AC Fan Switch Ground

It has been forever since I have posted here.
Anyway I have had my Pro5 since new in 02. Still driving it with 200K+ miles on it.
I finally figured out what the issue is with the fan switch that it seemed many a while ago were having. The ground connection for the fan switch is not a good one.

I wired in a switch that I had laying around and ran a ground wire to a bracket under the dash and have had zero issues.
In the picture you can see the wires with the yellow lug. One goes to the switch and the other the new ground. The factory connector is the original ground.

Does anybody know where the factory ground goes? I can see that it goes under the drivers side some where but from there I don't know. I would like to fix the factory ground connection so that I don't have to get creative with the wiring unless I have too.
 
I soldered in 1/2" braid to the ground wire and took that to the frame and its all good now. Still a slight blink at position 2, so I will replace the switch.

I was measuring 1.5 ohms from the ground to the negative terminal of the battery, which is way too much for a short wire like that.
With the braid it now measures <0.5 ohms.
 
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Mazda, Protege5
I have replaced my switch twice so far, and the latest one has only been in for about 2 years and it is bad.

The most recent new switch started having problems with position two blinking at the end of the summer last year, and now it only works in position 1, or in between 3 and 4. It is so annoying.


I think I remember a how-to a bunch a years ago where someone looked up the actual ohmic value of each position, removed the connector, and hardwired resisters to the wires.

I might try to find it and do this, as replacing the switches is a pain.
 
Hardwired resistors might or might not work. Since the issue is the ground adding resistance and changing the voltage the ECU reads as being out of spec.
There was also a thread on a forum where somebody added some active electronics to fool the ECU but this didn't address the actual issue of the bad ground.

You could use some standard crimp connectors and jumpers and a "T" for the ground wire and test it out. That's what I did originally to lead me to this permanent solution. You still might have to change the switch if its too damaged. Though it still might work as well.
 
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Mazda, Protege5
I am not doubting the ground issue at all, but according to many things I have read in the past, the issues had to do with the ohmic value changing of the resisters themselves.

Of course, if the guide I saw before if they hardwired a ground, instead of using the existing one, then I guess that could have fixed the issue like in your example above.

Maybe it is a combination of the ground and the resister going bad? Because if it was just a grounding issue, then a new switch probably wouldn't (temporally) fix the issue.

Either way, this switch is annoying, and of course when one needs it fixed, it is crazy hot outside. Maybe I will just deal with mine until the fall.
 
If its the blower motor resistors, you can get a new resistor pack and swap that out. Would be easy and cheap to do. I did this and it didn't change anything for the operation of the AC.
By hard wiring resistors you reduce the slight parasitic resistance for the wire and connectors from the switch to the resistors and back.
There was another fix where the person crimped and soldered the wires directly to the switch which again helps but doesn't fix the main issue.
The switch goes bad due to arcing and heat (from current) through the switch.

Run your AC on 4 and see how hot that ground (black) wire gets.
It would actually be interesting to measure the resistance from the switch connector to the negative on the battery to see what it measures.
 

pcb

The Diagram Dude
:
2002 MP5
The switch goes bad due to arcing and heat (from current) through the switch...

^^^ what he said...
That switch is only good to handle about one amp.
It gets hot and conducts the heat up the wires.





I still had continuity through the switch so I wired it up to relays, reducing the current through the switch to 20 milliamps.

 
I would actually be interested to see what resistance to negative on the battery is for several owners.
Before I did the mod it was >1.5 ohms, after <0.5.
If I can I will try to get a picture of the ground braid that I installed when I install the new bezel.
 
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Mazda, Protege5
I would actually be interested to see what resistance to negative on the battery is for several owners.
Before I did the mod it was >1.5 ohms, after <0.5.
If I can I will try to get a picture of the ground braid that I installed when I install the new bezel.

If I get some time, I will check it out. I am curious.

If a new ground and a new switch fixes the issues, that would make be very happy.
 
