I have an 88 Bronco with a 302. Since I have owned the truck the defrost and vent system has never worked right. When I turn on my A/C on it will start coming out of the vent no problem, but once my engine idle goes down the a/c will switch and start coming out of defrost. So I had to jam the vent door open to stop that. I have replaced the control head and have no vacuum leaks. I was told that I could put some sort of vacuum canister on the main line with a certain type of valve that keeps my vacuum up so it will stop that. Has anyone heard of this before? If so, what is the proper way to do it?

 

Carefully check the small plastic line that leads from the vacuum source to the top of the AC box in the engine compartment. This line goes into a hole in front of the firewall on the passenger side. The line cracks and it is hard to spot a small leak. You can not hear it. A small leak will be overcome while the engine is runnig by the amount of vacuum available. However, when at idle you may have a problem. these line crack and break all the time. I have fixed 15 or 20 of them in my shop. If this does not turn out to be the problem, please hook up a vacuum guage to the motor while it is at idle. You should have a lot of vacuum at idle, otherwise investigate problems like too low an idle or other problems related to engine vacuum. An 88 shold have a couple of vacuum canisters on it alerady. There is one on the side of the evaperator cover beside the engine. It is rectangular. There is a ball shaped one on the driver side fenderwell, and at least one, if not two, metal cans, one on left inner fender and one on right by the battery. I do not know which of them is connected to the vacuum circuit for the AC, but suspect it is the one on the evap cover.

www. autozone.com has vacuum diagrams online for free. Just go there, enter your vehicle, and follow the links through repair.

Good luck.

 

Hi! I have a vacuum leak and it is the line that goes in the hole on the firewall on the passenger side of my 98 expedition. I brought a replacement line for it but how do I get behind the firewall to install that end of the line. I looked on the auto zone website but the diagram for my year and model is not yet available. Thanks in advance for any help you can give me.