How do you use 6V Wipers in a 12V Truck?

[Gordon Maney]

My '48 has a 6 volt heater and when I converted it to 12 volts I got a dropping resistor from J. C. Whitney.

[Jerry Henry]

My '48 has a 6 volt heater and when I converted it to 12 volts I got a dropping resistor from J. C. Whitney.

Gordon. I think the heater fan motor will run okay with the resistor (obviously, because you are doing it), but some have had trouble running the wipers on the resistor. Apparently they draw more under load, and need more amps than a resistor can supply?

[W_A_Watson_II]

Jerry,

I had to put a new motor in the heater, so I installed a new 12 volt one, just not that easy of a sway for the wiper motors.

Will

[Jerry Henry]

I wonder if an electric motor shop could convert it to 12v?
We used to have an old guy here in town that could do almost anything with an electric motor. But he's been dead for 15 years or more.

[Gordon Maney]

Gordon. I think the heater fan motor will run okay with the resistor (obviously, because you are doing it), but some have had trouble running the wipers on the resistor. Apparently they draw more under load, and need more amps than a resistor can supply?

A generic dropping resistor is sized for one current draw. It's a guess on someone's part as to what current draw your load presents. Meaning, lots of times it will be wrong. To what degree we don't know.

You really need a different capacity resistor for each load of differing value. The resistor must also have a minimum power rating, wattage rating, or you will burn it up.

If the dropping resistor is properly sized there is no reason why you could not run wiper motors. I was just on the phone with my electrical engineer friend, discussing some calculations. I would go on at length now, but I am jammed against a printing deadline and have to leave you for now.

I will go over some of that some other night.

[Gordon Maney]

I wonder if an electric motor shop could convert it to 12v?
We used to have an old guy here in town that could do almost anything with an electric motor. But he's been dead for 15 years or more.

Yes, rewinding one for a different voltage is technically possible. Finding a shop interested and qualified is likely the problem, and then what would they charge?

I believe American Bosch made both 6 and 12 volt wiper motors for these trucks. A better solution than rewinding might just be buying the right motors.

[75Crew]

Resistors with variable loads

Using a resistor to drop the voltage on a fixed load is fairly simple as long as the resistor is rated for the power (watts) that it needs to dissipate and the resistance (ohms) is calculated to produce the needed voltage drop based on the expected load current (Ohm's Law). The problem comes in with a variable load, which is what a windshield wiper motor represents. Depending on the friction of the blades against the windshield, the amount of snow it is trying to push, ice...you get the picture, the load is quite variable. If you were to put an ammeter on the motor during these varying conditions, you would see the load swing quite a bit which means that the voltage drop across the resistor, which changes with the current, will also vary quite a bit. One way to solve this problem is to install a matched pair of 6-volt batteries in series (instead of the standard 12 volt battery) so that, to the charging system and 12 volt loads, the battery appears as a single 12 volt unit. The 6 volt loads need to be connected (with appropriate fusing) to the midpoint connection (where the positive terminal of the first battery connects to the negative terminal of the second battery) to operate properly. Both batteries stay charged this way but you can run both 6 volt and 12 volt loads without the need for dropping resistors. (Consider that this used to be done by connecting the 6 volt load to a screw that had been inserted into the middle connecting bar of a 12 volt battery, back in the day when the connecting bars were exposed on top of the battery case. After all, a 12 volt battery is just 6 cells connected together in series...and a 6 volt battery is just 3 cells connected together in series. So connecting two 6 volt batteries in series is not an issue if the cells are of comparable capacity to permit equal charging by the alternator or generator. The biggest issue is the cost of two batteries and the available space to mount them.) If anyone is interested in considering this further, I can post a wiring diagram showing how this setup would need to be connected.

[Gordon Maney]

That's a great idea. I can remember when people used to center tap batteries, which was easy on a tar top battery.

