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Relay 156, and a few things to try.
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 22 Jun 2001 09:22:19 Share Post by Email
In Reply to: I Can't Believe No One Knows Why, S.O., Fri, 22 Jun 2001 00:00:27
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Follow this logic:
AC won't turn on unless the fan switch is in position 1-4. The fan speed is controlled by a series of resistors connected to the switch. In position 1, the most resistance is in series with the fan, in 2 less resistance, and so on until setting 4, where there is no resistor. Less resistance means more current, and the fan blows faster.

Relay 156 is the AC relay that pulls in the compressor. The coil of that relay is taken through a variety of switches and connectors, but it gets the signal for the fan switch off Switch Position 1. That means that when the fan switch is at 1, voltage goes through a lot of resistance to the fan, but directly to the AC relay coil. When you switch power to position 2, there is less resistance between the power supply and the fan, but that missing resistance is now between position 2 and 1 - so there is some resistance in series with the AC relay.

As you turn the fan switch to 4, you have no resistance between the power supply and the fan, but you have the MOST resistance in series with the power and the AC relay coil. I figure that the resistance is too much for the relay to pull in. So - no AC in position 4.

Now, the tough part - there are a bunch of reasons why this could be. Perhaps the AC relay is just a little tired. If you can Borrow one and try it, that may tell you the problem. I'd borrow it, because they've got to be expensive. The problem could also be extra contact resistance in the switches on the power supply side of the fan - the fan position switch, and believe it or not, the air distribution switch. The air distribution switch turns the fan off if you set it to O. The fan takes a lot of power, especially at 4; a voltage drop in the line could mean the relay doesn't pull in. Or the blower is pulling a lot more than usual current, and that means the voltage available for the relay coil is reduced.

I would try the following tricks - remove the relay, and clean the lugs on the relay. A little Scotchbrite or fine sandpaper will do. It might be some contact resistance, and this couldn't hurt.
If you can borrow a known good AC relay, that would help. According to my manual, there were different relays depending on which engine control you have, so you may be able to filch a different relay from somewhere else in the system to test it. Cross check part numbers.
Here's a trick that may work if you're comfortable with electrical work. Pop out the fan switch. On pin 1 there should be two wires - one blue, one yellow. The blue wire goes to the AC relay (actually to the relay through the AC push button). Cut the blue wire, and splice it into the Blue wire on switch position #2. Basically, you're taking the power for the AC relay from the center of the resistance pack. The relay coil will always see some resistance, but it'll see less than it did before at position 4, and about the same at the other settings.

I hope this helps.
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