So after a couple of weeks it seems to be working ok. Though I have noticed the #4 wire still getting quite warm. I might try to do something similar to what PCB above did with the relays. It would be really nice to mount them on a PCB with a plug so that I dont have to worry about cutting and soldering wires.
 

pcb

The Diagram Dude
:
2002 MP5
I would worry about the circuit board having too much current going through it. It's normally just a thin film of copper and those breadboard things can't handle much current either.

However,... You can just buy relays that come with the connector that just plugs in. Then just twist the wires together and maybe use marr connectors if you don't want to solder.

 
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protege5
I dunno. I live in the south and use my AC pretty much year round. If i am in the car, it is on. I did the resistor trick in another thread ("Permanently fix the flickering AC" or whatever it is called). First i took the switch apart, and cleaned the contacts. that worked for a couple months but failed again. So I did the resistor trick. Still works like a champ and zero issues, no melted connectors, anything bad about it. That was a couple years ago, and probably about 70k miles in those couple years.
 

pcb

The Diagram Dude
:
2002 MP5
I dunno. I live in the south and use my AC pretty much year round. If i am in the car, it is on. I did the resistor trick in another thread ("Permanently fix the flickering AC" or whatever it is called). First i took the switch apart, and cleaned the contacts. that worked for a couple months but failed again. So I did the resistor trick. Still works like a champ and zero issues, no melted connectors, anything bad about it. That was a couple years ago, and probably about 70k miles in those couple years.

So I guess you changed your mind ??...

Clearly the original poster is of an engineering background, a well thought up work around. However out of curiousity, of those of you who have done this how has it held up long term? I'm sure the AC still works, basically the "acceptable range" for the compressor to engage after voltage drop, and understand that completely. So the AC light won't flicker anymore. In the engineering realm i understand what this does and why it works.

However the root cause that i've found real world (which doesn't always mesh with the engineering world) is that the contacts inside the fan switch are very thin, and the surface area of the contact point itself leaves a bit to be desired. When clean, everything works great. However over time dirt accumulates in there, which adds resistance to the circuit. This added resistance inside the fan switch itself is why the voltage drops as much as it does, to put the "AC Sense" circuit out of range, and turning off the compressor (AC light turns off). resistance adds heat, and enough resistance/heat melts connectors and wires. Normally, if the AC quits working, you'll stop and choose a different fan speed that DOES work. You don't LEAVE it on the setting that doesn't work (and if you do, you've likely got damaged/melted wires/connectors behind the fan switch).

While this work-around does increase the operating range of the AC sense circuit, and the AC doesn't cut out anymore, you're not fixing the increased resistance through the switch itself, which in many cases melts connectors/wires/etc at the back of the fan switch. So i'm curious to some of you who have done this, hows it work out long term? Any signs of melting parts because you're basically ignoring that it normally "wouldn't" work and forcing it to work anyway.

I still don't get how the resistor trick prevents melting ??

You're still getting almost 12 amps through that switch.

Low resistance + High current = High power.

Originally Posted by pcb

"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."



Even the horn has a relay... I'm sure it doesn't draw anywhere near 12 amps.

I like my bank of relays.. They're rated 40 amps...
Our fan switch shouldn't be rated more than 1amp.
 
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protege5
Yes.. I changed my mind lol. I live on a very dusty dirt road. I took the switch apart, cleaned it and it worked great for a couple months then started failing again. So i cleaned it again, its as if I never touched it to begin with. So, desperate for my AC to work correctly, I wired up the resistor hack. It's worked like a champ ever since. The blower resistor should be taking on all of the heat. You could measure with a meter but I am sure the switch itself is more than capable of handling the current demand. it's a solid copper bus bar, not a stranded wire. just like your ignition switch.
 
If i get a chance I will post the wiring diagram for the fan switch tonight.
The ground point is the high resistance point that was putting the AC sense line out of spec.....i think. But since I cannot find it and look at it directly I cannot say 100%.

