How-to: Permanently fix the "no A/C on fan speed 2/3" problem

V
03 MSP
^You can always use two 100 Ohm resistors in series, but 220 will work fine. I always thought 200 was a standard value...
ohh very nice i assumed i could use 2 100ohms but wasnt sure its been a while since iv toyed with resistors will go back tomarrow and pick up the 220 ohm and finish this project temp hit 92 here in cali thats hot for us lol
 

bazooka joe

No words
Moderator
Contributor
V
2016 Miata Club, 2014 CX5 Touring, Honda 1976 XL 175
awesome! i started ny car the other day (its a winter duty only car now) and i flipped the ac on and was reminded that my fan speeds were not working....i'd do this tomorrow but i'm only using heat these days... good job!
 
V
'13 Mazdaspeed3
awesome! i started ny car the other day (its a winter duty only car now) and i flipped the ac on and was reminded that my fan speeds were not working....i'd do this tomorrow but i'm only using heat these days... good job!
its the factory harness to the AC fan switches. Even the switch itself. I bypassed the barness and just used splitters to get all my fan speeds to work properly. I think i just placed it all tight and i had some wires come loose. Gotta go back in there for it to be a final job. Never will have to worry about it again. So far so good. 4 is pumping cold air. Love that speed.
 

Maxx Mazda

Resident Asshole
Contributor
V
Completely Stock P5
I'm going to do the fix directly on the PCB itself. My car doesn't get any wires hacked and spliced. Has anyone done the PCB resistor replacement? The write up mentions replacing the R48 resistor with a 4.7K ohm one. Can anyone confirm this is correct?
 
I'm going to do the fix directly on the PCB itself. My car doesn't get any wires hacked and spliced. Has anyone done the PCB resistor replacement? The write up mentions replacing the R48 resistor with a 4.7K ohm one. Can anyone confirm this is correct?
I haven't done it, but I'm pretty confident it would work. I'd recommend using a 2K resistor to match the performance of the solution I posted. Here are the simulated results with R48 being replaced with a 2K resistor:



The result is pretty much identical and there's no hacky wiring :)

The resistors on that board are all surface mount, but there's plenty of room to fit a through-hole resistor if you stand it up. Just make sure to glue it in place so it doesn't vibrate itself loose!
 

Maxx Mazda

Resident Asshole
Contributor
V
Completely Stock P5
I haven't done it, but I'm pretty confident it would work. I'd recommend using a 2K resistor to match the performance of the solution I posted. Here are the simulated results with R48 being replaced with a 2K resistor:



The result is pretty much identical and there's no hacky wiring :)

The resistors on that board are all surface mount, but there's plenty of room to fit a through-hole resistor if you stand it up. Just make sure to glue it in place so it doesn't vibrate itself loose!
Will do. I've got limited experience soldering PCB's but I do have some. 2K it is then.
 
V
2003 Mazda Protege5
The easiest thing to fix it is to get the latest revision of the hvac switches that doesnt have these problems out of a late production protege or p5. Imo way easier than wiring of any sort. Took me a half hour drive to a u pick junkyard and 10 bucks and a half hour to install. Works perfect for the last 2 years. They are really not hard to find.
 

Maxx Mazda

Resident Asshole
Contributor
V
Completely Stock P5
Yeah my local yards are littered with Protgs. Still, of I'm gonna go to all the trouble to rip the board out, might as well just modify it to be sure that it's fixed.
 

bazooka joe

No words
Moderator
Contributor
V
2016 Miata Club, 2014 CX5 Touring, Honda 1976 XL 175
how do you know the one from the junkie isn't bad?
 

Astral

who got tha funk?
Contributor
The easiest thing to fix it is to get the latest revision of the hvac switches that doesnt have these problems out of a late production protege or p5. Imo way easier than wiring of any sort. Took me a half hour drive to a u pick junkyard and 10 bucks and a half hour to install. Works perfect for the last 2 years. They are really not hard to find.
You'd have to find some really late production Protege or P5. I have a 2003.0 that was just before the butterfly recall--and I have A/C flicker.

IMO wiring up two resistors is pretty easy and beats ripping the board out.
 
V
02 P5
Doing this to my resonantly bought p5. Does it from time to time on fan 3.

EDIT: Just fixed it. The ac light would flicker from the fan 1 to 2 speed now no more, and so far 3 is sold. Will see how it holds up.

Thanks for the awesome write up! THIS NEEDS TO BE STICKY TO THE HOW TO!
 
Last edited:
This thread is definitely still alive.

My AC would only work solidly on 1, occasionally flicker but usually would be off on 2, and nothing on 3 or 4.
Went out to the local electronics surplus shop and spent $5 on materials. I'd actually bought two of everything but it's working on the first try.

I do notice that the A/C light will be on (and therefore the compressor will run) even if the fan is off. Does anyone else have this issue?

