HVAC fan won't run at No. 4?

[BadaBingM3]

So got a problem with my fan. I replaced it with some spin-off brand off of Rockauto a few years ago and ran fine up until lately it would not turn on intermitted on all settings, initially noticed it would stop running on highest setting. Doesn't matter what setting the vent settings are set at, A/C, panel, defrost, defrost floor, etc. Today I replaced the air sensor (can't think of the exact name) that is placed next to the fan on the fan shroud. Also found one of the air-vacuum lines that goes to the vacuum sphere located under right front wheel well disconnected. Plugged the tube back in, replaced the sensor and the fan comes on from setting 1-3 but won't run in setting 4. Very odd. Any ideas? Thanks.

 

[TelMinz]

Blower motor fan resistor?

Dorman 973-411 HVAC Blower Motor Resistor Kit for Select Ford / Mercury Models Amazon product ASIN B009IF2I7G

 

[BadaBingM3]

Blower motor fan resistor?

Dorman 973-411 HVAC Blower Motor Resistor Kit for Select Ford / Mercury Models Amazon product ASIN B009IF2I7G

Well that is the component I just replaced. I could try the relay itself next though. Thanks.

 

[TelMinz]

Usually that part makes you lose the lower speeds. If you have an ohm meter you could try and see if the switch is bad, I believe you could do it right at the blower fan resistor plug.

 

[BadaBingM3]

Usually that part makes you lose the lower speeds. If you have an ohm meter you could try and see if the switch is bad, I believe you could do it right at the blower fan resistor plug.

Good point that it could be the switch. I'll have to see if it manifests again not turning on the fan in all speeds like before.

 

[J_C]

It is most likely the blower relay, second most likely the high speed contact position in the dash switch (but it only grounds the relay coil so at such low current, shouldn't wear from this like a high current DC switch would, but given decades and oxidation anything is possible), 3rd less likely a wiring fault between relay and switch.

 

[Rdc#1958]

Blower motor fan resistor?

Dorman 973-411 HVAC Blower Motor Resistor Kit for Select Ford / Mercury Models Amazon product ASIN B009IF2I7G

Wonder if that's the problem with my 2000 Mounty?it only runs on high speed also, I had same problem on a 2000 Monte Carlo O had some years ago and the new resistor solved the problem so that's been on my mind as far as the problem as well as the dash switch but haven't had the to check it out yet.

 

[J_C]

^ Yes only running on high is usually caused by a blown resistor module (or shorted to closed-circuit transistor, module, if equipped with automatic (EATC) climate control).

With manual climate control, the dash switch failing cannot cause it to only operate in high mode. You could completely remove the switch and the blower should still work in the lowest speed which does not go through the switch at all. With automatic (EATC) climate control, a leaky transistor in the the console head unit could cause the fan to always work at high(ish) speed, but would do so no matter what speed you tried to set, possibly even if it was turned off.

 

[Mark Madden]

Blower relay most likely

 

[Rdc#1958]

Blower motor fan resistor?

Dorman 973-411 HVAC Blower Motor Resistor Kit for Select Ford / Mercury Models Amazon product ASIN B009IF2I7G

 

[Rdc#1958]

Dorman number for automatic temperature control is #973-567

Blower motor fan resistor?

Dorman 973-411 HVAC Blower Motor Resistor Kit for Select Ford / Mercury Models Amazon product ASIN B009IF2I7G

That is the number for models with manual temperature control,number for automatic temperature control is #973-567

 

[Rdc#1958]

^ Yes only running on high is usually caused by a blown resistor module (or shorted to closed-circuit transistor, module, if equipped with automatic (EATC) climate control).

With manual climate control, the dash switch failing cannot cause it to only operate in high mode. You could completely remove the switch and the blower should still work in the lowest speed which does not go through the switch at all. With automatic (EATC) climate control, a leaky transistor in the the console head unit could cause the fan to always work at high(ish) speed, but would do so no matter what speed you tried to set, possibly even if it was turned off.

I have the automatic temperature control in my Mounty and just wondering where the resistor is located,under the hood or inside the vehicle.

 

[TelMinz]

I have the automatic temperature control in my Mounty and just wondering where the resistor is located,under the hood or inside the vehicle.

Here is a video for a ‘97 explorer but it’s probably in the same location

 

[Jason94sport]

I gave up on mine OP. I have 1, & 4 only. 2 resistors, 1 fan, 2 switches & nothing disconnected behind the panel, or any bad vacuum lines.
I can deal with it in FL.