The motor, harness and switch are ready for diagnosis.
A friend of ours contacted us recently, seeking help for his problem. He has a 1938 LaSalle with a faulty heater motor. He's spent several hundred dollars in repair attempts so far, with no progress and a great amount of frustration. We told him to drop the motor off and we'd look at it (he lives nearby).
So why not buy a replacement, you might ask? There probably are some old replacements available if one searches hard enough, but the situation is complicated by the fact that the motor is very specialized – translation: expensive. It is a dual-direction motor that is operated by a switch on the dash. The six-position switch not only runs the motor in one direction for the floor heat but also in the reverse direction for defrost.
Six positions? Yes, it has all those positions to accommodate dropping resistors and dual poles for each direction so that the motor can be run slow, medium or fast in both directions. All of this results in the motor having four input wires, two of which go to individual brushes and the others to the field windings. This is all further complicated by the fact that the LaSalle used a positive ground electrical system. That's no big deal, but we had to remember to connect any power source correctly during the entire repair process.
It's all apart and we could see some problems already.
We put the motor on our workbench and made a few quick checks using a meter and a 6-volt battery charger. It didn't respond to any initial testing so we proceeded to the disassembly.
Despite the dual-direction nature of this motor, the fact remains that a motor is a motor. It consists of an outside case with bearings (or bushings, in this instance) mounted in the center ends, field windings, central armature and brush mounts. Wires feed the various components and the typical overall circuit demands that current be fed to the brush and field windings and from there to ground, otherwise nothing will happen.
Once we took the motor apart we found that the wires to the brushes were shorting out. They had originally been connected to brass tabs riveted to an insulated plate, but these had been broken off or removed. The wire connectors were now screwed to the insulator with sheet-metal screws that were long enough for the tips to touch the motor case, hence the short circuit.
The wires to the brushes were originally riveted (red arrows) to this insulated plate. The screws that replaced the rivets shorted out the brushes. We decided to solder the wires and dress them in hot glue to keep them from shorting in the future.
In addition, the screws really didn't hold the connections tightly, which means a high-resistance connection that would surely fail. We opted to remove the screws and solder the wires to the brush leads, followed by dressing the wires away from the spinning armature body and hot-glued in place.
We also found broken insulation on the field wires, very close to the windings. A new wiring harness had been soldered and shrink-wrapped to the wires, kinking them badly inside the motor case. We cut those off and soldered in some new, flexible wires that would dress neatly inside the case and lead out to the harness.
The motor going back together.
The armature shaft was in good shape but didn't spin very smoothly in the bushings. We took a few minutes to polish the shaft ends so that the motor would spin freely. Doing this helped to minimize the current draw necessary to spin the motor, thus helping keep the windings from overheating.
We took some time to correct the damage done to the case by past repair attempts. This involved re-rounding the case halves and sanding the rust off the field winding's steel cage. The work was necessary because of the difficulty we had in taking the motor apart in the first case. Once the work was done we turned to the switch.
Look at the circuit inside the switch carefully. As the wipers — upper left — sweep the contacts the current is reversed to the motor.
The control switch, we found, is a bit of an engineering marvel. It is very cleverly designed to reverse the connections internally so that the motor can run in both directions. Wire-wound resistors on the three speed contacts control the speed. There's also a light bulb in the control knob's mount that illuminates the switch position through an opaque plastic bezel. In all, it's pretty impressive for a 70 year-old design and we realized that we don't give credit to the engineering abilities of those long-gone designers.
We carefully took the switch apart and lightly sanded the oxidation off all the internal contacts. Afterwards we reassembled the switch and tested each position with an ohmmeter before connecting the wiring harness. Using a 6-volt battery charger for our power source, we tested the motor and switch for proper operation.
The old LaSalle now has a working heater/defroster again.