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Nie, dziękuję Przekieruj mnie tam1999 Mercury Grand Marquis as relay. Compressor clutch will not engage.
Applying 12 volts directly to pin 87 engages clutch. There is power at pin 30 and pin 85 but the power is not getting to pin 87 with relay in place.
• The relay itself is not being energised, so battery voltage on pin 30 never transfers to pin 87.
• In almost every 1998-2002 Panther-platform (Grand Marquis/Crown Vic/Town Car) A/C circuit, pin 85 of the clutch relay is the 12 V “feed” and pin 86 is a PCM-controlled ground that is routed through the low- and high-pressure switches.
• If pin 85 shows 12 V but pin 86 never goes to ground, the PCM is withholding the ground (usually because a pressure switch is open or a related input is out of range).
• Less often, the relay coil or contacts are open; a 30-second relay swap confirms or eliminates this.
Key points
Circuit topology (1999 Grand Marquis)
• High-current path: Fuse #6 (15 A) ➜ pin 30 of A/C relay ➜ (closed contacts) ➜ pin 87 ➜ C110/S157 splice ➜ compressor clutch coil.
• Control/coil path: Ignition-run feed ➜ Fuse #11 (5 A) ➜ dash A/C request ➜ pin 85 ➜ relay coil ➜ pin 86 ➜ WOT cut-out diode ➜ low-pressure cycling switch ➜ high-pressure cut-out switch ➜ PCM (low-side driver) ➜ chassis ground.
When all sensors are “happy” and the driver asks for A/C, the PCM pulls pin 86 low, the coil energises, and 30 → 87 closes.
What your test results prove
• Pin 30 hot = fuse and power feed are OK.
• Pin 85 hot = dash control, fuse 11 and ignition circuit are OK.
• Clutch energises when 12 V is forced on pin 87 = clutch coil, diode and harness to compressor are OK.
• Therefore the only missing element is a complete coil circuit (85 → coil → 86 → ground) or physically bad relay contacts.
Systematic diagnostics
A. Relay health
– Swap with horn or blower-motor relay (identical 5-pin, ISO-280). If A/C now works, replace relay.
– Bench test: continuity 85-86 ≈ 70 Ω; apply 12 V and verify click and < 0.3 Ω between 30-87.
B. Coil ground (pin 86)
– Back-probe pin 86 with meter referenced to battery –.
– Command A/C ON (engine idling, blower on, not at WOT).
– Result table:
• 0–0.2 V → coil has ground → relay/contact fault or socket corrosion.
• Battery voltage → PCM not grounding → continue at step C.
C. Why PCM withholds ground
• Low-pressure switch open (< 23 psi) – most common; confirms low charge or switch failure.
• High-pressure switch open (> 430 psi) – blocked condenser fan or over-charge.
• WOT cut-off circuit stuck (TPS > 3.8 V).
• ECT over-temperature (> 240 °F).
• A/C request signal missing (EATC head, blend door faults).
• PCM DTCs P0530–P0533 set, indicating A/C pressure sensor faults (for digital-EATC cars).
To isolate, jumper cycling switch connector: if clutch now runs, charge is low or switch is bad.
Socket/contact issues
Even a healthy relay will not pull 30 → 87 if the coil power cannot flow through corroded terminals. Inspect and tension female terminals; millivolt-drop test across each while coil is energised.
Voltage-drop check under load
With relay forced ON (jumper 30-87), measure voltage at compressor connector relative to battery: > 0.5 V drop indicates harness/diode resistance; repair if necessary to avoid brown-out overheating of clutch coil.
• Ford service bulletin 01-15-6 notes premature cycling-switch failure on 1995-2001 Panther cars; replacement part F6DZ-19E561-AA supersedes the original.
• Many technicians now use scan-tool PIDs “A/C REQUEST”, “A/C PRESSURE”, “A/C CLUTCH CMD” to see instantly whether the PCM is grounding the relay—eliminating guesswork.
• R-134a prices rose sharply; proper leak/charge diagnosis is increasingly important to avoid repeated low-pressure lockouts.
Analogy: The relay coil path is like a doorbell circuit. Pin 85 is the transformer’s hot lead; pin 86 is the button to ground. Until someone (PCM) presses the button (grounds 86), the bell (relay) never rings, regardless of how much power is available on the hot side.
Releasing refrigerant during pressure-switch bypass tests is illegal under EPA 40 CFR 82.154. Always recover/charge using certified equipment.
Potential challenges
• Intermittent cycling-switch opens only under vibration—use jumper wire or scope to catch.
• PCM grounding driver transistors can fail; verify with noid light < 200 mA load before condemning PCM.
• Continuously jumpering 30 → 87 to “get some A/C” risks compressor damage if charge/pressure is wrong.
• Wire colours may vary on fleet/California-emissions looms—always cross-reference schematic by VIN.
• Ford EVTMC CD-ROM service manual, section 12-00 for exact circuit traces.
• SAE J1627 and J2788 standards for A/C recovery/charge procedures.
• Study of smart A/C clutchless compressors (scroll/variable displacement) as future retrofits eliminate relay-controlled clutches altogether.
Your measurements prove the clutch, power feed and relay coil supply are good; what is missing is the coil ground (pin 86) or a working relay. Start by swapping/bench-testing the relay. If it is good, look for why the PCM is not grounding pin 86—almost always an open low-pressure cycling switch from low refrigerant. Resolve that condition and the relay will energise, restoring normal compressor operation.
