A note on DC motors.
Very often turning the shaft will feel like it's hitting something or dragging internally.
This is normal and would not let the shaft and blower free wheel turn to natural stopping point after many revolutions.
The reason is a DC motor is often an actual DC 'generator' in reverse depending on design so what you feel is the resistance to motion caused by it's internal magnetic properties between the stator and armature.
Sometime connect a DC voltmeter set to about one volt scale and turn the shaft. You may see an actual DC output voltage.
Good luck.

 

Just a note regarding the last fuse blowing much sooner than the one I used prior. The fuse was a MINI fuse. Same blade configuration, color, amperage as the prior ones. But the shape was not exact and I believe that the MINI may not have been time-delay (slo-blo) as I just discovered that the MAXI fuse (what is called for in the owner's manual) is. Unbelievable. Still think there must be another issue causing the blowing out problem but at least the fuse will now be correct. At $5 each, really struggling with just installing it and trying again. Maybe it was a bad resistor after all, but if it was and I replaced it, why did the fuse blow again even if it wasn't a time-delay? Very confused at this point.

 

A time delay fuse is made with the ability to stand short time high current draws such as when a motor starting current is higher than it's running current.
For example in your home the refridg motor takes nearly 2 time the current starting than it does running. Same with an air conditioner, oil burner etc that all have motors that take more current starting than running.
On a 15 amp circuit the starting current can be as high as 13 amps but running current will be 6 to 7 amps range depending on it's load.
The circuit has to be protected for 15 amps max draw with a circuit breaker that will stand the higher current for a short fraction of a second without tripping.
You get things right in your truck and it will work as designed.
There is nothing else to the circuit but the correct parts 'all present' at the same time. You already found on error to prove it.
Good luck..

 

Is there a thought, in this case, as to why it would blow when just running on hi for awhile and not when turning on or turning to hi initially? Could it have been "fatigued" or "stressed" by a few switch speed adjustments during it's brief (approx 10 min) period of service and just gave up at the point when it did? Or is it something else like a short to ground, etc. Not sure how to check for that exactly. Any grounds that were easily visible on the firewall (not sure if they are any of the grounds in question) appear tight with only light surface rust.

 

Going to try an amperage test of the motor just to eliminate whether the motor may be drawing too much. Does anyone know what the amperage draw is for this motor so I can check the value when running. Thanks.

 

Here is some ways to check out the system.
Measure the motor resistance as accurately as you can.
Use ohms law to calculate the parameters.
The system voltage is often around 14.2 volts.
Current is equal to voltage divided by the resistance.
For example 14.2 volts divided by 1 ohm equals 14.2 amp or nearly a standard 15 amp fuse value.
What that means is the motor starting resistance has to be greater than 1 ohm to keep the starting current below 15 amps.
For example a motor resistance of 1.2 ohms would cause a current to flow of 11.8 amps well below the burn point of a 15 amp fuse.
When on lower speeds the speed dropping resistors add there value to the motor causing the current to drop and the speed to lower.
Apply this same method to your circuit with the fuse value to see if the motor is indeed too low a resistance.
Example again at 14.2 volts with the motor resistance at .45 ohm (less than one) the circuit current would be close to 32 amps. voltage divided by resistance = current in amps.
This way you can see if the motor is responsible for the fuse not holding.
Tenths of an ohm makes a large difference in total current in these types of applications.
.
A fuse is made from an alloy that has a known melting point and a resistance per it's length inside the fuse mount.
The only way it blows is excessive current due to the voltage drop across the fuse element in it's housing from the current flowing.
Poor grounds etc are not the cause of blown fuses in DC circuits, it's excess curent.
Good luck.

.

 

I will check the motor resistance, etc, but am still curious if the above connection results point to anything. The wiring diagram, if I'm reading it correctly, indicates that the org/blk, blk, yel/red, and grn/wte should all be connected when the speed switch is set to hi. Does the fact that the grn/wte is not involved, indicate that one of the blower resistors is not part of the hi setting and thus allowing too much current through to the motor, thus blowing the fuse? Not sure if I understand this correctly. Thank you.

 

None of the resistors are in-circuit in the HIGH position by design.

In HIGH, full battery voltage is used to operate the blower.

