Odd Blower Speed Problem

[TrailerTrash]

sorry if this is long winded im just trying to get to the bottom of this issue

i have a 79 k10 that originally had A/C. everything AC related had already been removed when i bought it. the only thing that is still there is whatever is inside the airbox under the hood and the wiring around it.

all the electrical connectors are still hooked up except for one round rubber 2 wire plug with green wires coming from it. the plug is about the size of a ping pong ball. nothing appears to have been changed or jerry rigged as far as the wiring it all looks factory.

the blower motor has a good ground and always works(stays on all the time on low).

the first problem is that the blower only works on low and high. it stays on the lowest speed for the first 3 notches then will be on high on the high notch. thats not a big deal for me BUT i have to have the switch kinda "halfway" between the 3rd and 4th(highest) notch for the blower to actually run on high. so i have to switch it to high then back it down slightly and get it to stick just below high for it to run on high.

i have tried 3 different switches(2 used and 1 brand new) and the problem is still there exactly the same with all 3 switches.

i can deal with only low and high but when its on high all it takes is a bump in the road and the switch gets jarred out of the sweet spot

i am in no way an electrician lol

thanks for any input in advance

[hatzie]

Originally Posted by TrailerTrash

sorry if this is long winded im just trying to get to the bottom of this issue

i have a 79 k10 that originally had A/C. everything AC related had already been removed when i bought it. the only thing that is still there is whatever is inside the airbox under the hood and the wiring around it.

all the electrical connectors are still hooked up except for one round rubber 2 wire plug with green wires coming from it. the plug is about the size of a ping pong ball.

nothing appears to have been changed or jerry rigged as far as the wiring it all looks factory.

That's the Low Pressure Cut Off for the AC compressor clutch. It plugs into the LPCO switch on the dryer/accumulator. Nothing to do with the fan.

Originally Posted by TrailerTrash

the blower motor has a good ground and always works(stays on all the time on low).

the first problem is that the blower only works on low and high. it stays on the lowest speed for the first 3 notches then will be on high on the high notch. thats not a big deal for me BUT i have to have the switch kinda "halfway" between the 3rd and 4th(highest) notch for the blower to actually run on high. so i have to switch it to high then back it down slightly and get it to stick just below high for it to run on high.

i have tried 3 different switches(2 used and 1 brand new) and the problem is still there exactly the same with all 3 switches.

HMMM... A new switch should fix the detent problem with the HI position. You might've gotten a bum blower speed switch... Or the mode lever may not be passing volts to the blower speed switch till you jostle it by playing with the blower speed switch.

Originally Posted by TrailerTrash

i can deal with only low and high but when its on high all it takes is a bump in the road and the switch gets jarred out of the sweet spot

i am in no way an electrician lol

thanks for any input in advance

The blower fan has four things in the circuit;

1. The mode selector lever.
2. The blower speed switch.
3. The blower speed resistor.
4. The blower relay.

Givens:

1. You have switch positions LO, M1, M2, & HI.
2. LO works
3. M1 & M2 don't work at all.
4. The detent for HI seems to be in the wrong place.
5. HI works if you can jigger the switch to the right position between detents.

The resistors are likely OK...
LO is not really a switch position it's a default.
Battery voltage from the Yellow wire on the resistor runs through all three resistors in the package (R1, R2, & R3). The LO blower even runs with the mode lever set to OFF.
Since LO works I'd guess the resistors and the wiring between the resistors and the blower are OK.

The blower relay is likely OK...
Since HI works, when you can find the sweet spot on the blower speed switch, and LO works all the time we can assume the blower relay is doing its' job.

The blower speed resistor is mounted through the AC box in the engine compartment. It will have a T shaped 4 position Delphi-Packard 56 plug with Yellow, Light-Blue, Dark Blue, and Tan wires.

Setting the blower switch to M1 or M2 is supposed to pass power to the blower resistor M1 & M2 positions.

The first thing to check... Power should route to the blower switch plug on the Brown/White wire from the mode control levers in every position except OFF.
Does the Brown/White wire on the blower speed switch have 12V when the mode lever is in any position besides OFF? I'd check em all.

