Diode type cutout getting really hot!!

[mcgarrett]

What would cause the "diode" type cutout and the little add-on fuse block on the starter switch to become very hot? A friend of mine has a Model A that has been giving him electrical trouble...battery had decided to go DEAD. We did some troubleshooting today, but aren't any closer to a solution. Battery is new NAPA unit installed on Thursday of this week. If the battery is topped up to full charge and left overnight, it will be dead by morning. If it were simply a short circuit, logic says the fuse would blow, but this is not happening. I can't seem to find a parasitic drain anywhere with my test light probe attached to ground. We checked for bare screw heads touching the firewall behind the terminal box - all is well there.
Helpful feedback appreciated.

 

[Kurt in NJ]

is the polarity correct?—- for the cutout

 

[mcgarrett]

Kurt,
My friend has been driving the car with no problems as far as I know until last week. We replaced the starter switch and fuse holder with new parts because he had them available. As we were wrapping up out troubleshooting session, I put my test light probe to the terminal opposite the wire going to the battery terminal of the cutout, I noticed the light came on. I didn't think it was supposed to back-feed current. It was too hot to touch. We will continue to investigate...

 

[mcgarrett]

Kurt, actually I'm not sure if the cutout is mounted correctly (polarity). Which terminal should the battery wire be attached to? The cutout is not marked anywhere. Should the battery wire attach to the terminal going directly to the diode or the other way around? Never worked on one of them before.

Since it's not my car I've noticed several problems, but we're working them out. The most pressing problem is getting this battery drain problem solved without ruining a new battery.

 

[ndnchf]

By their nature, diode cutouts generate heat. Good quality diode cutouts have a large heat sink. On the other hand, original Ford cutouts are very high quality and when serviced periodically, very reliable.

I offer tefurbished, adjusted and tested original Ford cutouts. PM me for more information.

Steve

 

[nkaminar]

Disconnect the battery when the car is not in use. Keep the battery charged using a battery maintainer.

Remove the fan belt. Hook the battery back up. If the generator works like a motor, at a show speed, then the electronic cutout is backwards or burned out. Try turning the cutout around and if the generator does not motor then the cutout was backwards. If the generator sill motors then the cutout is bad. In either of these cases, buy a mechanical cutout from NDNCF, post right above this one. Polarity does not matter on the mechanical cutouts.

 

[mcgarrett]

Great information guys, thanks for the helpful info.

 

[rotorwrench]

Power diodes, such as used on 3-brush generators to replace the OEM type mechanical cut out relays, can fail by burning out to an open circuit condition or they can start to leak current through the wrong way. They should have a heat sink. Power diodes fail on alternators too so don't be surprised it it fails. A person can check a diode with an Ohm meter. It should pass current one way but not the other. There are diodes in cut out cans that are made for negative ground such as the model Ts had or for positive ground. A person just needs to know the difference if they are marked on the bottom. They should be marked in some way to tell the difference.

 

[oldspert]

They should be marked in some way to tell the difference.

I think they usually have a small arrow showing the direction of flow.

 

[mcgarrett]

Power diodes, such as used on 3-brush generators to replace the OEM type mechanical cut out relays, can fail by burning out to an open circuit condition or they can start to leak current through the wrong way. They should have a heat sink. Power diodes fail on alternators too so don't be surprised it it fails. A person can check a diode with an Ohm meter. It should pass current one way but not the other. There are diodes in cut out cans that are made for negative ground such as the model Ts had or for positive ground. A person just needs to know the difference if they are marked on the bottom. They should be marked in some way to tell the difference.

rotorwrench,
Thanks for the clarification on the diode cutouts. I don't know if the one on my friends car is connected correctly. There are NO markings on the cutout indicating which terminal is connected to the battery lead, or any indication whether it is pos. or neg. ground. I'll check it to see if it passes current correctly or if it's burned out.

 

[ndnchf]

On all Ford model A cutouts, the open end of the feet point towards the generator's output terminal.

 

[mcgarrett]

On all Ford model A cutouts, the open end of the feet point towards the generator's output terminal.

Well, that's where things get weird. As previously stated, the cutout is an aftermarket diode type with no markings to indicate battery or field connections, and has NO open end "fork" connection to attach to a terminal; just a 3/8" bolt on either side. And instead of an actual output "terminal", his generator has 2 wires wrapped in shrink tubing coming out of the generator case. Obviously, someone has been tampering with the generator. I'm going to recommend that he replace the generator and cutout with new or known good parts to give us a new baseline before we go any farther.

 

[ndnchf]

That sounds like a model B generator that uses two wires coming out. One goes to the cutout terminal, the other grounds the fields to a cutout mounting screw. Shrink tubed wires? It could be a real can of worms.

If its any help, I can provide a proper rebuilt generator with original refurbished ford cutout. PM me for details.

 

[Keith True]

He means the open end of the mounting feet of the cutout should face the terminal,not actually connect to it.You have a 32 up generator.They work fine.One of those wires would have been connected to the mounting foot to ground it.Also,you say it is a diode cutout.Are you positive it is a diode type,and not an electronic regulator?Seems like I see the two names get mixed up quite often.They are completely different animals.If you tried to motor the generator with a Funprojects EVR on it you most likely fried it.

 

[Bruce of MN]

On a generator that I acquired, the power output stud was farther from the right angle terminal than stock, so I modified a brass plated angle bracket to reach to the stud from the cutout.

 

[mcgarrett]

He means the open end of the mounting feet of the cutout should face the terminal,not actually connect to it.You have a 32 up generator.They work fine.One of those wires would have been connected to the mounting foot to ground it.Also,you say it is a diode cutout.Are you positive it is a diode type,and not an electronic regulator?Seems like I see the two names get mixed up quite often.They are completely different animals.If you tried to motor the generator with a Funprojects EVR on it you most likely fried it.

Keith,
Sorry, I misread ndnchf's post, now I understand what he meant. Since I removed the cutout, I don't remember which wire went where since I assumed they were wrong to start with. How can I tell??

To your other question; I'm not certain if it's an electronic regulator or a diode type cutout, but the photos below will help confirm what we have. I'm curious to what kind of critter we are working with and since removing the 2 wires coming out of the generator, I forgot how they were connected and need to know where they connect!! Any help greatly appreciated.

 

[ndnchf]

The wire from the field coil connects to one of the cutout's mounting screws. The other wire, from the power brush connects to the cutout terminal on the side with the open feet.

 

[mcgarrett]

The wire from the field coil connects to one of the cutout's mounting screws. The other wire, from the power brush connects to the cutout terminal on the side with the open feet.

Roger that.

Since the generator is still in the car and the wires are covered with shrink tubing and disappear into the generator housing, I can't tell which is which, so I guess I can try to trace them with a continuity tester unless you know a better way to identify them.

Thanks for the help.

 

[ndnchf]

If you remove the cover band, you should be able to see the wires with a good light. The power brush will be visible at the lower edge of the opening, 9 0'clock position when looking at the back of the generator. Red arrow points to it in this photo. The wire from the power brush goes to the cutout input terminal.

 

[ndnchf]

One other thing, be sure both wires are clear of the spinning armature.