1996 Eddie Bauer Explorer Blower Motor Only works On High

[Marcus Behel]

I have a 1996 Eddie Bauer Explore with an automatic temperature control system. The blower motor is either off or at full speed. The blower motor speed controller (not the standard resistor) seems to work as does the motor.

The fan speed dial (a potentiometer) on the EATC (electronic automatic temperature control) works, however when a certain level is reached (about three clicks from the top of the dial) the relay on the blower motor speed controller engages and the fan is powered at the full 12V. At the same position (of the dial) the blower motor speed controller's input wires have 10.8V (and the relay switches). Before the threshold level of the dial, the blower motor speed controller gets 0V and the fan gets 0V.

I suspect the issue is with the EATC, but I am not sure.

Does anyone know how this system works or what might be wrong?

 

[1998Exp]

I don't know how the power transistor in the controller is driven, but it's a good assumption that this is in PWM fashion (rapid on/off). Your 0V reading might be because this transistor is burned and shorting the control line to ground. Before you suspect EATC failure (which is much less common than controller failure), I would unplug the connector from the controller and repeat your measurements.

 

[J_C]

I am unfamiliar with the EATC setup on a '96. Is the fan speed pot supposed to be able to completely turn off the fan or only adjust it down to a very low speed? If it is supposed to be able to turn it off then I suspect the pot itself is worn (wiper springiness lost or oxidation on contacts) or has hardened grease on the conductive tracks.

The other common faults in this system are transistor failures. If either the transistor(s) in the EATC head unit or in the firewall controller were to fail open then the fan shouldn't run at all, UNLESS 100% mode goes through a relay instead - I didn't think that was the case but I would have expected the fan relay to turn on with the ignition/accessory key position, not dependent on the pot setting.

If either the head unit or the firewall module have a transistor failing to a short circuit condition then the fan should be on full speed all the time.

Again I am unfamiliar with the EATC setup on a '96 but it seems more of an analog transistor biasing setup, not PWM, but I could be wrong. Anyway you should be able to measure a base drive voltage that varies with the adjustment of the pot on the head unit. With PWM a meter reading might jump around but the values should still get higher.

If the head unit looks like it can be non-destructive opened, I'd first try spraying some contact cleaner into the pot and rotating the pot back and forth while still wet inside. There are more things that could be tested with it open such as a continual, varying resistance between the pot wiper and either end of the conductive track as you turn the pot shaft.

 

[1998Exp]

I am unfamiliar with the EATC setup on a '96. Is the fan speed pot supposed to be able to completely turn off the fan or only adjust it down to a very low speed? If it is supposed to be able to turn it off then I suspect the pot itself is worn (wiper springiness lost or oxidation on contacts) or has hardened grease on the conductive tracks.
The other common faults in this system are transistor failures. If either the transistor(s) in the EATC head unit or in the firewall controller were to fail open then the fan shouldn't run at all, UNLESS 100% mode goes through a relay instead - I didn't think that was the case but I would have expected the fan relay to turn on with the ignition/accessory key position, not dependent on the pot setting.
If either the head unit or the firewall module have a transistor failing to a short circuit condition then the fan should be on full speed all the time.
Again I am unfamiliar with the EATC setup on a '96 but it seems more of an analog transistor biasing setup, not PWM, but I could be wrong. Anyway you should be able to measure a base drive voltage that varies with the adjustment of the pot on the head unit. With PWM a meter reading might jump around but the values should still get higher.
.

If it's anything like my '98, the '96 blower controller has an internal relay, which is used to bypass the transistor-based circuit at full power. The controller is low-side, so the power transistor connects to ground. How it's driven is anybody's guess; If I were to design that circuit, it would be PWM, for sure, to reduce power dissipation in the transistor, and for simplicity. If I get ambitious enough one of these days and run out of yard chores, I will stick a scope probe on the control lines and find the definite answer...

 

[J_C]

I'm assuming it's analog instead of PWM because it's (the firewall module) a darlington transistor instead of a mosfet. There's no reason to use a darlington if it's PWM.

 

[bigred4x4]

My 96 has a resistor in the blower motor housing that's known for going bad on some vehicles(engine bay) that controls the lower fan levels max level is essentially achieved by supplying full power to motor via a relay located in the small black relay box under the air intake filter housing.

 

[bigred4x4]

Sorry after reading your post again I see we do not have the same system mine is just the basic knobs for ac.

 

[Marcus Behel]

I have connected the BMSC and measured all of the connections at each position of the potentiometer (with the BMSC connected). The BMSC seems to be working properly. I have attached a graph of the output of each pin below based on the following pin-out diagram form here. Based on this I suspect the blower motor itself might be the problem.

Any ideas?

 

[Marcus Behel]

Well, the power inputs to the motor are only powered (12V) when the relay is engaged (otherwise 0.15V). So I assume the BMSC is not working properly?

 

[J_C]

The terms being used are giving me a headache. The speed controller is the head unit. The firewall unit is an active slave of the controller, only mounted remotely for heatsinking purposes in the airflow. I can understand them being called different things.

Pin 3 should be the transistor base drive. It is getting progressively higher voltage as the knob is turned. If I understand the setup correctly, power is live through the motor and the firewall module grounds the motor negative to complete the circuit. We see the "Motor Negative Output" has power, so it appears the firewall module is not grounding that despite receiving the base drive voltage on pin 3.

