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Blower Not Working On High - Help!

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pwjimmy

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I have a 1974 Jimmy (w/A/C) and I am having a problem with the blower not working on the high setting (it works on the three lower speed settings). I have tried everything I can think of, but it still isn't working. I am hoping one of you has the answer. Here is what I have done so far:
  1. Bought a new Temperature Control Assembly from LMC (which included a new switch)
  2. Bought a replacement switch thinking maybe the LMC one was faulty - no luck
  3. Replaced the blower relay and blower motor resistor
  4. Replaced the 30A in line fuse off the junction block (shows 15A in the diagram below but the original was 30A)
  5. Tested directly powering the blower off the battery (ran at high) so the ground is good
Here is the wiring diagram I have. Any thoughts?


Screenshot 2024-01-20 at 5.45.30 PM.jpeg
 
At the hi blower relay the orange wire (circuit 40) should have battery voltage at all times. When you select Hi blower at the switch, the dark blue (circuit 101) should have power. If you do find battery voltage at these 2 circuits, unplug the purple wire at the blower motor and check for voltage. If there is power plug the wire back on blower motor, and then back probe the connector. With hi blower selected, (key must be on for these tests) check for power. If there is no power with blower motor load in circuit, there is hi resistance somewhere in the circuit. Try cleaning all the brass spade connectors, give the motor a better ground for testing, any splices are suspect.
 
There are two sources of power. Low and Medium pull from the instrument cluster. The orange circuit 40 is only used for High. Inspect the relay connector for melting/damage and double-check the ground circuit 150. If you have power/ground at 150/152 (key on, heat selected, high speed selected), you should be able to plug the relay in and hear it click.

When you put the switch on high, does the blower stop or it does it run at a lower speed?
 
I'm sorry the smaller orange wire circuit 52 is the Hi blower relay control wire. not the dark blue as stated earlier. Had another look at the diagram.
 
Gentlemen,

After trying several things I am still essentially at the same place, but I did learn a few things tinkering tonight.
  1. When I turn the key to accessory some power is getting to the blower regardless of the Temperature Control Assembly settings. The blower is turning even when it should be off.
  2. I disconnected the purple wire from the blower and inserted my multimeter proben into the connector. With the key on, I cycled through the four speeds at the Temperature Control Assembly switch. I got the same power reading at settings 1-3 (about 11.82) and slightly higher at high (11.98), so I am perplexed as to why the blower isn't going at high.
  3. I disconnected the 52/150 connector from the blower relay. Using a jumper cable, I connected circuit 52 directly to the blower connector. With the power on, I cycled through the Temperature Control Assembly switch setting. The blower was off at settings 1-3, but went on high when I put it on setting #4
  4. When I cycle through the speed settings, there is no clicks moving from speed settings 1-3, but when I move it to high I hear the blower relay click.
I obviously have more trial an error stuff to do tomorrow, but I was hoping that this information might give you some ideas regarding what is going on.

Thanks again for the help. I really appreciate it.

Paul
 
Ok, some if not all the early factory ac trucks always run the blower with key on. Don't recall if accessory should allow this, think ign switch problem/adjustment.

Your jumper test tells us you have voltage on orange 52. Please don't jumper 52 to the purple 65 again, you may overload and melt circuit 52.
The dark blue wire at 101 should have variaring voltages from the resistor block.

We don't know if the relay ground is good from these test. Red lead from meter, probing 40, and black lead to 150. You should see battery voltage on your meter. This test won't prove the ground can carry the load to run the relay.
The other speed pass through and do not use that ground.
With everything reconnected and test leads black to battery negative, red back probing 150, blower motor on high. Meter should read lower than 1 volt, if it is closer to 12v then the ground is bad, somewhere between battery and relay.
The battery voltage is a bit low nothing alarming, and to be expected while troubleshooting, I possible put a battery charger on when not actively troubleshooting.
Feel free to ask questions.
I some times say stuff, w/o realizing everyone doesn't have the same back ground knowledge.
 
This troubleshooting is good progress. With the relay clicking and the low speeds working, everything is verified except 40 ORN and the relay. You wouldn't think a new one would be bad, but these days some electrical parts are bad out of the box. I used to get the opposite problem, that only HI works, which makes sense since NO contacts usually last longer than NC, due to contact force. The contacts get pitted until the resistance is too high and even though the relay is working mechanically, you don't get a good electrical connection.

This is easy to test, if you can get the multimeter probe in the back side of the relay connector with everything set for HI. The key here is that everything is connected while measuring. All of your voltage measurements above are normal even with a bad connection because there is no load (open circuit = no current = no voltage drop, so a junk connection at 200 Ohms tests the same as a great connection at 10mOhm. Your DMM essentially draws no current). If BLU DK (101) is ~12V and PPL (65) is much lower, the relay is bad. If the voltage at BLU DK (101) is low, measure at the terminal block. Once you've backtracked enough to find where the voltage is dropping, you've found the bad component (fuse holder, wire, ring terminal, etc.)

If this leads you to the relay, you might try disassembling the original relay to clean the contacts with fine sandpaper. The aftermarket relays are often built a lot lighter and won't last as long.
 
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