Question about 1998 Ford Mustang

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98 Mustang v6 - no AC

98 Mustang v6 - top radiator hose gets very hard and engine heats to normal operating temp. Cooling fan doesn't run and AC no longer engages. Stuck thermostat? Bad temp sensor?

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  • alauer2 Sep 09, 2009

    Thanks drallim. I left the rad cap off and found the coolant in the rad got hot almost instantly and the coolant level never dropped as expected when the thermo opens. This made me think the thermo failed and stuck open but figured this would lead to overheating (which it never did - guage reads dead center). Put the rad cap on and the system pressurized. Hoses are supple and in good shape but both appear to plump a bit after the engine heats up. Top and bottom hose are almost instantly hot (again making me think the thermo is stuck open). Yes, blower motor runs for heat and AC.

    I haven't checked the fuses and relays yet but will do so. I thought I was initially dealing with an AC problem because the AC would blow cold when driving and heat up while standing at a stoplight, then cool down again while driving. Over a few days, this evolved into no cool air so I started looking closer and found the elec fan not working. I heard the AC will protect itself and not turn on when the condenser is unable to cool.

    Sanity check... I think the thermo might be stuck open and maybe we blew the fan fuse or activating sensor. Odd that the car maintains temp when driving and standing... what do you think? Seems like a mechanical (rather than electrical) failure to me. Might also need a new water pump but see no weep discharge and hear no noise.

  • Anonymous Mar 22, 2014

    why are there 2 fan sensor switches

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  • Master
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Make sure the radiator pressure cap is the correct one and that the hose is not soft--check the lower hose and heater hoses for softness as well. They should be firm, but not hard or cracking. The temperature sensor is reporting normal operating temperature, so it and the thermostat are working correctly (hot water would blow out of the radiator if it were stuck). Check the cooling fan for free turning and no noise. Check it's grounding connection. Check the fuse box for blown fuses to the fan and to the A/C compressor. Make sure all the relays and fuses are firmly pressed into their sockets. Check the A/C compressor clutch winding for resistance--should not read infinity. Does the blower motor for the heater/A/C run when either is turned on? The A/C switch on the dash may be defective.

Posted on Sep 08, 2009

Testimonial: "Excellent suggestions for diagnostic checks! Thanks!"

  • drallim Sep 09, 2009

    You need to work on the radiator fan problem first. If the thermostat is stuck open, water flows all the time (which you would be able to see) and should warm slowly with the engine running. If the water is suddenly hot, then the thermostat appears to be working properly. There is a small weep hole in the thermostat that allows pressure under it to bleed through over time, so I wouldn't expect great level changes in the water when it opens. With the fan inoperative, I would expect the upper and lower hoses to be hot, but why isn't it boiling over with continued running (but not moving--do you have a lot of wind blowing all the time?) Even if it had an over-sized radiator, it should eventually boil over. I think you can rule out the water pump as a problem, it seems to working fine. Your description of the hoses bothers me, they should get firm with pressure, but not expand in size. Check a new hose--squeeze it and compare the feeling (cold) of yours. Hoses are made with an embedded fabric material that prevents expansion when pressurized. While this condition has nothing to do with the present problems, you could have a blown hose sometime down the road. Back to the radiator fan--the heat sensor resistance is read by the computer, which instructs the fan relay to operate, and it supplies power to the fan. Look for the associated fuse and relay. The fuse will show if it has blown and the relay can be swapped out temporarily with another similar one. The fan can be tested by pulling it's local connector and supplying ground and +12 volts to the fan. If you get the wires swapped, it won't hurt for the test as long as the fan runs. Does the temperature gauge rise from zero to midpoint as the engine warms up? If the fan runs (test) and fuse and relay seem ok, I'd try a new sensor. I couldn't find any information on what cold and hot (turn on fan) resistances should be.

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