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Discussion Starter #1
Trying to figure out a '93 EXT EFI 550. No spark, stator checks out fine, fuel pump works when powered but there's a diode that starts to smoke heavily. I've include two diagrams. One is of the wiring diagram with the red circled diode that starts smoking and the other is the parts diagram labeled #11, "diode, relay".

The kill switch harness seems to heat up but not to the melting point. I'm thinking maybe just a short on the red highlighted red/white wire or could it be the actual diode?

I'm not sure I can test for spark by unpluging main wiring harness because EFI. This is a closed system. ECU flashes once which is correct and shows no code.

Any idea's?

Dave in Maine

[attachment=157906:93EXTEFI...gcolored.GIF] [attachment=157907:11.gif]
 

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That diode does nothing more than to short the extra emf to ground. It's called a "shunt". You can look it up on google. it should describe the function. But basically it's nothing more than a damping diode. As for the harness heating up you'll have to put a meter in there to see if the current flow is excessive. If you have a short and chaffed wire it would cause some of the symptoms you mention. check the harness over closely. Based on the age of the machine I'd bet that's your issue.

Steve
 

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Discussion Starter #3
Thanks Steve. I'm thinking short to ground also. Do you think, looking at the highlighted wire in blue, that a short on this circuit could have fried the CDI? No spark but ECM, stator, coil and safety switches seem to test OK. Thanks for the info on the shunt. I should test resistance at EFI relay. Forgot that one.
 

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It depends on which blue wire you are talking about. One of them is just a ground anyway so that one would be no. The other one that goes to the ECU in blue is bigger deal. It could be a power or signal wire to fire the CDI from the ECU. If that one shorts then yes maybe it could cause a dead ECU or CDI. Depends on what that signal actually is and which way it feeds.

Steve
 

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Once you smoke a diode it is likely shot and in the case of diode if it still smokes then it is shorted. If you trace your red/wht wire you will find that it goes through you ignition switch tether and kill and eventually to battery positive. With the sled turned on you basically have the diode across the battery and if it is shorted then that is your problem. Pull the diode and test as follows. Set you multimeter on the diode test setting, usually has a symbol of a diode and is around the lowest ohm setting. Put the meter red wire on the anode (brown sled wire side) and meter black wire on the cathode (red/wht sled wire side) and the diode should conduct (meter reads a voltage) now reverse the wire and the diode should not conduct (meter reads open circuit), my guess is that the diode is your short. You can disconnect the diode and test as I believe it is there to protect the sled electronics if you happen to hook up the battery backwards.
On that note make sure your battery is hooked up right before you remove the diode from the circuit.
 

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Look at the bottom left of your posted wiring diagram where it says fusible links.
These are designed to open and cut power if that diode shorts to prevent wires from burning.
Check to see that these fusible links have not been replaced by regular wire, if they have you could
insert a fuse into the line to protect the sled
 
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