Jim Grant's Tech Tips
'88 Jeep Cherokee, A/C Vacuum Doors Close with Acceleration
Q:
My son and I have a project vehicle an ‘88 Jeep Cherokee with a 4.0
engine. The problem is with the air conditioning doors changing position
to vent upon acceleration. The engine and accessories feel good and are
running great cruising down the road, except during acceleration. When
the engine drops the vacuum level, the A/C unit controls lose vacuum supply
and the doors shift to their neutral positions. When we reach cruising
speed with traffic, the vacuum comes back in and pulls the doors to the
intended position to get that good cold air back in your face. We've visually
checked the main vacuum lines from the intake manifold port through the
reservoir under the front bumper up to the firewall. Thought I would ask
if this is a recognized problem before I bend over and go through the
dash, emissions lines or 4X4 systems.
A:
If the vacuum lines and vacuum reservoir are not damaged, you
have a sticking or missing check valve in the vacuum supply to the vacuum
reservoir. The vacuum reservoir is just what its name implies, a storage
device for vacuum. The engine creates vacuum. The vacuum is an energy
that is used as a power source to operate different emission controls
and accessories like the controls for the ventilation system. There are
times that an engine does not create a good supply of vacuum. Because
of this the vehicle manufacturers install reservoirs to store vacuum.
The hose from the engine or the reservoir itself will have a check valve
that blocks the loss of vacuum back to the engine. Why is this? When accelerating,
the amount of vacuum created by the engine drops to a level that will
no longer power any emission controls or provide for control of the ventilation
system. The reservoir is designed to hold enough vacuum in storage to
maintain control under most driving conditions. If the check valve fails
the vacuum reservoir will lose vacuum so quickly that there will be none
left in reserve. The result? A loss of control of vacuum operated devices.
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