Finished up the C4 blower motor swap tonight. Pretty happy with the results. I found quite a bit of information about this swap here on the forums - some accurate, some not at all accurate, so I thought I would share some info while it's fresh in my brain. Hope this can help save someone else from the delays and frustrations I ran into.

While this may seem like a big project due to the information below, it's really not, I'm just sharing those odd details of the job that people seem to forget or omit when talking about it.

The advantage of swapping to a C4 blower motor is that the "squirrel cage" has more blades and they are at a steeper angle, so it moves more air making your AC and heat systems more efficient.

I used an '84 C4 blower motor and "squirrel cage" for this project. The '84 blower motor is a single pole connector and a ground tab on one mounting screw just like the stock C3 fan, so no wiring hacks are needed. (If you can find an '83 Corvette blower motor, buy it, because they are very rare )
FYI - Regardless of what you read here on the forums, the C4 cage will not work on a C3 fan motor due to the depth / shape of the C4 cage. The C4 fan motor has a unique shape to accommodate the cage. Because of this, don't order just a C4 cage and expect it to work on your C3 fan motor. Tried this and it's a no-go.

Since I ordered the cage first and then ended up picking up a blower motor after the fact, I'll list the part numbers that I used here:
Cage / Wheel: ACDelco 15-8418
Blower Motor: Murray PM140

Spacer: Since the C4 cage is longer than the stock C3 cage, it sits too deep into the evap core case, and would rub on the firewall. Because of this, you need to make a spacer which mounts between the fan motor and the evap core case. I saw a BIG variety of recommendations on this from 1/2" to 2" in thickness.

• What I found to be accurate is a 1" thick spacer which put the fan at exactly the same depth as the stock C3 fan.
• For the spacer, I ordered a 1" thick plastic chef style cutting board from Amazon for $18 and cut the spacer from that. Worked great. Easy to cut, shape, drill, sand and paint.
• I chose to mount the spacer using the stock fan mounting holes (just get longer screws), and then offset the fan a few degrees on the spacer, and used separate screws to mount the fan into the spacer. This way, I can replace the fan motor if needed without having to remove and then re-align the spacer in the very limited space we have to work with under the passenger fender.

Fan motor cooling tube: (FYI - Yes, you need to have this cooling tube installed or you'll cook the fan motor quickly due to overheating)
I ran into two issues with this and ended up purchasing a used C4 cooling tube off of Ebay. I used this in combination with the stock C3 cooling tube.

• First issue - the hole in the C4 blower motor where the cooling tube snaps into place is smaller than a C3 so the C3 cooling tube won't attach to the blower motor.
• Second issue is it is clocked differently. The C3 hole is in the 6 oclock position, and the C4 is in the 9 oclock position, so the C3 tube won't reach the new location.

I used the C3 cooling tube for the evap core case end, and the C4 tube for the blower motor end. (Cut the unused ends from each tube and slip them together - see photo)




If you ever decide to go back to original, just remove the C4 fan and spacer and put a standard C3 fan and a stock cooling tube back in place.

Hope this helps!

Adam

 

Thank you!!
Nicely done; including the experienced insights on your calling out the need for a 1" spacer and your creative source (plastic cutting board) for it.

Dave V.

 

Why did you need to do this? Was it that the original C3 blower wasn’t strong enough? I hadn’t heard of that being an issue before...

 

The C4 fan puts out noticeably more air than the stock c3 fan. I did the mod on mine and would do it again. Not very hard, and completely reversible if someone else wanted to undo what I did.

 

The purpose is to gain an improvement on the somewhat weak AC systems in the C3. I'm in Arizona, so anything that can be done to make it more comfortable in the summer months is a good thing.

While this is not a particularly difficult project, I did run into quite a bit of incorrect information while researching how to do it, and that led to several delays and parts purchased that I could not use. That was the main reason I wanted to post this up - so others could avoid the same problems I had.

As mentioned, the car can be put back to original if needed.

 

I wasn’t complaining at all, I think it’s great you shared your experience and knowledge... In a normal temperate climate the stock C3 blower is good enough?

 

No worries Dave - didn't take it that way. I'm sure it's just a matter of preference on what is acceptable to you. If you're comfortable driving your car that way it is it's all good. You know the old saying "If it isn't broke, don't mess with it."

Part of this decision for me is that fact that we're putting in a completely new factory AC system front to back and since I had the evap case out of the car, now was the time to get it done.

