Big Blue's HVAC Blower Issues

[FordFETruck]

I just installed a new blower motor in my truck, it helped a bit with speed as the old one was worn out and the bearings were junk but the motor still does not spin as fast as I think it should. Defrost works but it's about half as effective as any other car I have, and the blender works. I guess I should take the connectors apart and clean them?

[ArdWrknTrk]

I'm still wondering about your voltage drop.
It's almost like there's a diode in the circuit...(at least those readings are about what you should see, if there was)

Did you ever locate the mystery ground that doesn't appear in the EVTM?
I don't have a Bullnose to look at.

[ArdWrknTrk]

FordFETruck wrote

The blender works. I guess I should take the connectors apart and clean them?

The blend door works, but are there any other foam joints where air could be leaking out?
You've got to remember these trucks were designed in the 1970's and used blow or roto moulded parts with more thought about how to make them fit and be produced on tools available 45 years ago.

Smooth flow and velocity were more of an afterthought.
There's no doubt that NVH have come a LONG way since then.

[Gary Lewis]

Right, looking at the ductwork they didn't give much, if any, thought to getting the air to flow smoothly or rapidly.  But I assure you that the wiring, connectors, and switches are a huge part of the problem.  However, fixing that is a pain - unless there are any of the kits left like Jim installed on his truck.

As for there being a diode, I sure don't know where it is.  You are right that the drop I'm seeing is about right for that, but since all the wiring now is mine save for the last few inches of the motor's whip I don't think there is one.  But I also don't understand where a .8V drop is coming from.  

Nor did I find that ground.  I really just ignored it and provided my own.

[FordFETruck]

I have not found any leaks in the system other than the vents themselves but listening to the motor I think it should spin faster especially on high. The reason why I started questioning it, is the heater in my 64 Galaxie, which is an even more crude HVAC system makes the trucks heat/defrost look and feel like a horrible joke both blower fan speed wise and the vent performance. I think a truck that's 22 years newer than that car should be able to keep up with it on defrost.

[Gary Lewis]

If you want to measure the voltage going to the blower motor you might discover why it is blowing so little.  But measuring the voltage isn't all that easy as the connector for the motor doesn't provide any easy access.  I made little jumpers that plug into the female side and have alligator clips to go on the male terminals to give me access.

Or you could just bite the bullet and put relays in.  I'm certain you'll like the results as I brought the voltage up by 50% on mine, which made a huge difference.

[FordFETruck]

I tested the voltage at the motor. The new blower motor has a rubber grommet over the motor end of the connector  and I could get right to the metal part of the connectors. I am getting .5 volt drop from battery to blower motor connector running on high with engine on or off. With blower motor disconnected I only get .1 volt drop through the factory wiring. I also ran jumpers straight from the battery to the blower motor WHILE it was running on high and the motor barely sped up enough to notice a change in sound. Looks like my system does work pretty well since there was barely a RPM difference from factory wiring to jumping straight to the battery.

[Gary Lewis]

Wow!  That's great.  I'm amazed, but happy for you.  

[Gary Lewis]

Time for an update.  As I've been re-wiring the passenger side PDB I was planning to use the 2nd spot in the box for a diode to protect the relay that grounds the blower in high.  But that got me to thinking about how a snubber diode should be used on a motor and I realized I'd implemented the first diode incorrectly.

The first diode, which is actually D2 in the PDB, was wired from the positive lead of the motor to ground.  But, the "low side" of the motor is only truly grounded in high, so if the ground relay isn't closed then the diode is trying to snub the excess current through the resistor pack.  And while that does some snubbing it won't be all that it could be.

So now I'm going to wire the diode from the positive lead to the negative lead of the motor.  So whenever the power relay is cut and the back EMF hits the diode will snub it regardless of the speed setting on the control switch.