Got a weird one guys.

My truck has a block heater installed, but the PO (the truck is from California) cut the plug off the cord. No big deal, just get a new piece of 3 conductor cord with a 3 prong plug, and splice it to the cord that's there, right?

So I did just that. Both cords have a two 14 gauge wires, and a lighter gauge green center conductor. Easy-peasy. Spliced 'em together and plugged it in and ...FIREWORKS! Sucker made a big flash, threw sparks, and tripped the breaker - but not before welding the prongs of the new plug into the socket of the extension cord!

So I managed to get the prongs out of the socket, and everything still looks usable, so I go get my volt/Ohm/amp meter. Now let's call the 2 flat prongs #1 and #2, and the round grounding plug #3. Using the Ohm meter I measure ~ 15 ohms between 1 & 2, and the same between 2 & 3, and the same between 1 & 3.

So I do a little math.

Volts/Ohms=Amps, right?
So thats 120 Volts / 15 Ohms = 8 Amps, and the 14 gauge wire is good for up to 13 Amps. So far so good.

Volts x Amps = Watts, right?
So that's 120 Volts x 8 Amps = 960 Watts. So it must be a 1000 Watt (nominal) block heater.

So why the HECK is it tripping breakers and behaving like a dead short? Is there something I'm missing here?

Anybody with a block heater installed got an Ohm meter and would be willing to take measurements between the plug prongs #1 & #2, #2 & #3, and #1 & #3, and post the results here?

 

I can do this in the morning for you. It is too dark outside and late right now to start futzing around outside. Sorry. I will post this up in the morning around 8am EST.

 

Thanks man - at 8 am EST it is still dark around here (5 am)

Even if someone beats you to it I'd still appreciate seeing what your readings are....

 

Well crap, I just tried to measure mine, and I got open circuits between any 2 prongs of the plug. I did however notice that the resistance of my body is about 250k ohms, lol. Think my heater's fubar?

 

That may be why they cut the cord.

 

Good, OBVIOUS point. I didn't think of that until you mentioned it.

 

Well, yeah, I thought of that too.

BUT, that still doesn't explain why its doing what its doing, or what's wrong with it to make it do what it is doing...

My math makes sense, right? Based on that it seems like 15 Ohms resistance sounds about right, and therefore it should work like it is supposed to.

 

Well it is 6am now and still to dark to take the measurement, but I will get it shortly. How are you reading the plug? Is the 3rd prong up or on the bottom? I assume the 1 & 2 are the side by side prongs and the 3rd is the outlier, right?

 

Ok, if I measured correctly, I got 17.6 - 17.5 ohms between the left and right prong, however I got no measurement at all when I tested between either the left or right prong and the bottom 3rd prong when using my multimeter. I hope this helps, but then again this is east coast resistance I am measuring.

 

The round, #3, is called a frame or case ground. If you get a reading from, if you are lokking at it, the left flat to the round, you have a short. This will give you your fireworks. My electrical expertise, dont plug it in It sounds like your heater is bad.

 

Same resistance between any two of the three terminals, yes something has shorted internally.

There should be an open circuit between the round ground prong and either of the flat prongs.

Yes, your math looks dead on.

 

Hmmmn, wonder what would happen if I just left the center wire (to the center prong) disconnected?

 

Did you get the same OHm's that I got?

 

Unplug the cord from the block heater itself and ohm the cord first before you assume it is the block heater.

 

I agree. I have seen a more than a few that had simply a bad cord, shorting out right before it goes into the heater.