fact AC genius help needed

[Blazer Steve]

OK guys need some help

Have great working factory ac with exception of high speed. I know the relay is normally the culprit, but it looks new and have tested and it seems to work. I haven't swap it out, was wondering if someone had any other ideas

Also, was there a factory heater core shut off that was controlled by the ac controls? my ac guy says he believes these trucks had one, and I have holes where one would be mounted on the rt inner fender in line with the heater hoses
Thoughts?

[N444TX]

The most common GM high speed blower problem on older cars/trucks is a poor connection at an inline fuse to the high blower relay. Usually melted from high resistance connections.

Many early GM vehicles had no heater control valve. If colder air is desired install a vacuum controlled heater valve and tee into the fresh air door vacuum hose. This will shut off heater coolant in the max position.

What year is your truck.

GM

[Dead Parrot]

The switch in the dash is another suspect. But as mentioned, check the line from the junction block to the blower first. Easier to see and fix.

My 75 never had a heater valve. The holes for the jack and handle might be confused for a valve mounting spot.

[Bandit130]

Originally Posted by Blazer Steve

OK guys need some help

Have great working factory ac with exception of high speed. I know the relay is normally the culprit, but it looks new and have tested and it seems to work. I haven't swap it out, was wondering if someone had any other ideas

Also, was there a factory heater core shut off that was controlled by the ac controls? my ac guy says he believes these trucks had one, and I have holes where one would be mounted on the rt inner fender in line with the heater hoses
Thoughts?

When my high speed blower was not working, this fuse was blown. Not sure if your model has the same fuse in the same location.

[Blazer Steve]

Thanks for the replies... Will check the fuse/wire to the relay! And I will just go with the original idea of installing a shut off valve, since summers in the central valley there is no need for more HEAT!

N444TX My truck is a 73

Dead Parrott I still have all the jack etc assy... These holes are a tad lower on the inner fender

Bandit THX I didnt even think of that fuse

[S10Fan]

Regardless of the AC, very COOL truck!

[Accelo]

Yes nice truck.
I admit I am biased on the color!

Typically the heater blower motors are supplied by two different supplies.
GM got me on that one the first time I worked on a NO High Speed Fan issue.
High speed provide by one circuit and the other speeds by another.

It is very difficult to trouble shoot electrical issues over the internet. It can be done but it requires diligent and preservation. And a lot of work on both ends.
Cheers.

[Just call me Sean]

I used a vacuum actuated heater shutoff valve from a 92 Firebird, but it also required the switch from the climate control. The switch swapped right to my climate control, it was the same except for the vacuum ports. This was on my 85 S10 Blazer though.

[Blazer Steve]

Originally Posted by S10Fan

Regardless of the AC, very COOL truck!

Thanks man!

[Blazer Steve]

Originally Posted by Accelo

Yes nice truck.
I admit I am biased on the color!

Typically the heater blower motors are supplied by two different supplies.
GM got me on that one the first time I worked on a NO High Speed Fan issue.
High speed provide by one circuit and the other speeds by another.

It is very difficult to trouble shoot electrical issues over the internet. It can be done but it requires diligent and preservation. And a lot of work on both ends.
Cheers.

Thanks... Yeah I know the whole separate circuit driving me nuts... Then, if you move the heat selector all way over to cold, that kicks the high circuit as well as the fan switch Its driving me nuts

[temp190]

i had troubles with high speed, mine was the ground above blower on firewall.

[N444TX]

The blower circuit is not complex. Hook up a 12 volt test light and test the light to the battery to verify proper operation. I assume your lower blower speeds are working.

Power on all low speeds goes into the firewall high blower relay through the blue wire (from the blower switch and resister) and to the blower motor through the purple wire. When high is selected power comes from the blower switch to the orange wire on the relay. The orange high blower signal and black ground wire on the relay now energize a magnetic coil in the relay and pull a gate closed. This gate now connects the red hot wire from the firewall junction block to the purple blower wire, supplying full 12+ voltage directly to the blower motor and thus high speed.

Like stated before, the easiest way to hook up a heater valve that requires no driver intervention on any non heater valve GM vacuum control duct vehicle is to install a vacuum controlled heater valve and tee into the fresh air door vacuum lines. On the trucks the hose connects to the under hood plenum vacuum pod which also connects to the vacuum pod at the right kick panel. When max ac is selected you will have recirculated air and no hot water flow. Exactly what is needed for the coldest air.

GM
ASE certified master technician and shop owner
GM

[Blazer Steve]

AWESOME response! I will try that all on Saturday Thanks!!!