What got me down this path was the over heated wires and somebody elses proposed fix where they used a couple of transistors to force a correct sense voltage. I saw this as a case of fixing the symptoms and not finding the source of the problem.
I am not familiar with the resistor fix in detail so I cannot really comment on exactly what is going on. Maybe its a degrading connection between the resistor pack and the switch that is actually causing the issue. They use higher wattage resistors and basically force cool them with moving air so they don't pop.


pb4ugotobed: This added resistance inside the fan switch itself is why the voltage drops as much as it does, to put the "AC Sense" circuit out of range, and turning off the compressor (AC light turns off).
I don't completely disagree with this statement, yes the resistance between the switch contacts goes up as arcing occurs and carbon builds up. I still think the bad ground connection is more of a culprit vs the switch. It would essentially add another small resistor between the switch and ground moving all of the voltages from the switch up. I could be wrong though, kind of comes with engineering.

I think I mentioned above that while on 4 the ground lead still gets warmer than I would like. And this is with everything working correctly. I am not familiar with the resistor mod. I wonder what it actually changes in the circuit to make things work vs what the factory has.


Looking at the data sheet for a quick connect wire terminal, I cannot find what I think is a reasonable current rating (only example is 250A), its more of a max operating temperature (221f). This is the type of connector used in the factory switch. I was looking to see if I could possibly back out a current rating of the switch itself.

You can get high currents through a PCB easy enough 10A but the trace needs to be 300+mil wide.

In the end there might be different solutions, but I am trying to find the source of the problem.
 
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Yes.. I changed my mind lol. I live on a very dusty dirt road. I took the switch apart, cleaned it and it worked great for a couple months then started failing again. So i cleaned it again, its as if I never touched it to begin with. So, desperate for my AC to work correctly, I wired up the resistor hack. It's worked like a champ ever since. The blower resistor should be taking on all of the heat. You could measure with a meter but I am sure the switch itself is more than capable of handling the current demand. it's a solid copper bus bar, not a stranded wire. just like your ignition switch.

https://www.mazdas247.com/forum/sho...the-quot-no-A-C-on-fan-speed-2-3-quot-problem

Is that the resistor mod that you did? This just forces the voltage on the sense wire to be within range. This doesn't fix the problem itself.

With my original switch and creating a better ground, I was able to get 3 to work all the time and 2 to work some times. I haven't opened the switch to see what the condition of the contacts are in, so I imagine that's part of the issue that I was experiencing after getting a better ground. With the new switch it works 99.9% of the time. I had one hiccup on 2 when the inside of the car was really hot.

I suspect that there are other connection points etc that might not be the best, and being a budget oriented car I would not be surprised.
 

pcb

The Diagram Dude
:
2002 MP5
https://www.mazdas247.com/forum/sho...the-quot-no-A-C-on-fan-speed-2-3-quot-problem

Is that the resistor mod that you did? This just forces the voltage on the sense wire to be within range. This doesn't fix the problem itself.

Yea that's the resistor mod. And I don't like it either... Everything still gets hot.

My relay pack has been working great for over 6 years now.

BTW... Mazda did upgrade the switch in later models. It's a bit beefier inside and works a bit better.
I'm pretty sure a replacement switch from Mazda would be the better one.
 
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protege5
https://www.mazdas247.com/forum/sho...the-quot-no-A-C-on-fan-speed-2-3-quot-problem

Is that the resistor mod that you did? This just forces the voltage on the sense wire to be within range. This doesn't fix the problem itself.

With my original switch and creating a better ground, I was able to get 3 to work all the time and 2 to work some times. I haven't opened the switch to see what the condition of the contacts are in, so I imagine that's part of the issue that I was experiencing after getting a better ground. With the new switch it works 99.9% of the time. I had one hiccup on 2 when the inside of the car was really hot.

I suspect that there are other connection points etc that might not be the best, and being a budget oriented car I would not be surprised.

Yes that's what i did.

if it breaks again in the future i'll look further into it then. but it's been working great for 2 years now like that with zero issues. ultimately, i don't really care as long as it works lol. it's an old car with 260k miles on it. as long as it's blowing cold and the car runs/drives i have no plans to change anything unless it breaks :)
 
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