Question: What could cause A/C not to flow thrugh the dash vents on a 2000 GMC Safari. Air only comes out the defrost vents regardless of switch position.
That's a familiar complaint for 1995 and later Astro and Safari vans. There's a pattern here, and it's common to find a broken vacuum line at the intake manifold as the cause of this complaint. The plastic supply line becomes brittle and cracks from engine heat. Without enough vacuum, these systems, like many others, will default to the defrost mode.
The first step when searching for a broken vacuum line is to gain access to the front of the engine by removing the air filter box and snorkel hose. There will be two electrical connectors (IAT and MAF) that need to be unplugged from the rubber snorkel tube. Once the snorkel tube and air filter box is removed you'll have a better chance of seeing the vacuum supply lines. Do not attempt to start the engine with IAT (intake air temp) and MAF (mass air flow) sensors unplugged as that will likely set a check engine light if the computer sees an open connection.
There are two common things to look for. One is the rubber vacuum hose to the vacuum reservoir. Our first photograph here shows the location of the vacuum reservoir. Itís a plastic ball hidden under the evaporator box on the passenger side of the engine compartment along the frame rail. You should find a single rubber hose connected to the ball. It acts as a reservoir when vacuum supply is low, like during uphill driving. Itís common for the vacuum line to come loose from this reservoir. You may find it just hanging. We see this most often after alternator replacement since these vacuum lines run very close to the alternator.
The most common place to find a broken vacuum line is directly behind the thermostat housing and EGR valve. You can see this broken line clearly in the photo below. The manufacturer utilized a hard plastic line which becomes brittle after years of under hood heat. If a break is found, and the line looks to be in good condition, the two sections can be joined with a small section of vacuum line. We find the plastic line itself is best joined together with 7/64" vacuum hose. That size provides a nice leak free fit.
It's our experience, if you find a break in the vacuum line as depicted in the photo above, you may have to replace a large section, as most of it could be very brittle and break easily when trying to make repairs. On the 1998 model that we photographed, we found it easier to remove the cowl, as the entire section needed to be replaced. The supply source is behind the compressor on this model, and may be difficult to reach without removing the engine cover. At the very least, if attempting to reach behind the compressor, make sure the engine has completely cooled down.
When replacing large sections, as we did, we felt it was best to use 5/32" instead of 7/64" hose as the larger diameter may last longer.