ranger a/c and heat weirdness

[Boscobilly]

Hello, I have a weird problem with my 2010 Ranger. The a/c and heat both work, but the fan runs with the load of the engine. If I'm sitting still, you can hardly tell it's running, but if I'm running down the freeway, both the heat and a/c run as if they are on high. Fan speed switch doesn't seem to do anything. What gives with that??? Never heard of such!

 

[Sgt. York]

It could be a faulty blower resistor.

 

[mindless1]

Faulty blower resistor will usually just make the fan run only when the speed switch is set to highest. If you have auto climate control there is a transistor module instead of the resistor module but it tends to either fail open circuit or closed so no fan or full speed, not varying with engine RPM.

If the interior dash module for auto climate control goes out, it depends on which components in it have a fault but again it shouldn't vary with engine RPM.

That it changes with engine load suggests two possibilities, perhaps in addition to an auto climate control issue.

1) At low engine speed you have max vacuum. If you have a vac leak in the HVAC system, it is common for vehicles to default to a duct door position that outputs to the defrost duct. However you don't mention where you are checking for airflow, if from the main front cabin ducts, airflow would be highest at engine idle with possibly enough vacuum to overcome a leak, so it seems probable this is the opposite of what you're observing?

2) More likely you have an electrical system problem, are losing voltage *somewhere*. It could be a failing alternator is dropping voltage. A bad enough battery can put a severe load on an otherwise healthy alternator but that would cook the battery pretty quickly, odds are you would not have more than a day before the battery couldn't even start the vehicle.

I would use a multimeter to check the battery voltage with engine off for at least an hour (should be close to 12.6V) and with engine running for at least a couple minutes (should be close to 14.4V).

You can also check the voltage at the fan connector while a helper varies the engine RPM and fiddles with the climate controls.

I am assuming you can repeatedly replicate this problem, not just that the fan barely blows when you start the vehicle then you get on a main road and it's sped up by then. In that case you might have your fan bearings worn out and as they heat up there is more play in them. This can be confirmed by no ("no" meaning very minimal) change in voltage at the fan connector at various engine RPMs.

 

[Boscobilly]

Faulty blower resistor will usually just make the fan run only when the speed switch is set to highest. If you have auto climate control there is a transistor module instead of the resistor module but it tends to either fail open circuit or closed so no fan or full speed, not varying with engine RPM.

If the interior dash module for auto climate control goes out, it depends on which components in it have a fault but again it shouldn't vary with engine RPM.

That it changes with engine load suggests two possibilities, perhaps in addition to an auto climate control issue.

1) At low engine speed you have max vacuum. If you have a vac leak in the HVAC system, it is common for vehicles to default to a duct door position that outputs to the defrost duct. However you don't mention where you are checking for airflow, if from the main front cabin ducts, airflow would be highest at engine idle with possibly enough vacuum to overcome a leak, so it seems probable this is the opposite of what you're observing?

2) More likely you have an electrical system problem, are losing voltage *somewhere*. It could be a failing alternator is dropping voltage. A bad enough battery can put a severe load on an otherwise healthy alternator but that would cook the battery pretty quickly, odds are you would not have more than a day before the battery couldn't even start the vehicle.

I would use a multimeter to check the battery voltage with engine off for at least an hour (should be close to 12.6V) and with engine running for at least a couple minutes (should be close to 14.4V).

You can also check the voltage at the fan connector while a helper varies the engine RPM and fiddles with the climate controls.

I am assuming you can repeatedly replicate this problem, not just that the fan barely blows when you start the vehicle then you get on a main road and it's sped up by then. In that case you might have your fan bearings worn out and as they heat up there is more play in them. This can be confirmed by no ("no" meaning very minimal) change in voltage at the fan connector at various engine RPMs.

I can replicate the problem. It runs as slow as it will at idle and runs full open running down the road. It's slower if I'm going thirty on a side street, slows to almost nothing at a stop sign, and gains again up to speed. On the freeway, it runs on high. I keep the control on high to get what I can. I don't have climate control. I'm going to try the resistor, I guess. Not an expensive part, if it doesn't work. Thanks.

 

[Boscobilly]

It could be a faulty blower resistor.

Going to try that, I guess. Shouldn't cost too much, if that's not it.

 

[mindless1]

If you don't have automatic climate control, the resistor only effects the switch-set, deliberately reduced speeds.

The fan would still run at full speed all the time on the highest switch setting if the resistor module is faulty, not have any reduction in airflow. This is because the highest fan switch speed bypasses the resistor module to run the fan at the full electrical system ~14.4VDC. If you have significant corrosion in your engine bay it wouldn't hurt to pull the fan plug and harness wires off the resistor module to check for corrosion but it's not likely the problem unless they've completely changed how that is set up. (you might see if you can find a wiring diagram for it).

It would be cheaper to buy a multimeter if you don't have one. Sometimes Harbor Freight even gives them away for free with a coupon, though the last time I saw their coupons they were like $3 or something. They are low quality meters but plenty good enough to tell you where you're losing power with voltage measurements.

 

[Paperdoc]

I agree with mindless1's point #2. What you describe really sounds like, at low engine RPM when the alternator output is reduced, electrical supply to your entire vehicle drops off. At higher revs it comes back. To test this, try other electrical devices under the same idle-versus-revved up comparison. I bet your windshield wipers do the same. If you have electric windows, same thing about how fast they move. At night you may be able to see this in the brightness of your headlights.

Three things could be involved.

1. Poor output from the alternator, and there are a few items in it that could contribute to this. But obviously it has not failed completely, because you get good electrical power (voltage) when RPM's are up.
2. Dirty / corroded electrical connections. IF the problem is isolated to only your fan, then examine its connections. If instead it affects several things, check through all the heavy cabling between the alternator, the battery, and the fuse boxes. High resistance at a contact point (e.g., corroded battery terminals and clamps) can interfere with delivering power.
3. Almost-dead battery. If your battery is really weak it can be difficult to charge up, and also it will not be able to deliver heavy current when there is no chaging happening at low RPM's.

Around here there are a few auto parts stores that will do a free test of your battery (some also will test the alternator) if you remove it from your truck and take it in to their shop. Testing usually takes most of a day, so you can't use the truck in that time. I'm always a little skeptical of what they say because they do these services to persuade you to buy a new one to replace the failing unit, but still it is useful info. And if the battery really is almost dead, there will be little doubt about that. So phone a few shops (if you don't already know about this service for free) and ask who does that.