Electrical MYSTERY? Blower motor runs when key is off

My K10 is acting spiteful now after sitting for 2 weeks while I was out of town. I typically drive it on an almost daily basis. It's an '85 with A/C (non-functioning, but pretty complete). The blower motor continues to run when the ignition switch is off.

Here's the details:
1. The truck starts good and the factory radio behaves normally when the ignition is in the run, off, and accessory positions. This leads me to believe my ignition switch is just fine.
2. The blower motor only continues to run when in the "Hi" position. If the fan is set to "Lo", or any position in between, the blower motor will shut down properly when the ignition is turned off.
3. With the ignition off and the blower motor continuing to run, the fan speed can be changed from "Hi" to any other speed and it will turn off the blower motor. Turning the fan speed back to "Hi" will not restart the blower motor.
4. The blower motor will continue to run when set to "Hi" with the ignition off, regardless of "A/C", "Vent", "Heat" settings. It does not continue to run when the switch is set to "Off".
4. With the ignition off and the blower motor still running on "Hi", moving the heater/vent switch to "Off" will kill the blower motor. Moving it back to "Vent" or any other position will not restart the blower motor when the ignition is off.

I think that covers all possible test scenarios for the switch positions, but I'm happy to clarify if there are questions. The resistors used for the fan speeds all look normal when I took them out of the A/C box last night.

Is the fan relay stuck or otherwise bad? I tried tapping on it with a hammer last night - no affect.

Is my blower motor *****'d and acting like some sort of bizarre perpetual motion machine?

What's going on here? I don't want to start throwing random parts at it just yet.

@scrap--metal Easy answer. There is a relay for the high speed blower fan, it's stuck closed. It's just as simple as that. I can take a look a little later and snap a pic of mine if you need to know where it is, but if memory serves me it under the hood attached to the HVAC box or close by. There is nothing else in the system that would cause your symptoms.

@SirRobyn0 Thanks for the quick answer. I know where the relay is at on the passenger side of the engine compartment, so no picture is necessary.

I tried tapping it with a hammer last night thinking that it may have been stuck. That didn't have any affect though, and I didn't want to start buying parts in case it was an issue with my blower motor or control panel.

I appreciate your input.

If you have a DMM you can check the relay.
GM used different relays depending on model and year. This MAY help.
If you are going to use a DMM to test the relay, trust me on this, it's easier if you remove the relay from the HVAC case. Leave it plugged in and test it.

Snoots said:

If you have a DMM you can check the relay.

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Good suggestion. I have a DMM and will plan on doing that tonight (before spending any money).

Thank you.

scrap--metal said:

Good suggestion. I have a DMM and will plan on doing that tonight (before spending any money).

Thank you.

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Check my edit.

Snoots said:

Check my edit.

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Thanks for the additional info. This did help.

A new relay took care of it last night. Just under $20 at O'reilly and I'm back in business (P/N STD-RY117).

I'll be hauling a canoe for an evening paddle after work today. Maybe even wet a line if I'm lucky. Thanks @SirRobyn0 and @Snoots !

My K10's not doing as well as I thought on Thursday night.

I went to start it on Friday night, and the battery was stone cold dead. I swapped a different battery into it so I could drive it (I already had the canoe loaded on top). This morning it was dead again.

I drove it on a couple short trips today, and the new blower relay stuck on twice (same issue as earlier this week). Something odd is going on, and I'm not sure if the new draw is related to the relay issue, or coincidental.

I hope to have time to pull some fuses tomorrow.

A bad diode in the alternator can run a battery down overnight, so have that tested to make sure it's working properly.
The way I start testing for electrical issues is remove the ground cable at the battery, put a test light between the battery post and the cable end.
If the positive lead is connected to the alternator, it will show up as the test light coming on, so disconnect that wire from the back of the alternator.
Then see if you have continuity across the cable end to battery post.
If the light glows, pull fuses, one at a time and repeat the test.
When the light doesn't come on, that last fuse you pulled is the circuit to investigate farther for a short to ground.

mtnmankev said:

A bad diode in the alternator can run a battery down overnight, so have that tested to make sure it's working properly.
The way I start testing for electrical issues is remove the ground cable at the battery, put a test light between the battery post and the cable end.
If the positive lead is connected to the alternator, it will show up as the test light coming on, so disconnect that wire from the back of the alternator.
Then see if you have continuity across the cable end to battery post.
If the light glows, pull fuses, one at a time and repeat the test.
When the light doesn't come on, that last fuse you pulled is the circuit to investigate farther for a short to ground.

