blower motor

[SSChevy]

i have a 78 T/A auto with A/C  (and T-Tops, just sayin, lol). I bought "ALL" A/C related NEW harnesses from American Autowire and thought that would get my Blower Motor spinning, it hasn't yet worked. Q. shouldn't the new motor I bought spin, when I jump it to a 12vdc source while grounding the case? It wont spin this is my 3rd blower motor. I just want to know I have a good motor before I expect it to work with the cars system.

YouTube channel is: ""Jeff's real estate diary"", look at my install here, ""1978 Trans Am A/C wiring harness, every component location show by name"".

[5th T/A]

12 volts to the spade lug connection on the blower and grounding the blower housing will give max blower speed. Without even installing the new blower motor you could do the same test with a set of jumper cables.

Hard to believe you had three bad blower motors.

[SSChevy]

thank you 5th T/A I put 12vdc on the new/3rd motor and it spun right up. I also did 12vdc to the motor in the car but no spin took place.
Tomorrow I'm going to Ground the one in the car better and see if that will make it spin.
If it still doesn't spin I'll put the new motor in and hope it gives three speeds and goes off as asked by the Fan switch on the dash.
Thanks for the reassurance that direct voltage will make it spin, I thought it needed a capacitor. Now I know it doesn't.
I will post a YouTube video showing the whole diagnoses resolve.

[5th T/A]

You could be on the right track by not having a good ground. This can easily be checked with a DVM. Measuring from the positive battery lead to you blower ground will read approximately 12.5 volts with engine not running. If your meter says 0 VDC, you don't have a good ground.

If this is an AC car it will have four blower speeds. The lower three speeds get their power from the blower switch that connects to a resister pack that is installed in and cooled by the (suitcase) evaporator housing. The resistors reduce the voltage to the blower causing it to run at reduced speeds. High/max speed comes from a relay on top of the evaporator housing. While the relay is energized by the blower switch all the current comes from the alternator output post, essentially the same electrically as the battery. These relays were famous for failing, but usually the symptom is no high blower speed.

[Wallington]

I'll check out the YT Diary later, cheers.

Not sure if it applies here if fully replaced with new harness which is meant to be the same, but often when guys remove their A/C gear then complain the fan doesn't work is because the ground wire for the fan is in the harness they just removed, and mounts on the compressor bracket, they also just removed.

Having said that, and not knowing much else, check that this ground wire exists, normally mounts on locating bolt for compressor in brackets. Also, if A/C has been replaced or 'upgraded', check that these aren't painted up or alloy, or a bad grounding. If you find the wire, run another wire off it to a better ground just to test. Otherwise, can start looking at the positive feed side of things, and if fan is dead on all speeds. High speed is full direct power and bypasses the resistors. If you have one and not the other you can pinpoint something, just like you can pinpoint other issues if no power there at all. That is an easy access area, before you try testing power at fan switch. Do you know when it last ran, prior to any changes?

[SSChevy]

ty yes the harness when I got it was a mess of splices and an added relay who knows why? It was not working when I got it. It came with a new Bl Motor thats how I had "two" so I just put it in without testing it first. Found out its not working, so to say, and that is why I now have 3 Bl Motors, lol ty 5th T/A.
Wallington, I didn't think of testing the harness grounds located on the compressor bracket (the screw that holds the dip stick) I had to buy new brackets so yea I may need to scratch some paint off.
Before I scratch paint thou I will Ohm it out. Bat + to the screw holding the GND Terminals should show 12vdc and then the screw to the motor chassis GND.
Thanks guys I did not know haw to check GND like that.
Tomorrow I will post a video of me doing those checks and I studied the A/C Electrical Schematic very hard so I'll hopefully be able to make diagnosis easier for all Trans Am gurus like us. (I hope that's OK to say)?

[SSChevy]

Bl motor spins fine now. I took it out of the suit case/plenum and believe it was mounted crocked in the hole not allowing the cage to spin. it worked fine out of the car. I reinstalled it more toward center, and it works fine with direct 12vdc from the battery.
Next is getting it to work with the controls. I'm thinking Heat and Vent only because the A/C sys isn't charged thus the pressure switch wont pull in.

[Wallington]

The power to the fan is independent of the setting on the slider controls, so even in AC mode, which isn't operational at present, the fan still is. I'd be following the main power feed wire from the fusebox to the controls and where the heater control harness connects in the yellow.

[FormTA]

Others may know this but, do the 78 and earlier cars have the separate high fan circuit? I know my 79 has it and I have it disconnected so I have an actual off position for my switch.  The high setting is now off because even in the off position the fan still runs. I didn't want heat blowing on me in the summer so that is why I have mine disconnected.  I'm just asking because maybe that is one of your problems.

[Wallington]

A/C MAX to high-speed fan? Yes, has that on my 78. It bypasses the fan switch setting, which can be on low. Or was it the other way around, high speed only hooks up in A/C Max. Can't recall, one of them overrides.

[5th T/A]

For many years GM used a high blower relay, if my memory is correct this was on AC equipped cars only. They used a four-speed blower, with no off option. This provided a small amount of fresh air circulating through the car whenever the ignition was on, even if the mode was switched to off. If the mode selector is not in the off position then the fan switch selects the fan speed. When the mode selector is set to MAX AC, the vacuum servo in the passenger foot well closes a door/vent and for the most part limits the fresh air intake and the blower is recirculating the passenger compartment Air Conditioned air. I believe in all other modes the fresh air door is open.

As mentioned before the blower motor gets its power from the blower speed selector switch routed through resistors for the lower three speeds. For the max blower speed the fan switch energizes the high blower relay and the 12 volts comes directly from the alternator/battery. This max blower speed is the same for all modes except Off. Somewhere I recall seeing a chart explaining some of this. I will see if I can dig it up.


IMG_2797 by Lawrence Alexander, on Flickr

[jbanna]

OMG, I had this problem more than once in the past - one time it was the blower motor, and the second time after much blood, sweat, and tears troubleshooting the relays, resistors, dash switches, etc., it turned out to be a broken wire between the alternator and the relay on the a/c housing...I still had the three low-speed settings, but it cut out on the high speed setting - I had read about a fusible link there, but I cut that out so long ago I didn't remember, so finding the broken wire was a Eureka moment!