1987 F-150 5.0 4 speed 4x4. When I engage my AC the idle never increases. I have a Haynes manual for this truck but its worthless when it comes to AC systems. Any clues where to look to resolve this?

 

There is a wire coming back to the computer to signal that the compressor clutch is engaged. The computer should then kick the IAC up a notch to increase the idle speed. You need a EVTM (elec vac trouble shoot manual) to show you the wires involved. There is also a diode in the circuit to bleed any back-emf to ground. When the clutch electromagnet is turned off, the collapsing magnetic field generates electricity, just like a normal ignition coil. They use the diode to kill that curent so it doesn't go back and fry the ECM. If the diode has gone bad, it could let the juice kill the sensor circuit in the ECM, so it would not know the clutch was turned on...

Buy the book if you can get it, it is the best value I've seen.
tom

 

The most likely cause is a dead IAC (Idle Air Control) valve. They often bind internally and the PCM can't compenstae for the extra load.
The Haynes manuals are best used for paper stock in the outhouse. Watch Ebay for the appropriate paper manuals for your truck. There will be 3: Body Chassis and Electrical, Electrical Vacuum Troubleshooting Manual, and Driveability and Emissions (Volume H).

 

Thank you for the help!!! I have had IAC trouble but I cleaned it and got it working...... most of the time, sometimes it sticks. I will watch for those manuals and I didn't know about the diode till now.

 

I've never seen one of those diodes fail, they're pretty tough. Cleaning an IAC only works about 50% of the time and when it does work, it doesn't last. Carbon gets down in the pintle bore where it can't be cleaned and binds the shaft. Replace the IAC and you should be fine.

I took a look at the auction site for 87 truck manuals. (As I was writing this, an 87 BCE and Engine set ended for ~$30 and no bids).
Here's an 88 Body Chassis &Electrical #130128925013, an Engine manual #290133418527, and a Volume H reproduction #300124917861. No EVTM right now.
The 88 Manuals are close enough for anything you should run into. That's 3 of the 4 in the set. I forgot that the BCE and Engine manuals were separate back then. They can get pricy because these trucks are popular, but those manuals were hundreds of dollars originally. Take a look so you know what to look for. Anything that covers more than one model year isn't a Ford manual.
Just search for "1987 Ford Manual" and "1987 Ford EVTM"





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Thank you, I'll be sure to look at those. I am use to Cummins, Detroit and Caterpillar manuals so Haynes is all but worthless.

 

I looked and did find a 1988 EVTM, looks like I will have to stay off the Snap-On truck for a couple of weeks and get those manuals. My local Napa says the IAC on the 87 f-150 is unique to a 87 truck but I think I will just order one from Ford just so I know what I am getting. Cleaning it helped but the fast idle still doesn't work most of the time and it runs terrible when the engine is cold.

 

Just to update: I tried to get a IAC from my local Ford parts dept, he said the part # was discontinued with no other p/n available. I went to NAPA and bought one and everything seems to be good now. Thanks for all the help!!!