[PWFFnut]

Will,I have the same problem as you, converted to 12v, and changed all the bulbs, put in new 12v gauges, but I am left with the heater and the wipers problem.
I know this sounds a bit extreme, but I am considering locating a small 6 volt battery somewhere on the truck simply to power the heater and the wipers. Obviously it would need to be charged at home, but I won't be driving long distances in the winter anyway.

What do you think of that idea? Jerry

Not sure where I saw the ad maybe My classic car 12v volt battery with a 6 volt tap

[wayneh]

So as new guy here, and if I may add a couple of ideas from an old school perspective that I learned in the dark ages...

There is an old formula called "ohms law" that gives the relationship to current, voltage accross the resistor, and the resistance measured in ohms.

So if you have: 2.0 Amps at 7.5 Volts on one motor and
2.5 Amps at 7.4 Volts on the other motor

You can figure out how many ohms you need to drop from 13.8 volts to 6 volts.

So... Ohms law says the resistance (R) is equal to the voltage drop (E) divided by the current flow (I). R=E/I

Since you want to drop about 7 volts and the current is about 2.0 amps, then you need a (7/2) or 3.5 ohm resistor. The reality is 3 or 4 ohms will be fine, the motor won't blow up.

Now the resistor will get warm though. The energy is measured in watts, and that is calculated by the ohms law equation "I squared R" or the current squared times the resistance. In this case, that would be

2.0 amps x 2.0 Amps x 3.5 ohms, or about 14 watts. That is a pretty big resistor and it will get as hot as a 15 watt light bulb...

Now please don't ask me how to rebuild a winch yet, I'm not quite there. But if I had 6 volt motors in a 12 volt system, I'd be trying to figure out how to wire them up in series, since then the voltage accross them will be roughly half the battery voltage.

Wayne

Wayne

[W_A_Watson_II]

Well,

I had installed the wipers and now am installing the wiring. I mounted the 50W 4 ohm resistors, and, well the wipers were real slow.

I measured the voltage, and it was only 4 volts at the wiper, while I had 13.5 volts at the power supply.

Measured the voltage across the resistor and as it should be the other 9.5 volts was there. Oh and yes the resistor sure got got quickly.

Did a little math using Ohms law, and tried to measure the current. Discovered my previous current measurements were bogus, as my current meter is only good for AC voltage.

Based on the math, it looks like the actual wiper motor resistance while working was only 1.68 ohms. If I run the motor on 12 volts, the amperage should be 8 amps at 13.5 volts.

I'm still thinking that the motors were re-built with 12 volt components and no label added to indicate the change, but the seller didn't believe so, but they were re-built and bought back in 2004.

Decided to hook the straight 12V up to it and do a quick test on a 7.5 amp fuse. Well the motor was a little loud, and after a short run the motor was getting warm, so I guess they are indeed 6V units. It never blew the fuse, but apparently I need a lower ohm resistor, perhaps only 2.5 or 3 ohms.

Will

[DavidGB]

12v Battery w/6v center tap

Thought you might find these interesting. http://www.antiqueautobattery.com/accessories.html
DavidGB

[Jerry Henry]

Thought you might find these interesting. http://www.antiqueautobattery.com/accessories.html
DavidGB

This is a very good option, although a little expensive. I called and got a price. $205 plus $35 (ship to Idaho).
Advantage would be simplicity in installation, if you need a battery anyway.

[im02crazy]

Cases grounded?

I keep thinking how simple it might be to run these in series if these units are not case grounded. I don't have a motor available to test so does anyone know (a) if these motors are case grounded, or is the motor winding isolated and gets its ground through a ground line? OR (b) can these motors be made isolated ground - ground strap or bonding wire removed??

I like the center-tapped battery idea and might go that route for a 6v radio I have on hand.

[W_A_Watson_II]

Mine were case grounded, I found out the hard way, accidentally hooked them up backwards the first time. these had both wire terminals, and the one odd ball I have in a parts stack, only has one terminal.