PCM not letting heat/AC blower blow?

Perhaps someone on the board has a ZJ manual. It is my somewhat fuzzy understanding that the slower speeds are controlled through a trouble prone multi-resister block and the high speed goes straight through the switch.

The PCM has nothing to do with your blower motor. What type of heat/ A/C controls do you have ? There are two choices, ATM ( Automatic temp control ) or manual control. The wiring is totally different depending on which one you have.

Manual switches.

I have a 97, this is for a 98:

"The PCM manages air-conditioning compressor and coolant-fan operation, but it has to see the correct incoming signals before sending outgoing commands. When the climate-control head is switched to A/C-on mode, a signal is sent to the PCM. The PCM must also receive a signal from the A/C low- and high-prcoolant-fan operationessure switches before it turns on the compressor and/or coolant fan. These switches prevent damage to the system. I'm not sure what method you used to check for a sufficient charge of R-134a refrigerant, but a low charge will kill the signal from the low-pressure switch and therefore prevent the system from working. It could be a low refrigerant charge, defective PCM, faulty climate-control head, one of the pressure switches on the blink, a break in related power, or an issue with the ground circuit wiring."

I am hoping for something else besides the PCM between the relay and the Battery(-), like an relay or some other $20 part.

That would be fantastic.

It is, but the resister block seems to be working fine:

and I can control the speed with the switch.

The above info is in reference to an ATC system not a manual system. The wiring is completely different from one to the other. Also, it talks about the PCM controlling the A/C compressor and the coolant fan. That is all the PCM controls and it is the same with either system.

No. The coolant fan motor is the radiator fan.

The PCM does not have anything to do with the blower motor.

From what you stated above:

"Pretty sure this is how my 97 Grand Cherokee is wired: (I may make it to

That's fairly close. It is actually alot simpler. Bat + -- 40A fuse -- blower motor -- resistor pack -- switch -- Ground.

The blower motor switch uses the resistor pack to vary the resistance on the ground side of the blower motor to change speeds. For high speed the resistor is bypassed comepletely.

From your other post, so this is easier to follow. You stated :

and I can control the speed with the switch.

The wiring diagram shows no relay in the circuit at all. Are you shure that you have a relay ? Were is this relay located ?

ah - I was wondering why it was called something other than "blower motor."

Whoops. Good Q - how did that happen?

Bat+ -- 40A fuse -- blower motor -- resistor pack == switch --x- relay switch

-y-- PCM? -- Ground.

The relay coil side is driven by the ignition switch and the air flow control switch (off and 7 on's: max, norm, bi, vent, heat, heat/def, def)

If I ground X, the blower blows and I can adjust the speed with the speed switch: low to high (no off).

If I ground Y, the blower only blows when the relay is engaged: ignition and air flow control switch on.

If the far side of the relay was just going to ground, where would the wire terminate?

The relay is mounted inside the box that the 3 controls are in: fan speed, air flow and hot/cold. if you look at the back of the box, you can see the 4 connectors, you have to disassemble the box to pull the relay out.

I am hung up on it going to the PCM just because of the XJ wiring diagram I have. but it is significantly different (like the relay is between the 40A fuse and the motor) , so it won't surprise me if it just goes to ground.

Carl K

That would be more likely one of the defective blower switches. When the resistor pack goes, the low speed go, when high goes, it is the relay or the switch, when they all go it is usually the switch.

Folks have been reporting bad out of the box new switches too. You can buy just the switch, you don't need the whole cluster.

Mike

Carl

I have a wiring diagram and it is not showing a relay in the blower motor circuit. It also is not connected to the PCM. Are you sure what you are calling a relay is not part of the A/C switch ? I believe that is what you are grounding. From what you are saying it soumds like you have a bad blower motor switch. It appears that the ground for the blower switch is opened inside the switch. I can send you a copy of the wiring diagram if you have a valid email address.

well, it is a relay that is part of the AC/air control assembly

picture of the relay, in it's socket (or whatever you call the place it plugs into)

I did ground everything on the ground side of the blower :)

except that when I ground 'something' the blower switch functions correctly. I am not sure a faulty switch would 'work' under some other configuration. Maybe if the thing was so broken that 2 contacts had a bridge between them it would get the blower to go, but the low, m1, m2, high wouldn't all work as expected.

snipped-for-privacy@personnelware.com

Thanks.

This looks like a subset of what I have been working from

"The blower motor relay is installed in a wire har- ness connector that is secured to the passenger side outboard end of the HVAC housing in the passenger compartment, next to the HVAC wire harness con- nector."

No, it isn't. I even pulled the glove box apart just to be sure.

But thanks anyway.

Carl K

L.W.(Bill) Hughes III wrote:

The ZJ's automatic temp control system has it's own microprocessor as part of the control head assembly. The blower speeds are variable and controlled by this processor. A relay is used for high speed operation only.

In the manual system, the blower is connected to fuse 7(40A) in the power distribution center. The resistor pack comes after and is wired through the fan speed switch which provides the ground path in series with mode knob. The resistor pack is bypassed at the speed switch for high speed operation. A relay which is part of the control head assembly completes the ground connection between the speed switch and the mode knob when the ignition is on. The relay gets power from fuse 12(10A) in the junction block which also powers the blend door and panel illumination. There are 2 ground wires coming off the control assembly, a VT/WT for the fuse 12 circuit and a DG for the blower switch circuit.

Fuses 18 and 21 > Mike wrote: