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94' Astro AC/Heat blower only works on highest setting.

12-12-2007, 08:12 PM
94 Astro EXT conversion
99K miles
4.3L V6
3rd owner
Totally amature but occasionally capable mechanic.

The front AC/Heat blower only works on the highest setting. Rear works fine at all settings. Seams like after searching that the problem is either the blower control switch in the dash or the blower resistor relay module under the hood.

Which one is more likely?

I took a quick look at the blower switch on the dash and nothing looked terribly bad but I haven't pulled it out and tested it with a multi meter yet. Which pins should I test on this one anyway? There's like 7 or 8 pin outs.

Tomorrow I'm going to locate the blower resistor relay module from directions found here and have a good look at it. I read here that they get corroded and the printed circuits get broken. Or that there's a fuse or thermal resistor that can be a gonner and maybe cause this.

Neither switch or blower resistor relay module seems like a hard or expensive fix but I don't want to order parts I don't have too.

What do you guys think? Which is more likely and how do I test that blower switch with a meter?


12-13-2007, 09:30 AM
By playing the percentages-- the high blower relay is the most likely culprit-- If you told me you have 1 or 2 of the lower speeds still working-- then I would say the blower resistor pack was bad--- the high blower relay will run 12 volts through the resistor pack in its normal state- and divert the 12 volts to directly to the blower in high speed mode-- But since you have NO lower speeds, then I would start at the relay-- to find it- follow the blower motor purple (big) wire backwards to it- it should be hanging on the multi-use relay bracket above the HVAC module.

12-13-2007, 03:35 PM
Thanks for the quick help brcidd. I checked everything today, the resistor module looked good and checked out on the meter as did the switch in the dash. When I finally found the blower relay module I put it on the meter and only got continuity between the unmarked pin and the pin labeled 5 which makes perfect sense. It's got to be bad. I ordered a new one for $22. It took me a while to find it because it had been un-mounted from it's original location on the mounting bracket and wrapped up with electrical tape and shoved under the overhang on the left side of the hood. Once I found it by tracing the purple wire like you said I new in a few minutes it was most likely the problem.

Thanks again for the quick help. Hopefully it'll be fixed as soon as the replacement shows up.

I love the internet.

12-14-2007, 02:41 AM
similar issue... wifes '02 recently lost 'high' on the blower fan. Lower speeds still work. As I understand the explanation above, (and my experience w/ electronics) I think the relay should close in the hi position and allow full battery voltage to the blower, bypassing the resistor. Am I right in this assumption? If so I'll diagnose the relay. I guess I should diagnose something, it's getting cold around here...

12-15-2007, 01:42 AM
From what I've read here that's hot it works. It's really easy to find that relay, pop it off and put the meter on it. If you find it passes current from the unlabeled pin to the 5 pin then it's something else. It makes sense to me at least.


12-15-2007, 02:30 AM
so wheres the relay? Underhood or under dash?

12-15-2007, 11:59 AM
In my 94' it was under the hood on the left hand side to the left of the blower motor housing and it looks like this:

Mine was loose and stuffed under the overhanging body panel on the left but I think it is supposed to be mounted to a mounting bracket over there probably right above the blower motor housing. If you find the blower motor housing right behind the wiper and coolant bottles, find the thicker purple wire and follow it to the module. It'll run through some wiring "hose" or wrap before it re-appears.

12-15-2007, 12:29 PM
Well the new module arrived and I put it on and....nothing. Same problem. It made perfect sense to me that that was the problem. Shows you how much I know.

Any other ideas?

12-15-2007, 10:21 PM
All speeds except high go through the blower resistor. The 3 lower speeds are fused separately from the highest speed. All speeds go through the high blower relay, they use two separate circuits in the relay.

12-16-2007, 11:32 PM
All speeds except high go through the blower resistor. The 3 lower speeds are fused separately from the highest speed. All speeds go through the high blower relay, they use two separate circuits in the relay.
So I checked the fuse in the cab under the drivers side dash and it looked good. Can't find the under hood fuse box you mentioned but from what you said I gathered that fuse handles the high setting anyway (which works fine) am I right? Is it behind the battery? I see a large wiring harness on the firewall but it doesn't look like a fuse box.

Since I replaced the relay module and that didn't fix it I'm thinking about the resistor module that I checked before. Since it handles the lower settings it could be the problem no? I'd meter everything at the pins but my meter won't handle that kind of juice I don't think. It's a cheap-o.

Could that resistor module be spent and I not be able to tell by looking at it? All the coils were intact and nothing burnt or broken or blackened.

What does the under-hood fuse box look like and where can I find it?

12-17-2007, 08:52 AM
No underhood fuse box on a '94 Astro-- that started in '95 models I believe--my '94 has none...

I'd check the wiring terminal connections at the relay and at the blower resistor and at the selector switch inside for burnt, melted, corroded contacts etc- the circuit is not that complicated- so the problem should be easy to find...

12-17-2007, 06:06 PM
brcidd, thanks, missed that.

Funk O Meter,
If high speed works, that means both fuses are ok, the switch is getting battery voltage, and the switch is able to activate the relay. To check the switch and wiring from the switch to the resistor for the low speeds: unplug the resistor, turn the key to run, mode switch on heat, blower speed as follows:

Low speed (1) Brown wire must have battery voltage
Speed (2) Tan wire must have battery voltage
Speed (3) Light blue wire must have battery voltage

If no voltage is present one or more wires, either the switch or the wiring from the switch to the resistor is faulty. If voltage checks out ok, either the resistor, or the dark blue wire to the relay has a problem. Post your results.

12-17-2007, 06:58 PM
Will do. I'm gonna probably take it to a parts place that might meter them for me as my multi-meter is 10amps DC max and unfused. Don't think it'll handle it. I would just buy a better one but 'tis the season to be broke you know.

Or maybe it'll be fine as long as I'm in volt metering mode?

12-17-2007, 07:29 PM
DC volt range should be fine.

01-19-2008, 05:05 PM
Allright finally got a chance to do more trouble shooting since the holiday lunacy has passed. Though weather is persistant at either producing rain or snow whenever I have a moment to spare to work on it. What can you do? I stood in the snow and 10 degree winds with my volt meter under the hood that's what.

Pulled the resistor module wiring harness out. The wires are arranged like this if your looking at it pins on with the odd 4th pin oriented on the bottom.

blue-green brown yellow
dark blue

Here's the results when tested for voltage.

speed wires with voltage

off none
1 yellow
2 yellow,brown
3 yellow,blue-green

I didn't test speed 4 because it's working fine as is. And my hands were numb. I couldn't figure out how to check for continuity between the resistory module wiring harness and the blower motor itself, but I did check for continuity on the dark blue wire that runs from the risistor module wiring harness and the relay module wiring harness. I'ts fine. Only thing I saw that looked a bit suspisious is that the dark blue wire on the resistory pack harness never showed any voltage on any of the speeds I tested. But I don't know what that wire does really so there you go. What does that wire do?

So I'm guessing I now know that:

a)The switch in the cab is fine

b)The wiring between the switch and the resistor module is fine

c)And I've already replaced the relay module so I can assume that isn't the problem

So am I right in assuming:

a)It could be the resistor pack module

b)A wiring problem between the resistor pack and the blower

c)A wiring problem concerning the relay module?

Thanks for the help guys.

01-19-2008, 05:56 PM
The dark blue wire carries speeds 1, 2 and 3 from the resistor to the high blower relay. With the resistor plugged in, ignition in RUN position, mode switch in HEAT position, the dark blue wire should show varying voltage depending on what position the fan switch is in. You can probe the dark blue wire at the high blower relay also, it should show the same results. If there is no voltage, the resistor is faulty.

01-19-2008, 07:04 PM
With the key in the run position and mode switch on heater I removed the relay module and checked for voltage at the relay module wiring harness. The resistor module was still plugged in.

I only got voltage at the big red wire and the smaller orange wire. Voltage at the red wire pin was constant regardless of switch setting and voltage at the orange wire pin was only present when the blower switch was on the highest setting. No voltage was present at any of the other wires including the dark blue one.

If I measured this right then it must be the resistor pack yes?

The resistor pack looks ok visually. I pulled it out and checked for burn marks or corrosion and cleaned it. It was a little corroded but all of the parts looked intact. I suppose it could fail and it not be obvious to the lay-man? There's really no way for me to test the resistor pack with my multi-meter is there?

Let me know what you think, if you concur I'm going to order a new resistor pack module. And hopefully that'll be it. If it isn't I think it's HVAC shop time.

01-19-2008, 10:24 PM
You checked the resistance of the dark blue wire from the resistor to the relay and it is good. No voltage on the dark blue wire at the relay means the resistor is faulty. You can remove the resistor and check each resistor separately if you want. You'll find that the resistor that connects to the terminal that the dark blue wire connects to is open.

01-19-2008, 10:48 PM
Ok, I didn't have a chance to pull the resistor pack back out and check it but I went ahead and ordered another one since I don't think it can be much else. I might pull it tomorrow and hit it with the meter.

Thanks for your quick replies, your a great help. I'll keep with the updates.

01-24-2008, 06:16 PM
New resistor module did the trick. All speeds are blowing now. Thanks so much for all your help guys. Especially Old Master. This forum and it's members are a great resource. I'm sure I'll have some more stuff to post about in the near future.


02-02-2008, 03:54 PM
Update.......two weeks later....

I lost seeds 2 and 3. CRAP!!

I can't figure why. I had already replaced the relay. Now with a new resistor module and it works fine for a couple weeks and now just the last two speeds work. I listened to the motor under the hood and it doesn't sound like it's stuggling at all. comes right on on the last two speeds so I don't suspect it as the problem. Course now I can't be sure it's not going and took the resistor coil for those two speeds with it. I don't know, the motor seams fine. What else could cause that?

I may just leave it be. The last two speeds are all I really need being that on the first three speeds the air blows slightly anyway.



02-02-2008, 04:07 PM
If the two highest speeds work, that means the relay and the resistor, (for the third speed) and all of their circuits are good. I would suspect the motor is drawing too much current and taking out the first two coils in the resistor. Those two coils resist the most current and are likely to be the first to crap out. Also check the blower motor ground connection, high resistance there would shorten the resistor and motor life. Post your results.

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