Heater Moter Resistor

[mikebryan]

I am restoring an August 1968 OTS ( S1.5 ). Along with most of the Dashboard electrics the Heater motor resistor is missing. After almost five years of restoration I an fully aware of how expensive E Type parts are but E120 for a simple resistor seems like a " Rip off" Anyone have any suggestions regarding what other resistor I can use in its place. ?????

Thanks
Mike

[Heuer]

SNGB sell them for £17: http://www.sngbarratt.com/ProductDetail ... de4670&l=7

[MarekH]

It's just a very large wattage wirewound resistor probably of roughly the same resistance as the heater motor coil, something below the 5ohm mark will probably do nicely.

You can probably buy five or six 1ohm resisitors which come in a metal heatsink casing for the same money and wire those up in series and then short them out against each other to tailor the fan speed to your liking, then replace them with a single chunky wirewound one of the same resistance.

(How large wattage do I need I hear you ask? Intercept the wiring with the meter set to amps and record how many amps it is pulling and then do the calculation. Double the answer for a safe margin of error. From eyeballing it I'd guess ~15-20 watts.)

kind regards
Marek

[cactusman]

Try rsdiospares are http://www.rswww.com. they will sell you a 20 watt 4.7 ohm wire wound for less than £17.

[Heuer]

They are about £4 and available from any electronics store such as Amazon, Maplins or Radio Shack if you don't mind the non stock look. You must rivet them to the heater box housing to act as a heat sink though:
https://www.amazon.co.uk/s/ref=nb_sb_no ... w+resistor

[mikebryan]

Thanks guys for all of your imput,really appreciated

Mike

[mikebryan]

Becoming a bit of an ongoing saga with the heater resitor. Purchased the resistor from SNG, riveted to the base of heater, to provide a good heat sink, viola..it works, both on fast and slow.

After a few minutes on slow speed I noticed that the resisistor was glowing, despite the good heat sink. A few minutes later the soldered connections melted. Put this down to my poor soldering ability. After cooling down, resoldered the connections and tried again. Same problem again, after about five minutes, the resistor glowing red hot and the solder connections failed, plus the heater motor was very hot.

Not sure what to do next. I know that the 1968 S1.5 originally had the resistor on the central dashboard section, but as I mentioned in my previous post, meine is missing .

Not sure what to do next, so any help would be really appreciated

thanks again

Mike

[MarekH]

It does somewhat suggest that the resistor was conducting straight to ground, pulling lots of current, exceeding its designed wattage dissipation and hence melting.

First I'd disconnect the 12v green wire at the switch before investigating what may wrong. I'd look at the wiring diagram and from there I deduce....

To work on fast speed, the fan gets its full 12v via the GY wire from the switch. The GU wire ought to also have 12v on it because it is connected via the resistor to GY. It is imperative that the GU wire doesn't conduct to earth anywhere along its length or at the switch when the switch connects GY to G. If it does, then the resistor will drop a full 12v across it and get hot when on fast speed.

To work on slow speed, the fan gets a reduced voltage via the GU wire from the switch. The GY wire ought to also have the reduced voltage on it because it is connected at the fan. It is imperative that the GY wire doesn't conduct to earth anywhere along its length or at the switch when the switch is set to connect GU to G. If it does, then the reduced voltage will attempt to short circuit.

With the fan turned off, if you disconnect the earthing black wire of the fan, then neither the GY nor the GU wires nor the resitor should have any reading on the resistance scale of an ohm meter with reference to earth. If they do, then that is where your short circuit is and what made the resistor glow.

The resistor should always be isolated from earth and it rather sounds like it wasn't. The switch should also always be isolated from earth.

kind regards
Marek

[Series1 Stu]

Or it might just be a badly made reproduction resistor, which wouldn't surprise any of us.

Someone may have decided to use a relatively short piece of higher resistance wire to get the rated resistance instead of a longer length of thinner lower resistance wire which would ultimately give less wild heat.

In other words, the solder may have melted due to the self heating effect of the resistor.

Regards

[johnetype]

riveted to the base of heater, to provide a good heat sink,
Mike

I wonder if this might be the cause of your overheating in that the action of riveting the resistor has shorted it to ground.

As an alternative you might consider using something like this:

http://www.ebay.co.uk/itm/40A-2000W-12V ... 0005.m1851

Concealed in it's black box under the heater, it can't be seen easily, allows you to preset any low speed fan setting you like and it you can be bothered to route the control potentiometer back to the cabin, you can have total control over fan speed all for less than the cost of the resistor.

John

[Hugo]

Is the motor ok? You said it was getting hot I believe? Could there be an electrical or indeed mechanical problem that would cause it to draw too much current?

[Heuer]

Indeed, the motor should spin freely by hand so worth checking the bearings - if it is stiff it will draw an excess current. A healthy Lucas motor will draw about 4 amps.