Blower motor confusion

[OldMan89]

My truck is an 89 2.3l 5spd for reference. My blower motor is very intermittent. Some mornings it will come on but seems week. Some mornings it will blow adequately. But most mornings it won’t do anything at all. My work commute is short at about 15 to 20 minutes and Tennessee winters are pretty unpredictable so I just put up with the cold, but it bugs me that it won’t just work lol. I just pulled it out to inspect it and blow the dust out of it and found that with the key on /engine off, it will turn strong in all settings. But with the engine running it would not turn??? I replaced the resistor last year thinking that was the problem but it didn’t help with the problem. Any insight would be greatly appreciated!

 

[RonD]

Blower motor's Speed is Ground controlled

With key on blower motor get 12volts on one wire from a 30amp fuse
Brown/orange 12v wire was common in earlier years
Unplug the 2 wire connector on blower and test this wire for 12v(battery voltage) with key on
Then test it again with engine running, should now be 13.5+ volts, alternator voltage, battery should also be this same higher voltage, means alternator is working

The Ground wire on the blower can be orange/black, also common color on older Fords
Test battery voltage, and remember it, 12.3v to 12.8v depending on age of battery
High Speed on cab switch is a direct Ground, by passing Resistor Block
So set switch in cab to HIGH(key on or off doesn't matter)
Then test blower's ground wire to battery positive terminal
Should see Battery voltage, 12.3 to 12.8v
If its lower then bad ground connection at switch in the cab
This is usually a Black wire on the speed switch, and can be grounded to firewall in the cab or in the engine bay, Ford did both

All the speeds use this 1 ground wire at the switch so if its loose or corroded all speeds would be effected

If ground looks good, start the engine and test again, should see alternator voltage, 13.5+ volts

The connector on Resistor Block gets very hot as does the block itself, so over time the connector can deteriorate, the blower motor's ground wire is connected to this connector along with HIGH speed wire from the cab, so 2 Orange/Black wires on same terminal in the connector, but Ford also spliced the HIGH wire to blower motor's Ground wire and didn't have it on the Resistor block, so could be either way

Then 3 other wires on this connector, each on one terminal of connector
Unplug the Resistor block connector
Select a lower speed on the switch, one of the 3 wires should now test as a full ground, so shows battery voltage when connected to battery positive terminal, just like HIGH speed did, or didn't
If its lower like HIGH was, then poor ground at the switch confirmed

 

[ericbphoto]

If all that is good, there is the remote possibility that the brushes in that 34 year old motor are worn and not making consistent good contact with the armature. But I would expect that to be a more consistent problem and less intermittent.

 

[OldMan89]

Blower motor's Speed is Ground controlled

With key on blower motor get 12volts on one wire from a 30amp fuse
Brown/orange 12v wire was common in earlier years
Unplug the 2 wire connector on blower and test this wire for 12v(battery voltage) with key on
Then test it again with engine running, should now be 13.5+ volts, alternator voltage, battery should also be this same higher voltage, means alternator is working

The Ground wire on the blower can be orange/black, also common color on older Fords
Test battery voltage, and remember it, 12.3v to 12.8v depending on age of battery
High Speed on cab switch is a direct Ground, by passing Resistor Block
So set switch in cab to HIGH(key on or off doesn't matter)
Then test blower's ground wire to battery positive terminal
Should see Battery voltage, 12.3 to 12.8v
If its lower then bad ground connection at switch in the cab
This is usually a Black wire on the speed switch, and can be grounded to firewall in the cab or in the engine bay, Ford did both

All the speeds use this 1 ground wire at the switch so if its loose or corroded all speeds would be effected

If ground looks good, start the engine and test again, should see alternator voltage, 13.5+ volts

The connector on Resistor Block gets very hot as does the block itself, so over time the connector can deteriorate, the blower motor's ground wire is connected to this connector along with HIGH speed wire from the cab, so 2 Orange/Black wires on same terminal in the connector, but Ford also spliced the HIGH wire to blower motor's Ground wire and didn't have it on the Resistor block, so could be either way

Then 3 other wires on this connector, each on one terminal of connector
Unplug the Resistor block connector
Select a lower speed on the switch, one of the 3 wires should now test as a full ground, so shows battery voltage when connected to battery positive terminal, just like HIGH speed did, or didn't
If its lower like HIGH was, then poor ground at the switch confirmed

Thank you!

 

[delaware74b]

Check the condition of your blower resistor. It's not a weather-resistant design and known to corrode.