Heater/blower motor issue...how to diagnose the problem

Posted By: Von

Heater/blower motor issue...how to diagnose the problem - 12/31/14 05:16 AM

My blower motor started popping fuses. Ive done some preliminary trouble shooting and need further advice.

I checked all 3 leads from the dash, and each seems to be doing what they should. Voltage at the corresponding terminal for low, mid, high. Wired the blower motor straight off of battery power and blower motor seems to be fine. No major arcing when I lite the motor?

Motor seems to run on low speed OK through the dash, but if you move the switch to med or high, it blows the fuse.

Resistor in the heater box possibly bad?

Thanks for any help.

Posted By: Golden-Arm

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 06:32 AM

did i miss the part where you said what this was in?

Posted By: RapidRobert

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 06:50 AM

You might jump from the downstream end of the fuse directly to the motor & see if it pops the fuse. that'd tell you if the prob is in the motor itself current draw or in the wiring/switch. I'm thinking the motor (not sure how the line circuit could increase the draw) but I'd wanna at least get the other eliminated before I went to the trouble of pulling the heater box EDIT motor/pigtail sticks out of the firewall on the eng side? jump fire to it there thru a 20 A fuse & see if it blows it & if so pull the motor as an obstruction on the vanes might be making it draw more than normal. If good so far then start at the switch as the prob is downstream from there to the motor

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 04:06 PM

Quote:

did i miss the part where you said what this was in?

Sorry about that. 72 RR.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 04:10 PM

Quote:

You might jump from the downstream end of the fuse directly to the motor & see if it pops the fuse. that'd tell you if the prob is in the motor itself current draw or in the wiring/switch. I'm thinking the motor (not sure how the line circuit could increase the draw) but I'd wanna at least get the other eliminated before I went to the trouble of pulling the heater box

I will try that. Thanks!

One thing that was weird....when I had the motor hard wired from an external battery, I touched the ground to a sill plate screw and it arched pretty dang bad. But, if I touch the ground to the heater box mounting bolt, no major arcing, the motor "seems" to start fine.

Posted By: therocks

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 06:08 PM

Check the gorund for it and also the amp draw for the motor.IIRc anything over the rated fuse is too much.Rocky

Posted By: MO_PA

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 07:05 PM

Check the engine to body ground strap to be sure it has clean connection on both ends.

Posted By: rustbuckett68

Re: Heater/blower motor issue...how to diagnose the problem - 12/31/14 07:06 PM

My old van had a similar problem. Turned out to be a corroded harness plug. Figured it out when a new blower motor did the same thing.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/01/15 03:36 AM

Quote:

Check the engine to body ground strap to be sure it has clean connection on both ends.

No issues with that. Engine is grounded to body 3 ways.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/01/15 03:39 AM

Quote:

My old van had a similar problem. Turned out to be a corroded harness plug. Figured it out when a new blower motor did the same thing.

Plug is good. No signs of corrosion.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/01/15 03:42 AM

Quote:

Check the gorund for it and also the amp draw for the motor.IIRc anything over the rated fuse is too much.Rocky

The ground for the blower motor grounds to the metal cover of the heater box, which is grounded through the bolts that hold the box into the firewall.

How exactly do I check the amp draw with the motor still in the box, in the car?

Posted By: RapidRobert

Re: Heater/blower motor issue...how to diagnose the problem - 01/01/15 05:07 AM

Quote:

How exactly do I check the amp draw with the motor still in the box, in the car?

set the meter on DC amps & you want the meter in series in the 12V power circuit between power and motor & fire it up

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/13/15 10:11 PM

Finally had some time to do some more diagnosing of my blower motor problem.

First thing is the blower wont run at all through the dash now. Motor still runs fine when jumped from an external battery.

I jumped from the fuse box to the blower and the motor runs fine. No excessive arcing when you touch the wire to the hot side of fuse box.

But....I verified that I had voltage at the plug end by the heater box coming from the switch. I attempted to jump from the plug to the lead that goes to the motor. As soon as you touch the jumped wire from the plug to the lead, the voltage reading goes to 0. Fuse is good. Pull the jumped wire off and voltage goes back to normal?

So...??

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/13/15 10:45 PM

I just pulled the connector off at the switch. Verified that I had voltage to the switch. I jumped from the hot wire at the switch back into the leads going to the blower. As soon I touched the wire, voltage went to O.

I assume there is an issue with the hot wire coming from the fuse box?

I guess I will check and see if when I connect the wire I have voltage at the fuse on both ends. If voltage is present, wouldnt that have to be an issue with the hot wire from fuse to switch?

Hope that makes sense. Thanks for the help.

Posted By: stumpy

Re: Heater/blower motor issue...how to diagnose the problem - 01/13/15 10:54 PM

Have you checked the blower motor resister attached to the heater box?

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 12:33 AM

Quote:

Have you checked the blower motor resister attached to the heater box?

Cant see how that is the issue?

Posted By: stumpy

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 01:08 AM

Ok, It can't be the problem. It was just a suggestion seeing that the power for the motor goes through it.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 01:39 AM

Quote:

Ok, It can't be the problem. It was just a suggestion seeing that the power for the motor goes through it.

After further testing, I guess it is possible. When I jump power at the switch from the hot wire to the switch to the high speed wire ( with all wires hooked up at the box) the fan runs. But, it wont run on low or medium.

Any other tests to do to verify that it is the resistor? Its sort of a pain to pull the box to get the resistor.

Posted By: RapidRobert

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 02:19 AM

Quote:

But, it wont run on low or medium.

there is an open in those resistor wire circuits plus it is popping the fuse which means it has a dead short to ground in there. No choice but to open it up to get at it

Posted By: stumpy

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 02:39 AM

Why do you have to pull the box can't you get to it through the glove compartment or radio opening or something?

Posted By: mikemee1331

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 03:29 AM

Quote:

Quote:

Ok, It can't be the problem. It was just a suggestion seeing that the power for the motor goes through it.

After further testing, I guess it is possible.

Stumpy, I think he is 'profiling' us older guys

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 05:11 AM

Quote:

Why do you have to pull the box can't you get to it through the glove compartment or radio opening or something?

It is very hard to impossible to get the resistor screws out with the box in the dash. Next time (better not be a next time) I will know I can get to them by just sliding the box back.

Posted By: Von

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 05:17 AM

So, I found the problems. Two separate issues. First the coils on the resistors were touching each other. Idk how in the world that happened.

Moved the coils away from each other and the fan runs fine.

The short was coming from a pinched/bare wire from the motor itself. It was shorting on the firewall.

I hoped to get it back together tonight, but noticed alot of air being blown around the metal case the blower motor mounts to. Took it apart to seal and the blower wheel is about to come apart. It didnt look near this bad when I replaced the blower motor some 20 years ago. It is rotten...Went through my parts and I have one other wheel that mice have attacked. So, I guess I'll find another wheel. No biggie as I dont want to take the box back out for another 20 years. LOL.

Thanks everybody for the help.

Posted By: stumpy

Re: Heater/blower motor issue...how to diagnose the problem - 01/14/15 06:33 AM

The reason they touched is the fact that all resistors get very hot. When the wires are hot they flex and if you just happen to hit a rough road the wires can bounce together. The excess currant draw because of the shorted motor makes the wires get super hot and make them almost melt and sag. That is why they are inside the heater box so the air flow across them helps cool them down.