High speed heater/AC blower fan
 

Father-in-Law just bought a '77 Trans Am. I bought a '77 Pontiac service manual (the official factory manual, not a dumbass Haynes or Chiltons, and not a "reprint") from eBay. I'm pretty sure it's genuine in that it has shop wear, fingerprints, and it smells like carb cleaner--the kind that comes in a five-gallon bucket with a dip basket.

So far as I can tell, the car was originally equipped with a high-speed blower relay, screwed to the heater/AC box in the engine compartment. It appears to be factory-original; with an old and brittle wiring harness that seems to be Genuine GM. The heavy red wire that powers the fastest blower speed via the blower relay was disconnected; some of the wire is missing. This red 12-gauge wire plugs into the high-speed relay at a multi-wire molded plastic connector. The wire is about 2--3 feet long, and ends with one side of a molded plastic connector and a spade terminal. The other part of the wire is either missing or hidden; I can't find it.

Problem is, the factory service manual doesn't show the high-speed relay at all! It also doesn't show the capacitor (for noise suppression???) that is right next to the blower motor under the fender.

Does the factory service manual have a mistake in the wiring diagram???

Where does the red wire from the high speed relay connect to + power??? I'm betting it connects to the alternator + terminal--but I don't know for sure, and I don't want to connect it to always-live power if it should be connected to power through the ignition switch. I'm also wondering if this wire should have it's own in-line fuse or fusible link. I'm guessing it is NOT intended to be protected by the existing fuse in the fusebox--but--I could be wrong.

The wiring under the hood of this car looks like a High School Shop project gone wrong. Unbelievable. I can fix it--but it'd be nice to have ACCURATE wiring diagrams!

Thanks for replies!

High speed blower power does come from the back of the alternator.
To me, wiring on these cars looked like an after thought.
Nothing really clean and neat.

agreed, always live via a fused link to the alt. plus terminal

Thanks. Greatly appreciated. That kinda explains why the harness at the alternator is similarly butchered.

As GM intended it? That surprises me some. I figured the OEM wiring system was entirely reasonable given the cost constraints--It doesn't look any worse to me than any other car of the same time frame. But the car my F-I-L bought has had at least one owner that should have his fingers broken with his own wrenches; he's made a rat's nest out of the wiring. Aided, I suppose, by the fact that the wires are 30+ years old and the insulation is getting brittle.


Glad to get the confirmation on the "always live" part. Circuits that are always live tend to scare me; look how many Fords burnt up because Ford didn't have the sense to un-power the cruise control circuit when the car is shut off. Add a slow brake fluid leak--and--call the fire department.

Fused link? As in "inline fuse" or as in "fusible link wire four gauge sizes smaller than the wire it protects"?

I'm betting on 16 gauge fusible link wire to protect the 12 gauge power feed wire. Simple, easy, and effective especially for short-term overloads like the inrush current draw when the fan is first started.

Thanks again, both.

As in fuse link wire, 4 guages smaller, soldered connections, link 5" long max.

Quote:
Originally Posted by "QUICK-SILVER" View Post
High speed blower power does come from the back of the alternator.

Thanks. Greatly appreciated. That kinda explains why the harness at the alternator is similarly butchered.

There are few if any inline factory fuse holders that havent cracked and broken after 35 years of underhood heat behind the alt. Some still look ok but will fall apart when you try to inspect the fuse. I replaced mine with 30amp circuit breakers as aftermarket fuse holders looked even less heat resistant than the toasted originals.
The factory fan speed switch drops fusebox supplied low amp voltage for the lower speeds and sends direct 12v from the alt to the blowe fan via hi speed blower relay