View Full Version : 12 Volt power loss

11 April 2012, 0958
I got my rig all set up (72V KZ440 with a ME1003)when I noticed a problem... It won't run.
My contractor won't move when I turn the key.
I hooked up the volt meter to the fuse box and see my problem.
After a quick diagnostic I see I lose almost all my voltage (12V) when I turn the key. Does this mean I have one of my positives from my ignition hooked up to a ground somewhere?

11 April 2012, 1254
Post a schematic please. What controller are you using? Sounds like something is shorting out your 12V or drawing more than you can supply...DC-DC or battery for the 12V source?

I am also building a 72V KZ440 with an ME1003, what year and what is your pack like?


11 April 2012, 1431
If you do have a positive hooked to ground, that would do it, but it should also blow one of your 12v fuses in the process.
What is providing your 12v supply and what is its capacity?
What do you have drawing 12v (contactor, headlight, etc.)?

11 April 2012, 2016
I have the 1980/81 KZ440 LTD
I will get my schematic up asap. I am using the
SW200 72 volt Solenoid,
but I have a cheap DC2DC. Green Galaxy 10amp (http://www.thunderstruck-ev.com/green-galaxy-36-72v.html).

Drawing off the DC converter... Headlight, warning light (3.2w), contactor, horn, controller.

I think I will try to hook up a 12 car battery to the output of my DC2DC instead of the converter and see if it is just underpowered.

Thanks for all the help so far, you guys are awesome!

12 April 2012, 0721
can you just use a 72 volt contactor???

run a DCtoDC converter from the 72 volts to power the 12 volt...

12 April 2012, 0907
I do, the SW200 is a 72V contactor http://www.evdrives.com/contactors_SW200.html

12 April 2012, 0930
I don't think this is part of my power loss problem, but I do need help wiring my contactor. I have the sw200, http://www.evdrives.com/contactors_SW200.html
What wires should I connect where? four contacts, four wires (12V +,- and Coil #2 anode, #1cathode)3019

12 April 2012, 0935
You said it's a 72v contactor. That normally means the coil is 72v. Why are you using 12v?

12 April 2012, 0954
I think we are getting somewhere. I need to activate the contactor with 72V? How do I do that? I am getting a diagram up now.

12 April 2012, 1003
That's why we usually use 12v coils. You have to run pack voltage through the key switch to the contactor coil.

Read the manual for the axe7245. It shows how to wire it.

12 April 2012, 1003

12 April 2012, 1009
I have that here, I see it now. Can my key switch handle that power? Can you help me wire my contactor? All I can find is conflicting information.

12 April 2012, 1019
Read the manual. I know for a fact that they tell you how to wire it. What "conflicting" info are ty referring to?

72v on a 12v isn't going to be the end of the world but it reduces the life of the switch. Again, this is why we typically use 12v coils. Will it work? Yes. For how long? No one knows.

12 April 2012, 1046
I found two web sites using my contactor with a 12V sysstm
Page 9 http://www.evamerica.com/curtisinstallationmanual.pdf

But I still don't know how to wire it. And can I run 12V to the key switch of the controller?

12 April 2012, 1102
You are the MAN! I see what I did wrong, when I ordered the contactor, I thought I was selecting the rating of the final connection 72V, but I was selecting the rating of the coil (I wanted 12V.)Sorry for being a rookie, I appreciate your help.

12 April 2012, 1256
Let me know what you think of this set-up, I am going to bench test this first, and then put the shunt in just off the ED250 Switch.


12 April 2012, 1310
:cool: im not sure what controller you are using, but maybe this will help...

12 April 2012, 1445
Put the pin 1 enable and the feed to the DC to DC converter on the load side of the contactor. Do not put that much load on that poor little keyswitch.

12 April 2012, 1451
That is the best digram I have seen yet. Would you say I should wire my contactor:


spade 1- to key spot controller
spade 2- 72V from ignition switch
spade 3- Anode of diode
spade 4- Cathode of diode

Thanks for the help in advance

12 April 2012, 1453
frodus, Got it good call

12 April 2012, 1459
Spade 2 goes to the output of the keys switch. The input of the key switch needs to be fused for a few amps and hook to B+.

Spade 1 goes to pack ground

Spade 3 goes to anode

Spade 4 goes to cathode

The DC to DC positive, pin 1 connection and controller B+ go to the output contact of the contactor.

B+ Goes to the input contact of the coontactor.

12 April 2012, 1533
Thanks, from this set up the key switch is only powered from B+, and only have power via the pre charge resistor? Is that enough to (after the key is turned) fire the contactor?


12 April 2012, 1552

14 April 2012, 1014
My 10A fues keeps blowing, do I have a problem with my contactor? Do I need to lower the amperage before the fuse and switch? Here is a simplified drawing of my new situation.3041

14 April 2012, 1118
What is the max voltage rating of the fuse (though I wouldn't think that should matter, voltage rating only deals with possibility of arcing once it's blown, I thought)?

What's the amp rating of the fuse? How many amps does the contactor coil draw on inrush and how many for holding?

What is the diode connected to? Typically a diode is run across the terminals of the coil to short out backwash from the collapsing magnetic field in the coil, but it looks like you have it on different terminals than those supplying power to the coil.

Anything else there in the circuit we're not seeing?

14 April 2012, 1132
250V, 10A Fuse

I use the 72 Volt SW-180 Contactor with 3 Amp Diode and 1000 ohm Resistor, I don't know the draw

I think the contacts (on the same sides) are linked, I will check for continuity. But I also connected them together so they were on the same spade, not just the same side.

Nothing else is in that circuit, I have worked down to the bare minimums to determine the problem.

14 April 2012, 1216
In your last drawing, just swap the B- and the diode Cathode (the banded end).
This is because I'm pretty sure that the four coil terminals are in two pairs which are shorted across the body not in line and so every time you've thrown the switch you've simply shorted the battery via the switch/fuse.

good luck

14 April 2012, 1310
You nailed it MagicSmoke.

Here's the datasheet:

"Coil connections are by 6mm spades of which two are supplied per terminal"

The line to B- on the controller and the line from the keyswitch are connected to the same terminal, creating a short circuit.

15 April 2012, 0744
Bingo Bango! thanks!