Couldn't find this one in a search. 1978 F150 with factory AC. The Heater/AC fan blows on all settings, low to high when the key is in the on position with the truck not started. When I start the truck the fan does not blow on any settings. Thanks in advance for any ideas.

 

Something isn't adding up. This circuit is powered directly off the ignition switch through a fuse, that's it. There has to be something else going on, be it some sort of previous owner wiring, accessory added, etc. In factory form, nothing upstream of the blower motor has any sense of whether the engine is physically running.

 

Yeah, seemed strange to me too. I'll pull the switch panel and look, was hoping this was a more typical type issue.

 

I think the issue is more in the ignition switch, fuse block and or under dash wiring. I doubt pulling the switch panel will show you anything cause you already said it works fine, just not with the truck running.

You do know how to get the headlight plunger out of the switch right? You take it out, to get the dash bezel off, to get the environmental control panel out.

Maybe some PO installed aftermarket stereo install job and he tapped power from the wrong place?

Very odd issue for sure, I will be interested to hear what the problem is.

 

Few electrical items have any sense that the engine is running (as opposed to stalled with the key on), and they are as follows:

1) RPM modulated through the coil power wire
2) Sending units
3) Alternator stator

All are isolated from the blower circuit. How repeatable is this issue? Can you repeat it 20 times in a row over a few days? Another explanation is that there is an intermittent failure in a switch upstream or the blower motor itself that coincidentally opens the last few times you've run the engine, and with time, you'll find a case where the blower motor works with the engine running (or not working with the engine off). The only other possibility is that the blower motor windings are so weak and tattered that simply the vibration of the engine running makes the blower motor open and fail to run.

I doubt you'll find anything behind the switch panel either. You're looking for egregious hacks and splices by a previous owner, if at all. However, my money is on coincidence, and your suspicion that the problem is related to the engine running is purely a red herring. There is an explanation for everything and there's always more to be found when something doesn't make sense. There's obviously a problem, all I'm saying is it's probably not worth racking your brain too much on why the engine running seems to have an effect. You'd troubleshoot this like any other problem (power going in and out of fuses, in and out of switches, power getting to the blower). If the problem only shows up while the engine is running (for now at least), then that's when you'll have to do the troubleshooting.

 

how do headlights/dashlights respond when you start the truck vs. just in run? Maybe there is a major voltage drop when the truck is running? Battery test good? Alt?

Maybe the ground for the blower is loose or corroded. With the truck not running, there is no vibration. As soon as you start it, maybe it shakes the ground loose.

 

Thanks for the input, it does it every time without fail since I bought the truck a few months ago. Just getting around to looking at that particular issue since the weather is nice today. Looks like I have some tracing down wires and ruling out to do. Thanks again, I'll report back when I figure it out.

 

Everything else is working as it should, charging system is at 14.6 all dash lights function as they should and no dimming down when things are running. Blower is a bit squeaky so I was going to replace that soon anyway. Might just go ahead and start there.

 

Let me see if I understand it.

If the truck is already running does if work fine or not at all?

And the issue is when the switch is on low/high and you start the truck the blower quits when you release the key from the start into the run position?

But when the key is on run only (not start) it does come on just fine?

Since you have a/c it might (big might) be a issue of the other switch (vent heat a/c defrost), have you had the blower switch on and changed that switch position?

 

You do know what you are getting into with that statement, removal of both a/c and heater core to just get to the blower motor....PITA.

You can not get it out by removing the metal cover on the box in the engine compartment.

See the pics, its still behind the heater core and what a pain when still installed in the truck.

 

Truck already running = fan doesn't work, fan running in on position then starting the truck = fan stops running, key in on position with the truck not running = fan runs fine in all positions, low to high. All blend doors function as they should when changing from heat to vent to defrost ect.

I'm actually going to drill out the rivits, remove the cover on the AC box in the engine bay, remove the fan from there and replace it then use small nuts and bolts to re attach the cover. It can be done, there are a few threads on here from people who did it.

 

Do you see the 4 nut plates on the edge of the circle in the last pic? That is where the fan/motor frame mounts. It goes in from the heater core side.

I am really sure that pulling off the metal plate will NOT let you get to the mount hdwr, or pull the motor/frame thru.

But if you get deep into the "advanced search" option I think there is a trick to get your idea to work. I will go look for the thread, its been a while.

I would figure out and fix the initial issue and then tackle the blower motor change.

 

I have the thread saved on my IPad, you have to cut the fan supports to get it out and get to the screws. Fortunately the truck is a weekend driver/project so these are just issues on a long list :-) since I was in the dash fixing the gauge lights I figured I'd throw it out there.

 

Well I took the complete assembly out (parted the whole assembly out) box one way, cage another....and I just can not see how you will access the screws.

The motor and frame supports are all on the back side of the plastic circle. And even getting an allen t handle in there to remove the cage to motor shaft lock screw was a PITA even with it completely removed from the box.

All this while standing on your head or laying on a valve cover sounds like no fun. Post pics of this upcoming adventure please.

Glad you have the thread link, I need to go look it up and refresh my failing memory.

 

Found the issue and it was none of the above :-) My ignition switch was touchy to the point that if you moved it at all toward cutting the truck off when it was running it would do just that, cut it off. I decided that I'd try replacing that first to see if something in the switch might be causing this issue as well. Sure enough, fan now blows, truck on or in run position and the truck doesn't cut off now when you touch the key. Thanks for the input guys!