Thanks for a nice write-up and an easy fix.
 

pcb

The Diagram Dude
V
2002 mazda protege 5
I have a bit of an issue with this fix,... I don't see the sense wire as the root cause of the AC problem. I see the fan switch as the root.
I'm sure that everybody's flickering light would be fixed with a brand new fan switch because there is no resistance across the new switch to trip up the sense wire. I think the flickering light should be more of a warning that there is a dirty, worn or bad connection in the fan switch or connector at the back of it. That switch is far too flimsy to handle the 12+ amps that run through it.
As soon as there is a bit of resistance across the connections in the switch, it creates huge amounts of heat that can liquefy the switch and connector at the back of it.
If all you do is "trick" the sense wire then you are probably still cooking your fan switch.
I would suggest taking apart the switch, cleaning and sanding it and checking the connector as well as doing this resistor fix.










There is only one true fix for this issue and I love it:
Now these switches are rated for a good 15 amps:




There are two other threads about this issue:

http://www.mazdas247.com/forum/showthread.php?123678787-Solution-To-Flickering-Fan-Speed-2-Or-3-A-c


http://www.mazdas247.com/forum/showthread.php?123710367-Solution-To-Flickering-Fan-Speed-2-Or-3-A-C-PART-2
 
Last edited:
This thread is definitely still alive.

My AC would only work solidly on 1, occasionally flicker but usually would be off on 2, and nothing on 3 or 4.
Went out to the local electronics surplus shop and spent $5 on materials. I'd actually bought two of everything but it's working on the first try.

I do notice that the A/C light will be on (and therefore the compressor will run) even if the fan is off. Does anyone else have this issue?

Thanks for a nice write-up and an easy fix.
You're half-way there :p

It sounds like you didn't connect the input (the white wire in my pictures). Please re-check your connections.
 
I have a bit of an issue with this fix,... I don't see the sense wire as the root cause of the AC problem. I see the fan switch as the root.
I'm sure that everybody's flickering light would be fixed with a brand new fan switch because there is no resistance across the new switch to trip up the sense wire. I think the flickering light should be more of a warning that there is a dirty, worn or bad connection in the fan switch or connector at the back of it. That switch is far too flimsy to handle the 12+ amps that run through it.
I disagree, the switch is definitely part of the of problem but the system as a whole is under-engineered. Have you read my analysis?

As soon as there is a bit of resistance across the connections in the switch, it creates huge amounts of heat that can liquefy the switch and connector at the back of it.
If all you do is "trick" the sense wire then you are probably still cooking your fan switch.
I would suggest taking apart the switch, cleaning and sanding it and checking the connector as well as doing this resistor fix.
This is a very good suggestion, especially if your fan switch connector is starting to melt. That being said, this fix doesn't put any extra current through fan switch, it just lets you get your A/C back. My fan switch connector looked absolutely fine, there wasn't even any visible corrosion.

There is only one true fix for this issue and I love it:
Now these switches are rated for a good 15 amps:

This is the most ghetto thing I've ever seen on this forum ... I love it :D
 

pcb

The Diagram Dude
V
2002 mazda protege 5
OK,.. I read your analysis and it makes complete sense. (I am reasonably proficient with electronics) However my main concern is still the same. If the internal resistance of the switch (which is where I believe most of the resistance is coming from followed by the connector at the back of it) is enough to produce a 1.1 volt drop, that is 1.1 V x 6.35 amps equals 6.985 watts of heat in the switch for fan speed 2. For fan speed three, 1.1 V x 8.8 amps is 9.68 watts of heat and 1.1 V x 11.75 amps is 12.925 watts for fan speed four.
That is plenty enough heat to start melting things.

http://www.mazdas247.com/forum/showthread.php?123826711-Broken-Heater-HVAC-Replaced-blower-motor-resistor-amp-relay-STILL-NO-AIR!&p=6121332#post6121332

If you crank up the cutoff voltage to 3 v then the heat in watts goes to 19.05 W, 26.4 W, and 35.25 W before the light would start to flicker. That's enough to cook a burrito on the fan switch. (consider how hot a 25 watt brake light bulb gets)

My AC light started to flicker last summer for the first time. I took apart my switch, cleaned and sanded the contacts and all the pins and connectors on the connector plug. That fixed everything and I got to it before the big meltdown. I kinda like the fact that the light flickers at 1.1 volts because it gives me enough warning to clean the switch and contacts again before everything liquefies.
I also noticed that the connector plate inside the switch is kinda spring loaded so I stretched the springs a bit and bent things a bit to help provide more pressure on the contact. I also put a bit of white grease in it.

Let me know if my math is right ,... I know that not all the resistance is in the switch, some is at other connections and some in the wire itself.
 
Last edited:

Latest threads

Latest resources

Top