Plastic Connector to Blocking Diode Melting with Heat. Help! By carefulrider (Read 2,412 times)

[carefulrider]

I'm slowly but surely getting my 2006 model year, JMStar/Jonway VIVA YY250T-2, Flagship-Style (1200-800) body, 244 cc (copy of Honda CN250, upright or vertically oriented) engine; 10" rear wheel, ready to ride next Spring.

While I was warming up the engine to change the engine oil, I noticed smoke rising into the cold almost-winter air. I followed it down to its source, & was dismayed to see the blocking diode's white plastic connector soft, shiny, looking like it was melting & smoke coming from it.

I should say at this point, that the headlights were not working at any time, from the time I initially turned the ignition switch on, untill I turned the key off to stop further heating, smoking & melting.

I suppose I should replace the blocking diode; is there anything else that I should check, as to the cause of the over-heating of the diode? Or is it that blocking diodes can over heat when they malfunction?

[JR]

Your headlights will only work with the engine running when one has the blocking diode system. Something caused it to heat up and I would start looking at the wiring/connections from the 3 wire (yellow) stator plugin and all the other connections around the R/R. Sounds like you possibly have a short?

After you unplug it I would check the voltage output on the stator wires which will be AC voltage. There are 3 yellow wires, unplug it for the R/R and start the scooter, test each leg to another, not to ground. Test as follows and you can mark the wires yourself, 1, 2 and 3

Check the output on the wires like this:

1 to 2
1 to 3
2 to 3

The output voltages should be very near the same.

JR

[carefulrider]

Thanks JR; shall check it next year, upon my return from visiting family over Christmas.

[ellpee]

I don't really know what a blocking diode sells for, but an alternative is to replace it with a simple relay from AutoZone or somewhere, under $10. That will mean your headlights come on as soon as you turn the key, rather than only after engine start, but no biggie there unless you forget and leave it on for a long time. The installation is pretty simple, four wires, and I found an easy how-to on the Internet.

[carefulrider]

I began the diagnosis process this week-end. I checked for current flow through the wires that connect to the blocking diode through a 2-blade connector. Both wires have current flowing through them. Is it normal? If not, what should I do? Thanks in advance.

[JR]

If the diode is fused together with a fully charged battery the headlights will be on even with the key off and the scooter not running.

If the diode is open or not connecting you will not be sending a charge from the R/R to the battery. The diode only feeds one way and blocks the battery voltage from going to the R/R.

JR

[carefulrider]

The headlights do not come on at all, with the ignition key turned on or not. Should both the wires that connect to the diode have current flowing through them? I am afraid to connect the wires to the diode without getting confirmation that it's normal to have power in both wires.

[JR]

Look at this diagram:  

Take note of the blocking diode labeled BD. The R/R feeds charging voltage to the battery and the arrow with the line points one way, the line on the back side indicates it blocks voltage from the battery feeding back to the R/R, why? If you note you see the light circuit tied into the line before the R/R. The head/tail lights will only come on with the scooter running.

So if the diode has fused together it's like jumping the line together and the lights would stay on even with the key off and the engine not running because the battery would then be sending voltage back through the red wire to the R/R.

So you are saying that when you put the diode back in the line it gets hot and starts smoking? If so then you have two things wrong, the diode is burnt up or fused together where it is passing voltage both ways and you have a short in the red line to the R/R on the input side of the diode. Now where the head/tail light circuit ties in this wire is generally yellow or brown not red as the diagram indicates.

Try this, leave the blocking diode out and start the scooter, watch carefully and see if you have any other wires get hot or smoke, check all the connections to the R/R first.

JR

[carefulrider]

Oops! I am so sorry: I did not state that I disconnected the plastic connector from the diode, & then when I connected the volt meter to each blade of the plastic connector, there was/is current flowing through each blade.
I shall do as you have directed on the next day it goes over freezing in this neck of the woods. Thanks for the help thus far.

[JR]

If current will flow in both directions on the diode it is bad. Unhook it put 12Vdc from any battery on one side then see if you pass voltage. Then reverse it and see if you have voltage going through the other direction. Again if you look at the diagram with a diode passing voltage both ways your headlights would be on all the time. I suspect a short in the system.

