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Old 03-29-2015, 05:22 PM   #1
JSW
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Can Diodes Fail?

In leaving a Hobby Lobby in Nacogdoches the rear end of my MH drug and crushed the cable to my toad light connector. I have replaced the cable and now I have signal through the pig tail cord to the toad and to the diodes but no lights in the back of the toad work. My question, could failed/shorted diodes stop current from getting through to my rear toad lights? The wires in the old, crushed cable were all raw and touching one another. Now I'm stuck in Tyler on my way to Tennessee. Help.
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Old 03-29-2015, 05:34 PM   #2
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My fist guess would be shorted wires would not cause diode failure. Look for a bad ground. Have you checked for voltage at diodes?
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Old 03-29-2015, 05:35 PM   #3
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If wires were ground down and touching check all fuses, diodes will fail but I don't see how that would cause a no light condition.They are there to keep turn signals from backfeeding into run lights. No ground would also cause a no light condition as previously stated.
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Old 03-29-2015, 05:40 PM   #4
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My fuses are good. All lights on both the MH and the toad work normally except when connected, then the toad lights do not work. I get signal all the way back to diodes but nothing beyond.
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Old 03-29-2015, 05:46 PM   #5
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Originally Posted by JSW View Post
My fuses are good. All lights on both the MH and the toad work normally except when connected, then the toad lights do not work. I get signal all the way back to diodes but nothing beyond.
Then the diodes are in backwards or have failed.
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Old 03-29-2015, 05:49 PM   #6
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My fuses are good. All lights on both the MH and the toad work normally except when connected, then the toad lights do not work. I get signal all the way back to diodes but nothing beyond.
Diode blocks have 3 pins. One pin goes to the bulb.
The other two are inputs from car and coach.
Swap the two input connections. From what you stated, the car should quit and the Coach should start working. Indicates one of the internal diodes is bad. Replace the diode block.

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Old 03-29-2015, 05:49 PM   #7
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My fuses are good. All lights on both the MH and the toad work normally except when connected, then the toad lights do not work. I get signal all the way back to diodes but nothing beyond.
If you have power to the diode but don't have power out of the diode it has obviously failed. To check for sure put a short jumper or just unplug the diode and touch the tail light wire coming to the diode to the wire going to the tail light of the toad with the mh lights on.
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Old 03-29-2015, 05:50 PM   #8
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The diodes only let voltage go one way if defective no voltage would go through sounds like you got to figured out as long as the ground is common to both is good.
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Old 03-30-2015, 07:02 AM   #9
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You have a new (or repaired) MH to TOAD cable.

Your saying you get signal all the way to the diodes. I am assuming your talking about the diodes on the TOAD tail lights.

Replace the diodes.

I don't see how the crushed and shorted cable would cause the diodes on the TOAD to open. I would also question why the fuses didn't blow if you basically stripped all the wires and tied them together.
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Old 03-30-2015, 07:08 AM   #10
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Diodes have two failure modes.. Shorted and open.

However the odds of either being caused by what you describe is near zero. But yes it can happen.

MORE LIKELY is there is a second break in the wires somewhere.
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Old 03-30-2015, 07:20 AM   #11
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FWIW If he grounded a hot light lead it could blow a diode open. Usually the bonding wire opens internally. The other option is the junction melts and the diode turns into a low value resistor. No idea what is in those blocks but I have build several steering diode combiners to convert separate turn and brake lights into the same wire. A 5 amp diode usually is sufficient but the circuit is fused for 20 Amps so the diode could win the fuse race. ;-)
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Old 03-30-2015, 07:20 AM   #12
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My fuses are good. All lights on both the MH and the toad work normally except when connected, then the toad lights do not work. I get signal all the way back to diodes but nothing beyond.
When you say "the toad lights do not work" do you mean just the tail lights, or do you mean stop and tail lights? There should be 3 diode blocks, one for the left, one for the right and one for the tail lights. Do what dpinvidic said and switch the inputs. That should rule out the diode block.

If it was me, and you don't have stop or tail lights, I would look for a ground connection between the motorhome and the toad. You can quickly check for this be running a wire between the negative battery connector or chassis ground on both units.

Good luck and keep us posted as to the outcome.
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Old 03-30-2015, 09:28 AM   #13
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I doubt the cable would cause a diode to go, they wither work, or they do not. I would guess it is a fuse, not sure what you are pulling the toad with, but most chassis have separate fuses for the toad.
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Old 03-30-2015, 02:57 PM   #14
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FWIW If he grounded a hot light lead it could blow a diode open.
Think about the sequence of devices

Motor home ---Umbicial----Some wire on car---Diode---Light

Now...The problem he had was with the UMBICAL

A short there will blow the fuse on the motor home

From the diodes point of view it is a disconnect.

Now what is possible is that the short did not quickly blow the fuse.. But it did cause a current surge in the wire.. Then the short vanished for a bit and all that current flowing simply did not wish to stop (NOTE: This is called Inductance) but when it ran up against the now non-shorted.. Well the voltage spiked,, and thus the current spiked,, and that... Blew the diodes.

But the odds of that are low.
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