99 GTP blower and vent problems

I've been tolerating several annoyances with my GTP for some time that I hope to fix. (The engine still runs great and I love to drive it.) The most annoying are the problems with the blower and vent control. (1) My blower no longer blows hard. (2) Sometimes no air at all comes from any vent - upper or lower - regardless of selected mode. I have automatic climate control.

Not getting any air from the vents is the biggest problem and the current situation is unacceptable. Sometimes, the air simply stops blowing from the vents although the blower motor is running; I can hear the blower motor change speeds as I change the fan speed control. I've recently realized that I can turn off the climate control for some period of time (up to half hour) and the vents will resume working when turned back on. I've checked the driver side blend door actuator; the door moves and the actuator motor works...at least it was when I pulled the actuator off.

I've run the CJ2 diagnostics that I found in a previous post. It reported trouble code 106 - RH electric actuator open or short. (Is that the passenger side?) The "no vent output" problem is intermittent, but recently seems to be happening more often...maybe a couple times a week.

I'm suspicious of the climate control head, but that's probably pretty expensive to replace. I'd hate to replace it when it's not the cause of the problem. And is it even possible for the control head to position actuators such that the blower runs but the air doesn't make it out any vent? Or could there be an obstruction in the air feed somewhere?

Regarding the weak air flow, the blower speed does vary with the control, but at the lower speed settings, there is hardly a breath of air coming from the vents. On the highest setting, the air speed is mediocre. I have replaced the blower motor twice with units from Autozone. I pulled the blower motor resistor today to inspect it but there was nothing obviously wrong. (How can it be tested?) A replacement resistor for the auto CC is $190 from Autozone. Again, I don't want to replace it without having some confidence that it is indeed faulty.

While I had the blower motor out, I measured the output voltage of the blower motor resistor (which actually has a busy circuit board in it) while varying the fan speed switch. Here's what I measured:
Fan off 0.02V
1 bar (lowest speed) 11.55V
2 bar 11.50V
3 bar 11.45V
4 bar 11.40V
5 bar 11.35V
6 bar 11.33V
7 bar (max speed) 11.33V

The difference between voltages for the min and max speed seems very small. And it surprised me that voltage decreased with increased fan speed selection. So I don't know what conclusion to draw from that test.

I've read that the "ignition switch" can cause climate control problems. I have had the "stuck key" problem for years and I've simply used the manual release under the steering column when it sticks. I will try to diagnose and repair that problem as well, but it's a lower priority than the vent problem. Any chance there's a common root cause?

I appreciate any guidance anyone can give to help isolate and repair these problems.

Cheers,

Hello.

You cannot read the motor voltage with the motor disconnected.
It's based on ohms law. the resistor is meant to drop the the voltage to the motor. Essentially, no motor means no current flow (except for the minuscule meter current ), therefore no measurable voltage drop. No voltage drop with be basically battery voltage. Try to measure the voltage with the motor connected. I know it helps to be a contortionist.

To test the resistor block, you will need to measure resistance and either have a wiring diagram or poke around. It will only be up to a few ohms (maybe 5) and will increase/decrease by say 0.5 ohm per change in speed depending how you measure it.

If your plenum dampers are functioning then check for plugged cabin filter etc.or watch the motor outside the airbox. You said you can here the speed change but no airflow, I think you need to verify the motor or dampers is your problem.

However, If you try to run the motor out of the plenum , hold the motor base as it will spin hard/twist under high speeds and do not run it too long outside as the resistor needs to be cooled and it gets its cooling from the squirrel cage air flow while they are inside the plenum.

cheers

You cannot read the motor voltage with the motor disconnected.


I should know that...:banghead: Thanks.


Try to measure the voltage with the motor connected. I know it helps to be a contortionist.


Good idea...thanks again.


To test the resistor block, you will need to measure resistance and either have a wiring diagram or poke around. It will only be up to a few ohms (maybe 5) and will increase/decrease by say 0.5 ohm per change in speed depending how you measure it.


Seems like measuring voltage across the motor while running should be done first (since it's easier). I don't know how I could check the resistance while it's running since the circuit current will interfere with the meter measurement.


If your plenum dampers are functioning then check for plugged cabin filter etc.or watch the motor outside the airbox. You said you can here the speed change but no airflow, I think you need to verify the motor or dampers is your problem.

However, If you try to run the motor out of the plenum , hold the motor base as it will spin hard/twist under high speeds and do not run it too long outside as the resistor needs to be cooled and it gets its cooling from the squirrel cage air flow while they are inside the plenum.


I'll check the motor before I bolt it back in, but I'm confident the blower motor is working.

I don't know how I could check the resistance while it's running since the circuit current will interfere with the meter measurement.

Yes, the res. cannot be checked with power on . That's why a wiring diagram would come in handy.
Your thinking is correct, verify the motor works and appears to be at the right speed to move the air and then you can move on from the motor and resistor.
cheers

Reconnected the motor electrically, but left in on floorboard. The motor speed varies according to the fan speed control and seemed to pushing a fair amount of air. (I didn't measure the voltage.) But when I remounted it, there was still very poor air flow: Low air flow when lower vents (heater) selected, very low when upper vents selected, and undetectable when defrost selected.

Earlier I took off the outer windshield cowl to verify nothing was clogging the intake. Nothing was in there and the rubber deflector shield seemed intact.

I did find that the vacuum source line under the glovebox had been pinched at some time, but it did not appear cut. I attempted to restore its shape. Vacuum is present: At idle and slightly above, the vacuum source will stick to my finger and support itself. But I don't know how strong the vacuum should be. I should've checked the strength of the vacuum at the engine for comparison, but didn't think about it at the time...I'll do that next time I'm under the hood.

Number one cause of no air to vents is vacuum loss- On W cars vacuum loss is very common due to a broken/cracked vacumm line to vacuum reservoir as it is routed near the car battery- look for a vacuum leak in black plastic vacuum line there-splice in a rubber hose if needed. I've fixed numerous W cars this way.

Oh and to test vacuum amount- use a vacuum gage

I haven't had a problem with no airflow through the vents since I attempted to correct the pinched vacuum hose. If the problems recurs, I will remove the crimped section and splice it with vacuum hose.

For now, I'm going to tolerate (what I perceive to be) the reduced airflow and attribute it to a less than optimal replacement blower motor.

Thanks to pfofit and bcridd for their feedback. The contributors to this site are awesome. The help in understanding and fixing our problems is very much appreciated.