This suggests the head unit is working and the firewall module is bad. That is also the usual failure mode, that the firewall unit's transistor fails from long term heat.

Depending on where you're located you might be able to purchase and replace just the transistor. It is about $12 on Digikey in the US, part # MJ11028 but you'll need a beefy soldering iron and to drill out the existing rivets then some replacement nuts/screws or rivets, and heatsink grease. There is more detail in other topics, search the forum for the keyword "darlington".

In case I am wrong (about it grounding the motor to complete the circuit) if someone has a proper schematic of this circuit it might help.

 

[96eb96]

The terms being used are giving me a headache. The speed controller is the head unit. The firewall unit is an active slave of the controller, only mounted remotely for heatsinking purposes in the airflow. I can understand them being called different things.

Pin 3 should be the transistor base drive. It is getting progressively higher voltage as the knob is turned. If I understand the setup correctly, power is live through the motor and the firewall module grounds the motor negative to complete the circuit. We see the "Motor Negative Output" has power, so it appears the firewall module is not grounding that despite receiving the base drive voltage on pin 3.

This suggests the head unit is working and the firewall module is bad. That is also the usual failure mode, that the firewall unit's transistor fails from long term heat.

Depending on where you're located you might be able to purchase and replace just the transistor. It is about $12 on Digikey in the US, part # MJ11028 but you'll need a beefy soldering iron and to drill out the existing rivets then some replacement nuts/screws or rivets, and heatsink grease. There is more detail in other topics, search the forum for the keyword "darlington".

In case I am wrong (about it grounding the motor to complete the circuit) if someone has a proper schematic of this circuit it might help.

Good analysis. It is a common failure point, I replaced mine too. Dirt gets on the cooling fins, probably because there is no cabin air filter.

 

[koda2000]

On the EATC equipped models, the "blower motor control module" takes the place of the "blower motor resistor" found on the manual HVAC models. It is quite common for these modules to fail with age and when they fail they typically leave you with high-speed blower operation only. You may be able to open the module and replace failed internal electronic parts, or re-solder the circuit board if your good at that kind of stuff, or you can just replace it with a new module.

 

[1998Exp]

The terms being used are giving me a headache. The speed controller is the head unit. The firewall unit is an active slave of the controller, only mounted remotely for heatsinking purposes in the airflow. I can understand them being called different things.
Pin 3 should be the transistor base drive. It is getting progressively higher voltage as the knob is turned. If I understand the setup correctly, power is live through the motor and the firewall module grounds the motor negative to complete the circuit. We see the "Motor Negative Output" has power, so it appears the firewall module is not grounding that despite receiving the base drive voltage on pin 3.
This suggests the head unit is working and the firewall module is bad. That is also the usual failure mode, that the firewall unit's transistor fails from long term heat.

As usual, you were right and I was wrong: the O/P's diagram clearly shows that this is an analog servo loop, and not an open-loop, PWM-driven motor. This sort of arrangement dissipates substantial power on the driver transistor - reaching a maximum when the motor is at around one half the maximum current (which is probably the prevailing condition). Obviously, the heatsink and/or the air flow through it were insufficient to assure long term reliability -- those things have already failed once on both my Explorer and on my Vic (which is barely at half the Ex's mileage).
The O/P's measurements confused me a bit too. Mostly because they seem to be almost normal, but the circuit doesn't work. In any case, it's my guess that the feedback is simply the voltage on the negative lead of the motor, and the small difference between the two is just measurement error. And now that I got curious, it's time to stick a probe on those pins and see what happens when everything is working correctly (a DVM will suffice because it's all DC). Will report when I am done.

 

[Marcus Behel]

The terms being used are giving me a headache. The speed controller is the head unit. The firewall unit is an active slave of the controller, only mounted remotely for heatsinking purposes in the airflow. I can understand them being called different things.

Pin 3 should be the transistor base drive. It is getting progressively higher voltage as the knob is turned. If I understand the setup correctly, power is live through the motor and the firewall module grounds the motor negative to complete the circuit. We see the "Motor Negative Output" has power, so it appears the firewall module is not grounding that despite receiving the base drive voltage on pin 3.

This suggests the head unit is working and the firewall module is bad. That is also the usual failure mode, that the firewall unit's transistor fails from long term heat.

Depending on where you're located you might be able to purchase and replace just the transistor. It is about $12 on Digikey in the US, part # MJ11028 but you'll need a beefy soldering iron and to drill out the existing rivets then some replacement nuts/screws or rivets, and heatsink grease. There is more detail in other topics, search the forum for the keyword "darlington".

In case I am wrong (about it grounding the motor to complete the circuit) if someone has a proper schematic of this circuit it might help.

That makes sense. I have ordered the part and will attempt to repair it. Thanks for the help.

 

[1998Exp]

The O/P's measurements confused me a bit too. Mostly because they seem to be almost normal, but the circuit doesn't work. In any case, it's my guess that the feedback is simply the voltage on the negative lead of the motor, and the small difference between the two is just measurement error. And now that I got curious, it's time to stick a probe on those pins and see what happens when everything is working correctly (a DVM will suffice because it's all DC). Will report when I am done.

Well, it's been a while and I forgot how deeply the BSMC is buried - no easy access to back-probe those pins, and I'd rather not poke through the insulation, just to satisfy curiosity. HOWEVER, if the O/P is kind enough, he can repeat his measurements with the new BSMC hanging loose and just the harness installed (carefully, so that it doesn't overheat).