Next thing I'd do is test the blower speed switch with a meter set to the continuity buzzer.
The blower speed switch contacts are arranged in a T with Common and HI in the top of the T and M1,M2 in the upright leg... See below.
-----------------
Common HI

M1
M2

-----------------
Set the switch on HI. The meter should buzz between Common and HI. Set to M2 and buzz between M2 & Common. Set to M1 and buzz from M1 to Common.
When I've gotten bad "New" parts I'm not shy about testing the next supposedly OK part at the store... Get a Standard Motor Products or AC Delco blower speed switch and I bet the problem goes away.

[TrailerTrash]

thats alot to process but thanks for the reply.

im not putting my money on the switch because ive tried 2 used ones and the brand new one and all 3 have the same issue. buuut it still could be the switch im not ruling it out.

i did test for voltage when i had it apart earlier. of the 4 wires that hook up to the speed switch only 2 had power(i forget which ones) but it was only 5.7volts or so. not 12. if that makes sense.

and you lost me about checking the switch with a buzzer lol

[hatzie]

Scroll to the 10:12 minute mark. This is what I meant by buzzing it.

http://www.youtube.com/watch?v=InJhgwmj2So

If the two wires are the TAN and LIGHT-BLUE wire 5V-6V is probably about right.

You should have 12v on the BROWN/WHITE wire to the blower speed switch when the ignition is on and any position on the mode selector lever except off.

I assume the HTR fuse is good...

[TrailerTrash]

i got ya now. ill check it out and post my findings.

thanks dude

[TrailerTrash]

ok my cheapo multimeter doesnt have the buzzer and i cant figure out how to test the switch the other way in the video...

but i do have the results of the voltage test on the wires.

these are the wires that go to the switch.

tan = 4.5v
light blue = 6v

brown/white wire. it gets a little funky here
blower speed postions
1(lowest) = 10.6v
2 = 6.1v
3 = 4.2v
4 = 11.5v

those are the same readings no matter which mode the heater is on and it was exactly the same when i did it with my current switch AND a known working one from my other truck. the 11.5v on the 4th setting is only when i get the switch in the sweet spot. if it isnt in the sweet spot i get no power at all

im just as stumped as when i started this thread

[hatzie]

It sounds like the mode selector switch is borked or it's not getting full power from the fuse panel on the Solid Brown wire #50.
You should have full battery voltage at three places in the AC wiring... The wires for circuits 2 , 50 and,64.
The Brown/White wire #64 should have full battery voltage when the mode slider is in any position except OFF regardless of the fan switch position.
#64 Brown/White is just a short chunk of wire between the mode selector panel and the fan switch. #64 gets power through the mode selector switch.
The mode selector switch gets power from the fuse panel on the Brown wire #50.
You can follow the power on the 1979 AC wiring diagram below.

[TrailerTrash]

alright cool. im grateful for all your help so far man!

ill check it out as soon as i get a chance

[hatzie]

In 1979 the fan runs on low whenever the igntion is switched on.
The Yellow circuit 51 wire that goes to the terminal marked BAT on the resistor in the wiring diagram should have full 12-14.5V power as long as the ignition is on. It's connected directly to the circuit 50 brown wire from the fuse panel at the mode switch electrical plug. This is the low speed fan wire.

The mode switch is marked SELECTOR SWITCH on the diagram. It's the heater control slider that controls Defrost through AC and OFF selections.

[TrailerTrash]

well it is most definitely the mode selector switch. ever since i got the truck ive only turned it from defrost to heat once of twice. since i started playin with it to check the voltage and all ive noticed that the speed switch will work fine once i do a bit of jiggling around with the mode switch. and when its on high and i switch the mode switch it will shut off then i gotta play with the speed switch again like before.

so i been chasing a bug that was right in front of me the whole time.

i have a spare HVAC panel that looks to be in good condition ill have to swap it out here soon.

thanks tons for your help dude

[hatzie]

You could try a shot of Caig Deoxit if you can get near the contacts. It might just be crud and dirt from not being moved.

[TrailerTrash]

i didnt even think about that. thats the trick i used in my 77 on the blower speed switch

[riz]

three simple words......... blower motor resistor.

RIZ

[hatzie]

If the resistor was bad then it would only run on High and not at all on Low because High bypasses the resistor pack. It runs on low all the time as it should. Low is power going through all three resistors.

When he plays with the Mode slider the fan comes on in some position other than low... Pretty good indicator of what the problem is.