I have a '92 B2200 that does not idle up with the A/C on. The A/C works well but the idle drops with the A/C on. I cannot find anything regarding the idle up function in my repair manual. Can anyone direct me as to what to look for and/or check to fix this. I have had the vehicle for about five years and the idle up has never worked during this time. I've gotten pretty good at the 'foot on the pedal' thing while deccelerating or sitting at a stop light but would really like to fix this problem. Any help as to how this works, what to look for and check, and possible pics would be greatly appreciated.

Thanks in advance!
mth

 

Still at work on the old gal eh mth?. Do not recall you ever mentioning if truck was an auto or manual trans?
Idle up for the A/C is performed by a vacuum operated diaphram on the carburetor near where the throttle cable would attach to the carb. Just look for a diaphram pot near that area that has a linkage piece going to the throttle lever with hopefully a vacuum line connected to a nipple sticking out of the end . If you apply manifold vacuum to this diaphram the idle speed should increase to around 1400rpm. Vacuum is applied to this diaphram by a solenoid over on the right fender area near but more rearward of where that PTC heater relay was located. Should have black two wire connector going to it with wire colors Brown and Black w/ Red stripe. Two vacuum lines at this solenoid as well. One should run to the intake manifold for a vacuum source and other line runs to the diaphram. When A/C is turned on the computer grounds the solenoid coil, energized solenoid then open an internal passage that allows vacuum to that diaphram on the carb and idle speed increases.
See if that idle speed will increase when applying vacuum to diaphram first and give a post back if run into problems or have a question. Not able to post a pic but could fax you a fairly decent idle up checking procedure if needed.