 1987 to 1991 Bronco wiring page:
https://www.supermotors.net/reference/ford/bronco/1987-1991/wiring-diagrams
Dana
The engine coolant temperature sensor (the one for the computer, not the gauge sender) has two wires: a signal wire the computer holds at a reference of around 5 volts, and a signal return. Key on, engine off, sensor unplugged:
1. Measure across the two harness terminals, signal to signal return (not signal to chassis ground). You should see about 5 volts. Zero volts means the signal wire is open or not getting reference.
2. Check the signal return wire for continuity back to the computer's signal return pin with the battery disconnected. An open return gives you the same code 51.
3. Jumper the two harness terminals together and run the self test again. If the code changes to 61 (the opposite, shorted low reading), the wiring is good and the sensor or its connector contacts were the problem.
Corrosion inside the connector shell is common on these, so clean the terminals and check that they grip the sensor pins.
The pinout for the computer and its sensors on the 1986 to 1995 speed density V8 trucks is on our 1987 to 1991 Bronco wiring page:
https://www.supermotors.net/reference/ford/bronco/1987-1991/wiring-diagrams
Dana
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