Attempting to measure the DC resistance of a DC motor probably won't be very productive.

 

Well finally got around to doing more specific/accurate tests with a Fluke meter and present the following results for any reviews/comments.
Test 1: This first series of results was to observe voltage drop. With key in "ON" position (engine not running), battery voltage read 12.45V. Selector switch positions were as follows: Lo= 12.16V, Med-Lo= 12.12V, Med-Hi= 12.08V, Hi= 12.01V.
Test 2: Motor resistance was 0.6 ohms.
Test 3: Checked resistance from the negative terminal of the blower motor connector to ground which read 0.8 ohms.
Test 4: This series of tests was to check amperage flow to the blower motor at the connector with the engine running. The results were as follows (Battery voltage= 14.45V): Lo= 3.8A, Med-Lo= 6.2A, Med-Hi= 9.4A, Hi= 15A.
It would appear that the motor is not drawing excessive current to blow the fuse. Even calculated, 14.45V divided by resistance of 0.6 ohms shows 24A current which is below the 40A fuse which had blown. I have replaced the fuse and will be driving around and running the circuit one more time to see if it will hold. The only thing different this time is the fuse I'm using. I mistakingly used a mini fuse (non slo-blo, I believe, but it looked very similar to the maxi fuse and very similar in shape) previously not a maxi fuse as is called for. As was stated earlier by Bluegrass 7 "the correct parts all present at the same time. Will see if this makes the difference. Does anyone see anything off in these test results that may indicate whether this circuit could blow the fuse again? Thanks guys.

 

Other than Test 1 being made incorrectly so as to yield it irrelevant, everything else looks normal.

 

I performed this test in the "ON" position and not with the engine running, but I would think that it doesn't matter. Just that it showed that voltage dropped with each change of selector switch position, right?

 

Just an update. So far with correct 40A fuse, new blower motor and resistor (done over the summer), with sporadic use of all motor speed settings....the darn thing is holding and has not blown anything. Could change at any moment, of course, but it's been almost 2 months and so far so good. Thanks for everyone's input along the way.

 

I just can't believe it. Well, I thought all was well but the blown fuse problem has returned. Went all last winter, spring, and most of the summer with no problems until early August. It happened when the motor was on HI. I replaced the fuse (didn't have time to test) back in early August and ran for about 3 weeks on all speeds EXCEPT HI with no problems. I finally gave HI a try and about 10 or 15 minutes later the fuse blew. I did an amperage draw test (like in the video I listed earlier in this post) and came up with the following values with the engine running:
Lo = 3.6A
Med-Lo = 5.7A
Med-Hi = 8.1A
Hi = 11.5A, but the value continued to slowly climb up to about 13.5A when I smelt rubber/plastic burning. I shut it all down and noticed my test leads which were attached to the connector that plugs into the blower motor had melted and were extremely hot to the touch.
I am again asking if anyone has any more thoughts based on the current amp and melting information. Thanks guys. Winter's coming.

 

Upon reviewing the blower circuit, the 40 amp fuse is powered at all times.
1.The selector switch in any on position operates a power relay to close power through to the blower motor.
Position 1 selects the full resistance all in series for low speed.
2. Position 2 cuts out one part of the resistance to increase the blower speed.
3. On high, no resistance is in series with the blower motor enabling the blower to run at full speed.
All you have left is the power relay as a fault or a short with some other lead in the harness that keeps getting lower and lower as the lead heats.
If a short develops between the 40 amp fuse and the blower motor 'input', current will rise and blower may even begin to slow down for lack of power being supplied ahead of it due to a shunt to ground via some other part of the harness.
Be careful of a fire until you find the fault..
That's all there is left to suspect.
Good luck.

 

Spoke with my general mechanic today to set up for him to take a look at the blown fuse issue, but he was booked for about 2 weeks. So he tried to walk me through some things on the phone to try in the meantime, such as to check the resistor connector, the motor connector, and the switch connector for melting. All were intact. So, while I have the dash apart and the selector switch out, is there a way to test if the switch is the culprit while it's out? I just did a continuity test on the various selections (using the blades on the back of the switch) and it seems to be functioning accordingly. Maybe barking up the wrong tree but thought I'ld ask how to check it while it's out and eliminate it from the suspect list for the next guy. Thanks.