 

Very good info...

thanks for taking time to post up as I'm at the beginning of the update process on my '73 C3 AC system.

mardyn

 

Very nice write up and very clever creation of the spacer and the cooling tube!!

This subject has always confused me, as the C3's -- when NEW -- blew plenty of air. In fact, it was very rarely required to have the blower set to HIGH position, because it has so much airflow. But, I know that, with age, things begin to change. I have always suspected that the major contributors to the "low airflow" syndrome were the following:

1. Leaves & debris collected in the air plenum so that it blocked most of the air the blower was TRYING to push;
2. Bad grounding of the blower motor. The grounding is accomplished with a simple metal tab the is assembled under one of the blower mounting screws. If the various metal interfaces between the wire and the blower housing are dirty, painted, corroded, etc, adequate current will not reach the motor. ;
3. The original blower motor has been replaced by a cheap, poorly designed replacement motor...which does NOT have the capabilities of the original motor.

But, that's just my "suspicions". If someone feels it necessary to go to the C4 blower motor 'fix', your write up is certainly a good model to use.

 

Hey 7T1 - I think you may be right on some of the reasons for poor performance of the blower motor. Especially #3 given the poor overall quality of replacement parts. When I bench tested the two fans, I will say that I was surprised at how much more air the C4 fan moves. Here is AZ at 110 plus I'll take every bit I can get.

Adam

 

And I suspect that the original C3 blower motor was about as powerful as the C4 motor. I have had both the C3 and C4 cars when new and I don't have any recollection of any deficiency of airflow in either one.

 

Very possible. I never did look at the amps etc. The difference I could see is in the cage. It's an inch longer and has more blades at a much steeper pitch. Definitely moves more air.

 

i had an old accord. fan sounded funny. dropped it and pulled a mouse nest out of the squirrel cage. damn rodents took it literally. anyway, it may be a good idea to stick a shop vac hose in the hole when the blower is removed and see what leaves leave... and i think the nice thing about the plastic spacer. it won't look like a bubbafied piece of wood!

 

The difference that I've noticed is that the motor windings are substantially smaller in the repro parts. Smaller wire and a LOT less of it. I just think it's "puny" compared to the original motor.

If that is really the case, it would make sense to buy a 'real', over-the-counter GM blower for a C4 (if you can still get them at dealer service parts), then do what is necessary to marry it up to the C3 housing. I'm pretty sure that you can't buy any new OEM C3 blower motors.

 

Great info Adam.
Thanks for the share
We're also in the triple digits here pretty regularly. I've been stalling on this upgrade for my '77 Cheyenne, but after reading your post, I'll do it this week.
Let me ask you this, did you just order a blower motor from your local parts store or did you buy it from one of the Corvette vendors?
Greg

 

Hey Greg,

Nice truck! Hopefully this will help a little, although I don't know if the fan from your truck is the same as the original C3 when referencing the cooling tube location and fan depth. You'll want to check the depth of the squirrel cage before you make the spacer. The fan motor and cage I used is just a new one I picked up at a local parts store - nothing fancy.

Good luck with your project,
Adam

 

Nice write up Adam. Two items that were new to me, the offset of the spacer and the requirement for 5he cooling tube. Well done

 

Hey Derek - that was my thought on the spacer as well. I didn't want wood for a couple of reasons. 1 - IMO wood belongs on the side of a woody, not in a Corvette and 2 - wood will deteriorate, hold moisture and warp when it gets wet. The plastic spacer should be good for years.

 

7T1 mentioned in a previous post about debris in the plenum. This is definitely an issue with cars from this period. Mostly due to the fresh air door on the passenger side of the firewall.

Thought I'd share a photo of what I found when I took apart the evap case. To be fair, the fan motor never did work in our '77 because of bad wiring, so I don't know the effect of all this debris, but it certainly couldn't have helped anything.

The orange silicone was a previous owners solution to mounting the fan.

 

The 'fresh air intake' door is under the right side of the windshield cowling. It is supposed to close, when the engine (and manifold vacuum) is OFF. But, that door can freeze-up in the 'open' state and continually allow airborne "junk" to enter the HVAC plenum. And, over the years of just driving the car (when that door is ALWAYS open), seeds, leaves, pollen, etc. all get captured by coming in that door--and never leaving. If you have NEVER cleaned out that plenum area, yours NEEDS to be cleaned. Just sayin'.....