Hi guys,
I have a 75 F250 with factory A/C. I have two problems with my A/C. First the A/C blows rather pathetically, second, after the A/C is operated for a length of time, the relay gets real hot and after a couple hours will start to melt its plastic parts and the connector. Now that relay has been replaced twice, so it seems to me that the blower motor is pulling too much juice and overheating the relay. The Haynes amnual says something about pulling the motor and hooking it up with an ammeter to see if its pulling more than 10 amps, and if so, replace it. So I figure I ought to pull it out, but my Haynes manual doesn't give the procedure and says that if I have A/C I should have it looked at professionally. Now I don't do much of anything professionally, so any help before I tear into this this would be greatly appreciated. I am not even sure if the motor is in the big plastic thing on the passenger side of the engine compartment or inside the cab.

 

Is the blower motor behind the glove box or in the engine compartment?
Can someone give me an idea of what's involved?
Thanks

 

I might be able to give you an idea. I can take you as far as the heater coil. Perhaps someone else here can tell you more about the motor or you can figure that part out yourself. My truck is also a 1975 model

Your right about the motor being inside the big plastic box looking thing mounted on the passanger side fire wall under the hood. But you can't get to it from there.

Actually its not that hard. Unless someone else knows a trick to getting at the motor from the outside, you have to access the motor from inside the cab. I haven't taken the motor out of mine but I have replaced the heater coil and the motor is just behind that.

First remove the golve box. Then under the dash, remove the plenum from the firewall. Disconnect the control cables and vacuum lines. Just take your time and remember how it all comes apart. You will now have access to the a/c evaporator. There is some kind of sensor that goes into the middle of the evap. coil. Be careful with it. Just pull it out and tie it off out of the way somewhere.

Now you don't have to completly remove the evaporator. Remove the sheetmetal screws holding the evap. What I did on mine is carefuly pull the evap. coil out and up from the bottom. You shoulden't have to disconnect the a/c hoses from the evap. Just pull it out far enough to get to the heater coil. You may have to tie it off up and out of the way using some tie wire. Now you have access to the heater coil. Remove the screws that hold the hold down straps. Note the orientation and which one goes where. Since you have alreay disconnected the heater hoses from the coil under the hood, remove the heater coil. There should be just enough room to pull it pass the evap. A couple of rags would be handy.

From there your gonna have to figure it out. Hopefuly someone else will respond to your thread with more information. Just to replace a heater coil will take a couple of hours. Good luck.

EDIT: Check this out. www.ford-trucks.com/articles/heater.php

 

Big Dave,
I have a 77f150 with factory a/c and I have the same problem with the relay getting realy hot and then melting it. I have burnt up about 5 relays. I replaced the blower motor next but am still having the same problem. Was wondering if you figured out what your problem was. I have been dealing with this for the past 2 years and could realy use some help before I go crazy.

Thanks

 

I really haven't done much to it, but last year the relay melted. It seemed to be caused by how it was constructed. The contacts are riveted together with plastic sandwiched in between. When the plastic got soft is colapsed and the rivet was no longer tight causing high resistance and making it get even hotter. and give out completely. I took it apart and with a little patience, put it together with a new rivet and so that there is metal to metal contact without the plastic part in between. I seems to work much better now, but it still gets warm. When I first got the truck, I had to drive it from California to Utah with the A/C on. after several hours i started smelling something burning and finally figured out that it was the relay. Since I have fixed it, I have yet to drive it more than couple dozen miles with either the A/C or heater on, so I don't know if I fixed the problem or just patched it up enough that it hasn't reoccured. But if you replaced the motor and still have the same problem, I guess I wont bother with mine. Hope this helps a little

 

It's possible that the connector for the relay has loose connections which will heat up. This will transfer the heat through the connections into the relay causing it to heat up and melt. I would check the connection on the relay socket for tightness.

 

I have noticed the same thing with the relays I have bought. Small rivits with plastic in between. I have gone back and fixed the burnt ones with solder but after a while they will get hot again, melt the solder, and melt down to the next rivit. Then it connects the circut and when you turn the truck off, everything that is grounded to the dash stays on. (blower motor, radio, 4x4 lock light,) Then you go to start the dang thing after work and the battery is dead!

I have checked the connections to see if they are loose or not but are tight. Have also checked the main power wire to the relay from the battery and cant find anything wrong. Now I have bought a new blower motor speed control switch, but its been too cold outside to stick it in. Will let you know if it helps. Any other advice would be great. Thanks