Car shakes when front defrost is on

Hi and thanks for using 2CarPros. Com.

The AC compressor places a heavy load on the engine. As a result, the engine rpm's must increase when the defroster (which runs the compressor to help dehumidify the air) or the AC is turned on. If the engine does not compensate for the added load, the RPM's will get very low and in most cases, cause the engine to shake.

Do a little test. Start the engine and let it idle with nothing running. Look at the RPM's. Now, turn on the AC or defroster. Do the rpm's drop? If so, how low?

Do this and let me know what you find.

Take care and I will watch for your reply.

Joe

Welcome back.

Okay, if the idle remained constant, that eliminates the idle stepper motor. At this point, I suspect that you most likely you have a worn motor mount. The mounts are enclosed in rubber. If the rubber wears, tears, or just cracks, often times you will feel vibrations from the engine. The engine mount is located on the left side of the engine (from front of vehicle). Open the hood and inspect it for cracks, dry rot, if it looks compressed... and see if anything looks out of the ordinary.

Here are the directions for replacement if you find it is bad. The attached picture shows the mount location.
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Engine Mount Replacement
Removal Procedure

Caution: Refer to Battery Disconnect Caution in Service Precautions.

1. Disconnect the negative battery cable.
2. Raise and properly support the engine.

3. Remove the right side engine mount-to-engine bracket bolts and nuts (1).
4. Remove the 3 right side engine mount-to-frame bolts.
5. Raise the engine slightly in order to provide clearance for the engine mount assembly removal.
6. Remove the right side engine mount from the vehicle.
7. Remove the right side engine mount-to-frame bracket from the engine mount.
8. Remove the right side engine mount-to-engine bracket from the engine mount.

Installation Procedure
1. Install the right side engine mount-to-engine bracket to the engine mount.
2. Install the right side engine mount-to-frame bracket to the engine mount.
3. Install the right side engine mount to the engine.

4. Install the 3 right side engine mount-to-frame bolts. Do not fully tighten the bolts.

Notice: Refer to Fastener Notice in Service Precautions.

5. Install the three right side engine mount-to-engine bracket bolt and nuts.
Tighten the right side engine mount-to-frame bolts (1) to 54 Nm (40 ft. lbs.).
Tighten the right side engine mount-to-engine bracket bolt and nuts (3) to 54 Nm (40 ft. lbs.).
6. Remove the engine support fixture.
7. Connect the negative battery cable.
Tighten the battery cable bolt to 15 Nm (11 ft. lbs.).

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Here are a couple links that discuss and engine mount, how it works, and in general how one is replaced.

https://www.2carpros.com/articles/how-a-motor-mount-works

https://www.2carpros.com/articles/replace-motor-mounts

Let me know what you find or if you have other questions.

Take care,
Joe