Diode/Cutout Help Needed

C

conservationist

Junior Member
Diode help needed.

Replaced the cutout on my 1930 truck with a diode conversion because I just couldn't depend on the cutout (90% of the time no charge and continually low battery). But I have a question. I still am reading no positive amps on amp meter (in dash) but do get a major negative reading when I turn on the lights/horn, etc. so I'm assuming the meter works.

So, with the cutout (when it worked) you had to have some fairly high RPM's going to get a charge --- what about with the diode? Took the truck down the road, up to 35 mph (in town) and didn't register a thing. Same with reving engine but I'm hesitant to ramp it up too high without a load. I'm 99.2% sure I did the conversion correctly since it wasn't rocket science, but ........ Just so you know, I soldered the small diode wire to the bracket has the two small rivets. That would be opposite the side that attaches by "nut" to the generator housing.

I've read a huge number of posts speaking that this conversion is the best thing since sliced bread -- but not sure mine is working -- any advice?

Thanks much,

Bob in Boise
 
Short the generator post to the side of the generator(ground) and see if it sparks indicating a working generator (engine must be running). To check the cutout diode (engine off), set your volt ohm meter to continuity. put the black wire on the generator side of the cutout and the red wire on the battery side. You should have continuity. Reverse the wires and you should show an open circuit. If this is how it is working, the cutout is functioning normal.
 
The generator has "in the past" been polarized, but not since I changed out the cutout for diode -- I'll short across the terminals again to be sure. I'll also run continuity test on the poles. I'll post results again.

thanks for replies!

Bob in Boise
 
30Ford said:
If the diode is in backward it will be cool because it is not working
Click to expand...

If the generator is putting out battery voltage or more then a diode installed backwards won't be working, but if the generator has no output as when the engine is off, then the diode will be working by feeding battery voltage into the generator.:eek:
 
conservationist said:
The generator has "in the past" been polarized, but not since I changed out the cutout for diode -- I'll short across the terminals again to be sure. I'll also run continuity test on the poles. I'll post results again.

thanks for replies!

Bob in Boise
Click to expand...

If you are using an ohm meter for the continuity check be sure to disconnect the cutout output wire going to the terminal box, otherwise you'll send battery voltage into the ohm meter and may blow the fuse inside, or blow the meter movement.
 
Okay, started engine and shorted from generator side to case and had sparks, but I'm thinking I would anyways because battery is hooked to other side?? But if not, then generator is "generating". By the way, shorted across cutout first to polarize just in case it needed it.

Disconnected battery and tested continuity -- strange readings. No continuity going Red on Gen side, black on battery side -- However, I didn't get clear continuity Black on gen side, red on battery side.

Took it off generator, and it appeared that maybe the solder joint had "bridged" across to the metal that the diode "head" goes through. Took it apart and re-soldered so that the wire did not "bridge" across the gap from the contact point. However, I still have no continuity the one way, and less than good continuity the other direction where you all are telling me I should have. By the way, there is a fair amount of corrosion inside cutout shell. Maybe that's messing things up. I did clean all connection terminals of rust/paint and gunk in general.

Put it back on, started up and no go as to a positive readingr on dash amp meter, but still can go negative when brakes, lights, etc. turned on. Back to the drawing board I guess.

Any other suggestions??

Again, I am assuming I should be reading some positive amps. I can say that when the previous cutout (before gutted) was working as well as an after market one that I could run with top off, when I closed the points manually I was generating current up to 10+ amps so the third brush is set pretty high.

Thanks for both past and future help.

May just need to get some local club help but we're hosting Regional Meet in a week or so and everyone's pretty *******. We were hoping to participate with the truck -- maybe not as I'd hate to win that breakdown trophy with all those guests in town!

Bob in Boise
 
Bob. There is a postive & negative diodes,depending which earth you have, As you know current only flows one way as its an electrical tap, Check you used the right one, they allow curent to flow at low revs unlike a orig cut out, yes better than sliced bread, Trust you will attend the barn users meet sat night 7pm spud bar>
Derek from damp , colder NZ
 
Remove the cutout output wire (battery) and touch it to the generator post. If you get a spark, but no spark when you touch it back on it's output terminal, then the diode is installed correctly, or is OPEN due to a poor connection or bad diode.

Are you using an analog meter with an "R x 1" scale, so you can read low ohms?
 
Tom,

No, I'm using a cheap digital ohm meter. Numbers just fluctuate (but do go zero when I touch terminals to each other to test). Getting too dark in garage to work more tonight and have in-laws in town so need to keep head out from under the hood for a bit. This Diode was from Bert's Model A part A10505D -- but gotta say what came looks nothing like pic on their online catalog. The electrical system is 6 volt, positive ground.

I'll unhook battery side and try your test tomorrow. I'm 99.9 % sure that I put this thing together correctly as to wiring. However, I did drill the 1/2" hole a bit lower than I now would have liked to, so the edge of the diode (hat rim for lack of better description) is touching the base where the coil once was. From looking at how it is all assembled, not sure that makes any difference, but, would have liked it a bit higher.

