VoluntaryXJ

I'm currently doing a heater core/ ac system overhaul on my 98 XJ, 4.0L. The HVAC box is all apart and I want to fix everything all at once.

My problem occured whenever I would change which vent I wanted the air to come out of. I would move the ****, there would be a noticable delay, then I would hear the blend door thump into the new position. If I understand the HVAC system correctly, and I'm not entirely sure I do, the blend door actuator controls which vent the air comes through so I'm guessing some part of that is wearing out.

I have found tons of info about complete blend door/ blend door actuator/ dash controls failures but nothing about a delayed response.

Is this normal? Can anyone confirm that the actuator is failing?

TeXJ

since you have it all out, try it by hand and see if you feel any resistance.

dave1123

The blend door controls the temperature of the air coming out of the heater/A/C. It is controlled by an electrical stepper motor, not vacuum. The MODE control sends air to the various outlets and is vacuum controlled. If you aren't familiar with a stepper motor, it works by turning it's shaft just a few degrees every time it receives an electrical pulse. Lets just say it had 10 poles. It will rotate 36* with each pulse and stop after the last. In other words, give it 2 pulses and it rotates 72*. You can almost feel the notches in the temp control **** as you rotate it. It has more than 10 poles, but that was just to illustrate my point.

The MODE control is vacuum actuated and relies on vacuum stored in the accumulator and supplied by the engine. If the accumulator isn't connected or has a leak, the mode change will be slow. If the mode control valve itself has a leak, the same will happen. CAUTION! If you take the mode valve apart, there is a tiny smaller-than-BB sized ball and spring inside it. If it flies apart, good luck finding that ball! If you do, lubricate the 2 faces with Vasoline to help seal it. You can stick the ball on one face to help reassemble it.

VoluntaryXJ

Dave1123, thanks for this extremely helpful explanation. When I bought the vehicle the accumulator was just hanging by the vacuum lines because the aftermarket bumper wouldn't allow it to be re mounted.... and apparently the P.O. ran out of zip ties or something. I didnt see any obvious damage to it but i will look over the connections again.

I have a hand held vacuum tester that I suppose I could use to test a few things. I don't suppose you know what the operational psi is?

dave1123

It constantly changes with the engine, being as high as 22 inches at idle to practically zero at wide open throttle. There is supposed to be a check valve in the vacuum line just after it comes out of the manifold to prevent it from suddenly dropping and to hold a reserve to close the heater air doors when you shut the engine off. You can pull a decent vacuum for testing with a standard vacuum cleaner, although with the intake blocked it will heat up quickly and may burn out unless you're using a shop vac. You can pull about 28 inches with a shop vac.

Frank J

I had a 97 and I’m wondering if the stepper motor your referring to is near the passenger door under the dash, and looks like a small canister that pulls an arm attached to - for lack of a better explanation - a door, that opens and closes based on movement of the arm in and out of the motor?