JR

[carefulrider]

Any suggestions as to where & how to look for a short? The temperature might rise above freezing this week-end, & I shall attempt to diagnose it, if it's warm enough outside. Or should I say less cold?

[JR]

Feb 3, 2015 14:19:28 GMT -6 carefulrider said:

Any suggestions as to where & how to look for a short? The temperature might rise above freezing this week-end, & I shall attempt to diagnose it, if it's warm enough outside. Or should I say less cold?

I would start around the R/r area and trace out the head/tail light wire that ties in between the R/R and the diode.

JR

[wheezy]

JR, if I am reading his posts correctly, he has voltage on both sides of the diode. This may be perfectly normal, depending on the circumstances. Here's why.

If the bike is running, you have rectifier output on the anode lead of the diode. Whether the bike is running or not, you should have battery voltage on the cathode side (unless the 20A fuse is blown). If carefulrider is checking for voltage with the bike running, voltage will APPEAR on both sides. In normal operation, the diode will switch on when the anode lead voltage is higher than the cathode. This allows the R/R to charge the battery, but prevents drain on the battery if the R/R fails to output enough voltage.

This seems similar to insanetexan's issue. My thought is that the voltage regulator is either not shunting excess voltage to ground correctly, or the ground lead that it uses to shunt the voltage is open or high resistance. When this shunt circuit doesn't work correctly, the excess voltage will seek out the best ground it can, and that seems to be through the lighting circuit, as that is the connector everyone reports melting. If the ground lead from the R/R measures short to the chassis, I would replace the R/R.

A question I have, based on this wiring diagram, is why they designed a 3A fuse for the horn circuit, but no fuse for the lights? I wonder what current that light circuit is reading when the connector gets hot. I would bet it's over 15 amps. I don't think the lighting switch was designed for that. 25-35 Watts for each headlight plus about 20 Watts worth of running lights only equals between 4-5 Amps on a normal basis.

[JR]

Feb 10, 2015 16:53:46 GMT -6 wheezy said:

JR, if I am reading his posts correctly, he has voltage on both sides of the diode. This may be perfectly normal, depending on the circumstances. Here's why.

If the bike is running, you have rectifier output on the anode lead of the diode. Whether the bike is running or not, you should have battery voltage on the cathode side (unless the 20A fuse is blown). If carefulrider is checking for voltage with the bike running, voltage will APPEAR on both sides. In normal operation, the diode will switch on when the anode lead voltage is higher than the cathode. This allows the R/R to charge the battery, but prevents drain on the battery if the R/R fails to output enough voltage.

This seems similar to insanetexan's issue. My thought is that the voltage regulator is either not shunting excess voltage to ground correctly, or the ground lead that it uses to shunt the voltage is open or high resistance. When this shunt circuit doesn't work correctly, the excess voltage will seek out the best ground it can, and that seems to be through the lighting circuit, as that is the connector everyone reports melting. If the ground lead from the R/R measures short to the chassis, I would replace the R/R.

A question I have, based on this wiring diagram, is why they designed a 3A fuse for the horn circuit, but no fuse for the lights? I wonder what current that light circuit is reading when the connector gets hot. I would bet it's over 15 amps. I don't think the lighting switch was designed for that. 25-35 Watts for each headlight plus about 20 Watts worth of running lights only equals between 4-5 Amps on a normal basis.

JR, if I am reading his posts correctly, he has voltage on both sides of the diode. This may be perfectly normal, depending on the circumstances. Here's why.

If the scooter is "not" running you have no voltage on the output side of the diode due to the fact that voltage from the battery is blocked if the diode is good. If it fuses together then the headlights will be on until they run the battery down.

As I said earlier a good way to test it is to unhook it and put 12Vdc on the input side and then the output side and check it. They can burn up where you have no voltage either way too.

The R/R on this scooter has a black sensing wire and the R/R puts out according to what it reads for voltage need.

JR

[ellpee]

He may well have a short somewhere, but this whole thing sounds like a helluva good argument for doing what I did -- replace the diode completely with a simple $5 relay from AutoZone or Pep Boys. Yeah, that means lights on as soon as you turn the key, but that's not a big deal. Of course that short still has to be found and fixed, but at least the relay has no complex electronics inside to melt down.