So, when you say it is correct; or open due to bad connection or bad diode, I'm confused as it sounds like those are all the same reactions, but different results or conclusions??????

Bottom line, from what I an tell, it isn't working so I'm sorta SOL at this point if the diode will output amps at low speeds. I'll also "adjust" third brush again just to be sure, but at this point, I feel I have:

A good clean solder point from diode wire to terminal (all bright and shiny)

A solid connection (if it makes a difference) of diode head to the old cutout base

Good connections from cutout terminals to both battery side and generator side -- although I can't speak of how good the "inside" terminal connectivity is but can test tomorrow. Like I said, there is corrosion inside.

If I can break away, I may go buy a better meter at radio shack if you think that will provide answers

What would make a diode go bad -- other than whiskey and loose women??

Thanks,

Bob in Boise
 
Bob,
The tests indicate that your diode is working. Think of a diode as a one way valve. It lets current from the generator to the battery, but does not allow the battery to send current to the generator and make it try to be a motor. Since the Model A is positive ground, if you put the black lead of the ohm meter on the generator side of the cutout and the red lead on the battery side and you get continuity, the current from the generator will reach the battery. If you switch the leads and you get no continuity, it indicates that no battery current will reach the generator. Eureka.. The cutout is doing its job. Since you get sparks from the generator post when the engine is running, It indicates that the generator is putting out voltage. Remember, the diode will not let battery voltage flow backward to the generator, so the sparks are not due to the battery.
 
Bob,
I am a little confused.. You said that you had clear continuity with the black on the generator and red on the battery, however later it sounds like you do not have a solid connection that way. You should have no resistance (total continuity) that way. Reversing the wires should read totally open. You don't need a high tech meter for these tests. Even the $3 Harbor Freight ones will tell you what you need to know. The black on generator and red on battery should look just like when you touch both leads together. If not, there is a bad connection. With the leads switched, you should have the same readout as with the leads not touching anything. Good luck Bob, and don't get upset with the problem. It will turn out to be something simple.
 
I'll try and add to this discussion.

Step 1 - With the car running, and your meter connected to the output of the cut out you should read a voltage around -7v (red to cut out, black to ground). This indicates the battery is charging and the diode is functioning. Move the red lead to the generator side of the cut out. It should read about 1v higher than the reading you took initially. (say -8v, if your first reading was -7v) . If this is what you get, all is normal.

Step 2- If the battery side is reading -6v or less, with the car running, its not being charged by the generator. Either the generator is not working or the diode is bad (open cct).

Measure the generator output. If its higher than -8v the diode in the cut out is bad (open cct) or not connected properly.

If the generator voltage is the same as the battery voltage the diode is shorted (rather than 1v different) as in step 1... replace the diode

Step 3- Shut off the car but leave the ignition on. If the diode is OK, the battery side of the cut out will read about -6v, and the generator side will read 0v . If the diode is in backwards, the generator voltage will be 1v less than the battery side or about -5v. reverse the leads to the diode and try again

If its shorted, both readings will be the same, say -5v. Replace the diode.
 
I'll try and add to this discussion also.

If the diode was shorted (assuming it is connected properly),the generator
will still charge while the engine is running but the battery would discharge
when you shut the key off. Same thing as stuck points on a stock cut out.
Bob
 
One other thing I forgot. If you are using an analog meter rather than a digital one, reverse the red and black leads, as this type of meter can only read in the positive direction. Actually you can do this for either type of meter. The readings then all become "positive" (ie -7v becomes 7v etc)...... probably easier to understand that way too!
 
I don't wish to be a "wet blanket" but years ago (1970s) I experimented with power diodes before they were available in kit form. At first they seemed to be a great idea. As soon as the generator output voltage got above the battery voltage, you were charging the battery. The diode made the amp meter look like a tach meter! At an idle, the diode prevented the current flow into the generator from the battery. But because they are a solid state device, they are susceptible failure due to excessive heat and can fail. In my case, the failed diode disconnected the generator from the battery and the excessive generator output voltage burned out the coil. The generator itself was so hot you couldn't put your hand on it! It sure wrecked the tour for me. The bottom line here is, carry a new conventional cutout and a known working coil in your tool box.
 
Ed ..
I don't think there should be anything connected to the "generator" side of the cut out except the generator. The ignition should be on battery side, and therefore protected by the battery from going too high? The wiring must be such that the battery is always in the circuit when the car is running.
 
Ed ..
I agree that the diode can fail if it is not properly heatsunk in the cutout. When charging the battery, the power dissipation will be in the order of 8-10W at 10A charging current. This will produce a lot of heat which must be conducted away from the diode for it to be safe. You can see this with the large finned heat sink in a typical GM alternator
 
One thing I haven't seen mentioned in this thread is: third brush adjustment. Maybe you just need to increase the charging rate of the generator by moving the third brush. Just my two cents....
 
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