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Jim Grant Tech Tips Jim Grant Tech Tips
A collection of helpful articles based on reader submissions
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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
If you have any questions, please feel free to email us.



Lincoln - Mercury
  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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