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- Feb 23, 2023
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If you’re doing an engine swap into your W-body and running into coolant temp sender issues, here’s something that might help. On my 1994 Grand Prix, I swapped in a newer 3100/3400 hybrid, and one of the first problems I ran into was the missing port in the cylinder head for the factory 1-wire coolant temperature sender (the one used for the dash gauge).
The newer engine only has the provision for the 2-wire ECT sensor used by the PCM. So, rather than drilling and tapping the head or adding T-fittings in the cooling system, I went digging for a cleaner solution.
GM used a 3-wire coolant temp sensor in some later vehicles that combines both functions — one signal for the PCM and one for the gauge — into a single sensor body. This is a perfect solution if your swapped engine doesn’t have a port for the original 1-wire sender.
Here’s the sensor I’m using – ACDelco 12551708
This lets you keep both systems functional from a single coolant port — ideal for engines that only have the one sensor bung near the thermostat or upper intake.
If you don’t want to change wiring, the other route is to add a coolant hose adapter or a T-fitting to mount the original 1-wire sender separately. But if space is tight or you’re trying to clean up the install, the 3-wire sensor keeps it simple and clean.
The newer engine only has the provision for the 2-wire ECT sensor used by the PCM. So, rather than drilling and tapping the head or adding T-fittings in the cooling system, I went digging for a cleaner solution.
The Fix: 3-Wire Combo Coolant Temp Sensor
GM used a 3-wire coolant temp sensor in some later vehicles that combines both functions — one signal for the PCM and one for the gauge — into a single sensor body. This is a perfect solution if your swapped engine doesn’t have a port for the original 1-wire sender.
Here’s the sensor I’m using – ACDelco 12551708
How It Works:
- Pin A: Signal to the PCM (same as the 2-wire ECT).
- Pin B: Signal to the dash gauge (like the 1-wire sender).
- Pin C: Common ground.
This lets you keep both systems functional from a single coolant port — ideal for engines that only have the one sensor bung near the thermostat or upper intake.
Notes:
- Do NOT try to splice the dash gauge wire into your existing 2-wire ECT sensor — the signals are not compatible and you’ll end up with inaccurate readings or PCM issues.
- This 3-wire sensor does require a little wiring work — you’ll need to run the gauge wire to the appropriate pin on the sensor and ensure a good ground.
- Thread size and sensor body match common GM coolant ports, so installation is straightforward if you’ve already got an ECT sensor in place.
Alternate Option:
If you don’t want to change wiring, the other route is to add a coolant hose adapter or a T-fitting to mount the original 1-wire sender separately. But if space is tight or you’re trying to clean up the install, the 3-wire sensor keeps it simple and clean.
Anyone else run into this on your 3400 or 3500 swaps? I’ll post pics once mine’s wired in. Let’s make this thread a solid reference for other W-body builders dealing with modern engine swaps and missing gauge ports.
