What are the fan relay trigger temperatures shown in this schematic?

Fan relay low side triggers at approximately 207°F (165 Ω) and fan relay high side at approximately 226°F (125 Ω).

What ECT sensor voltages correspond to the fan trigger temperatures?

The schematic notes .300v @ 207°F and .221v @ 226°F at the ECT sensor input.

What resistor values were changed between the BEFORE and AFTER circuits?

The BEFORE circuit uses 1277 Ω and 220 Ω resistors (with a 470 and BIAS 2490); the AFTER circuit changes these to 2980 and 3340 while keeping the 470 and BIAS 2490.

How did the comparator switching thresholds change after the modification?

BEFORE: OFF = .483v, ON = 1.05v. AFTER: OFF = .239v, ON = .326v.

What part number is referenced for the sensor?

The part number shown is Denso 179700-0220, with a 3-pin connector type sketched.

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Ford Probe Cooling Fan Relay Comparator Mod Schematic - Denso 179700-0220 ECT

Hand-drawn before-and-after op-amp fan relay circuit schematic with voltage notes

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Hand-drawn before/after circuit schematic of an op-amp fan-relay control with voltage and resistance annotations.

AI caption July 2022 · Photo 673 of 676

This is a hand-drawn electronic schematic comparing a cooling fan control comparator circuit "BEFORE" and "AFTER" a modification. Both sides show an op-amp comparator circuit powered by VCC 4.8V with a BIAS resistor of 2490 feeding a signal labeled "MS CPU I/O CLT CIRCUIT" (apparently a MegaSquirt-style coolant temperature input). In the BEFORE circuit, resistor values of 1277 Ω, 470, and 220 Ω produce comparator switching thresholds of OFF: .483v / ON = 1.05v. In the AFTER circuit, the resistors are changed to 2980 and 3340 (with the 470 retained), yielding thresholds of OFF: .239v / ON = .326v. Reference voltages from the ECT (engine coolant temperature) sensor are noted as .300v @ 207°F and .221v @ 226°F. The BEFORE side shows a TIP120 transistor driving the HIGH FAN RELAY, with fan relay trigger points listed: Fan Relay Low Side ~207°F (165 Ω) and Fan Relay High Side ~226°F (125 Ω). A connector type sketch is included, and the ECT sensor is identified by part number Denso 179700-0220. The document lets a reader understand how resistor changes shift the fan-on/fan-off voltage thresholds relative to coolant temperature.

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Frequently asked questions

What are the fan relay trigger temperatures shown in this schematic?: Fan relay low side triggers at approximately 207°F (165 Ω) and fan relay high side at approximately 226°F (125 Ω).
What ECT sensor voltages correspond to the fan trigger temperatures?: The schematic notes .300v @ 207°F and .221v @ 226°F at the ECT sensor input.
What resistor values were changed between the BEFORE and AFTER circuits?: The BEFORE circuit uses 1277 Ω and 220 Ω resistors (with a 470 and BIAS 2490); the AFTER circuit changes these to 2980 and 3340 while keeping the 470 and BIAS 2490.
How did the comparator switching thresholds change after the modification?: BEFORE: OFF = .483v, ON = 1.05v. AFTER: OFF = .239v, ON = .326v.
What part number is referenced for the sensor?: The part number shown is Denso 179700-0220, with a 3-pin connector type sketched.