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Fan not working on hi

blazinzuk

blazinzuk

Buzzbox voodoo
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Works just fine on all the other settings. I think I had it working last year but I really don't remember. Just totally shuts off on hi.

Resistor motor thingy?

What is the shiny metal thing that mounts to the AC box. I think I replaced that with another I had a couple years back to get it to work.

Anyway what you guys think?
 
I think that's some sort of relay for it. I'm pretty sure when on "hi" it just passes through the resistor. Typically if it only works on "hi" and nothing else, its the resistor.
 
You have all speeds except high? How about AC on high?
I'd pull out the heat selector out of the dash and check it out.
Seeing you live up in cold, the high was more than likely all the heater control saw. That can burn thru a contact over 20 yrs. of use.
Once out, you'll see a tan colored wheel with contacts that is controlled by the lever on the face. The cover of the wheel pops off and you'll see the copper contacts.
I'm leaning towards the switch contact burned out.
Blower either works or not. Or screams then dies.
I got spare few of the dash controls if you need parts.
 
Resistor pack only comes into play at slower speeds. High is full battery voltage to the fan.
Could be the switch, on my Ford, it would be. But some GMs use a fan relay that bypasses the resistor pack on high.

Of course being a Ford guy, not sure where it is or what vehicles. But if you have a relay looking gadget next to the fan motor, I would be suspicious that it might be it.
What year and model schematic would be most likely to apply? I'll see if mine shows a relay.
 
Well, this is probably worthless. I have a factory shop schematic and trouble shooting manual around here somewhere for a 90, but can't put my hand on it at 1:15 am.

I do have an online schematic I downloaded some time ago, but there is nothing on it to indicate what year.

It seems to have a fan switch by its self, and no computer control module, so I suspect its going to be pretty close to yours.

It shows a high speed only relay.

All my other stuff is too old.

Hope this helps.

0900c1528003db7d.gif
 
It's probably the relay mounted on the side of the air box. It's easier to check than tearing the dash off to look at the switch. Just check for voltage on the two sideways terminals. I've had better luck opening the OE relays and cleaning the contacts than picking up parts store replacements.
 
I totally had the same thing... and it was the relay on the side of the A/C box in the engine bay. Super cheap part; under $10. Heck I'd put money on that being the problem.
 
You guys have just confirmed in my mind what I did last year replaced that and it worked for most of the winter it seemed.

So off to get a new one soon will be so nice to have hi again. Hi heats the whole thing up, med not so much
 
Any idea what those relays are rated for?

Wonder if stepping up to a better one, or rated for more, would last longer? You've gone through two of them, and others have obviously had them fail (albeit how many decades later? lol) maybe going with something a bit better would be worth the work?
 
Of course you can swap a different relay in - it's just a relay - but there are a couple of caveats. First of all, it is a SPDT relay and a lot of automotive relays are SPST. Also, most relays have less current rating in the NC contacts, than the NO and the NC is what allows low and medium speeds to work. It's usually those speeds that fail (i.e the NC contacts) so that you only lose high. So the swap should be as simple as backing the terminals out of the factory socket and putting them into the socket for the relay you choose.

Does anybody have a wiring diagram of the blower/relay/resistor circuit? A SPST relay could work if the switch is only applying power to one leg at a time. I suspect it may be tying multiple lines together, which is why they had to us a DT relay to prevent feeding power back through the switch in HI (HI power would have to move to a switched ignition source).

Anybody know of a 12V SPDT solid state relay?
 
My schematic on post 7 shows why they have to use a DPDT. The fan is common, and the NO and NC are hooked to the resistor pack and +12V respectively.

As for a DC output, DPDT SSR, I don't remember seeing one. DC output SSRs are rare enough as it is, and even in AC, you don't see many DPDT.

Most of the time, when you do, its actually two matched relays tied together.
Plus, you would need some heat sinking, since SSRs tend to run hot.

Heavy duty 12volt mechanical DPDT relays are just too common and cheap to go SSR.
 
I'm all about using "common" things when possible, was thinking along the lines of the fuel pump relay. If it won't work, it won't, but those things are rated for (I believe) 30A. If that's more than the rating of the AC relay, and it is of the proper configuration, that would seem a viable swap, particularly if you already use that style relay for the fuel pump.

Based on the schematic, it would seem that it would?
 
Sort of thread hijack, I hope no one minds. My heater fan was only working on low and high, so I replaced the resistor pack and now I have low, med-low, and high, but not med-high. Would everyone agree that it's most likely the switch in the dash heater control lever?

If so, does anyone have one for a 1991 K5? Or should I take this apart and there's some way to fix it?
 
I've had intermittent operation on LO and MED for years. I have a collection of 3 spare relays, so I've obviously swapped it a few times (granted, some being junkyard pulls). Now many relays have higher current ratings on NO than NC contacts due to the contact force, but these actually have smaller contacts for NO than NC. I can clean the contacts with fine grit paper unit they shine and it works great for a few months, then the lower speeds become intermittent again. So I tried something different a few days ago and shorted across the NO contacts. Reviewing the wiring diagrams, I don't see any problem having the resistor pack tied to the blower motor all the time and letting the relay short it out for HI. Instead of cutting up my wiring, I just modified the relay internally. Works great so far. Will report back if any issues arise.
 
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