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This is the method I use on older rigs like ours. It's not effective on newer vehicles with multiple computers, the simple ones like in the 90's are ok but the complicated modern systems it's a no go so keep that in mind if anyone decides to try to apply this to another vehicle. But on these old rigs, nothing beats the test light method, it's simple and effective. It's very likely that in just a few minutes you'll get the circuit narrowed down.

scrap--metal said:

@SirRobyn0 Thanks for the quick answer. I know where the relay is at on the passenger side of the engine compartment, so no picture is necessary.

I tried tapping it with a hammer last night thinking that it may have been stuck. That didn't have any affect though, and I didn't want to start buying parts in case it was an issue with my blower motor or control panel.

I appreciate your input.

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They are only about 10 bucks. Splurge and it'll probably fix your problem. Or your fan switch needs to be replaced.

Raybo135 said:

They are only about 10 bucks. Splurge and it'll probably fix your problem. Or your fan switch needs to be replaced.

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Lol. TLDR?

mtnmankev said:

A bad diode in the alternator can run a battery down overnight, so have that tested to make sure it's working properly.
The way I start testing for electrical issues is remove the ground cable at the battery, put a test light between the battery post and the cable end.

Click to expand...

I messed with the K10 a little yesterday and again tonight. I don't have a test light, but I have a DMM. I couldn't pick up any current between the negative battery terminal and the ground cable after I disconnected them. It doesn't make sense with how quickly my battery drains, but I'm now resorting to the process of elimination.

Yesterday, I disconnected the relay (since that was the new part in the system to fix the blower staying on). The battery continued to lose voltage several hours between checks.

Today, I disconnected the alternator and have not seen any voltage loss yet. I'll check it again in the morning, but I think @mtnmankev called it with the alternator. It would make sense, too. While driving in heavy rain last Monday, the alternator was not charging well. I've confirmed that it still charges, but it was acting goofy early last week.

scrap--metal said:

I messed with the K10 a little yesterday and again tonight. I don't have a test light, but I have a DMM. I couldn't pick up any current between the negative battery terminal and the ground cable after I disconnected them. It doesn't make sense with how quickly my battery drains, but I'm now resorting to the process of elimination.

Yesterday, I disconnected the relay (since that was the new part in the system to fix the blower staying on). The battery continued to lose voltage several hours between checks.

Today, I disconnected the alternator and have not seen any voltage loss yet. I'll check it again in the morning, but I think @mtnmankev called it with the alternator. It would make sense, too. While driving in heavy rain last Monday, the alternator was not charging well. I've confirmed that it still charges, but it was acting goofy early last week.

Click to expand...

A DMM will work as good as a test light in the DC volts mode.
If you get a reading, it indicates a loss.
Also, I'm curious as to the age of the battery, and is it clean and the battery tray clean ?
I have seen batteries lose juice from corrosion and crud being a conductor to ground.

mtnmankev said:

Also, I'm curious as to the age of the battery, and is it clean and the battery tray clean ?
I have seen batteries lose juice from corrosion and crud being a conductor to ground.

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Good thinking, but the battery is from 2020 and still healthy to my knowledge. It recharged fine and has held >12V while disconnected. The battery tray might be a little crusty, but it's complete with a functioning hold down. I don't think it's bad enough to cause this.

The draw killed the 2020 battery, as well as the battery from my Monte Carlo that I swapped into it. Same overnight drain. The wiring in the truck is relatively unmolested. I'm thinking it has to be the alternator, and I'm just doing something wrong with my DMM and can't pick it up.