MF40B Excitation Diode

[Rebrown]

Does anyone happen to know the part number for the isolation diode between oil pressure switch and alternator? Part number or type diode, or even a work around would be appreciated.
Thanks in Advance

 

[warbaby]

Get an inline fuse holder from your local parts store, and a GM fuse type diode. Wire that setup between the two components.

gm fuse

 

[JMOR]

(quoted from post at 20:05:42 08/23/21) Does anyone happen to know the part number for the isolation diode between oil pressure switch and alternator? Part number or type diode, or even a work around would be appreciated.
Thanks in Advance

ny diode with a PIV over 100 volts and current rating of 3 amps or more. Even a 10 Ohm resistor will work. R.S. 276-1661 or 1N5408 & many others. Could even use the diode trio from a Delco 10SI alternator & have 2 spares.

 

[Rebrown]

Thanks for the suggestions. I have to find the existing in the wire mold, if it is even there. The previous owner cut out and bypassed most of the wiring and I am trying to put it back the way it should be. The tractor runs, finally, but the alternator is not charging the battery. It is a new alternator, so I have some troubleshooting to do.

This post was edited by Rebrown on 08/23/2021 at 06:57 pm.

 

[Jim.ME]

(quoted from post at 21:54:36 08/23/21) Thanks for the suggestions. I have to find the existing in the wire mold, if it is even there. The previous owner cut out and bypassed most of the wiring and I am trying to put it back the way it should be. The tractor runs, finally, but the alternator is not charging the battery. It is a new alternator, so I have some troubleshooting to do.

This post was edited by Rebrown on 08/23/2021 at 06:57 pm.

A couple questions. Do you have a wiring diagram for it? Does it show having a diode in the harness? I ask as looking in the parts book to try to find a part number for you, I can't find a diode in the harness, only the diodes inside the alternator. If the gauges and alternator are the only things fed from the oil pressure switch it should work without a diode. Back feed should only be an issue if the excitation feed is shared with the ignition or a fuel shutoff. Using an oil pressure switch to excite an alternator is one way to do a conversion without a diode, resistor or indicator lamp. If the engine shutdown (ignition or fuel) is run through an oil pressure switch most will have a momentary switch you have to hold until the oil pressure closes the switch, that is usually found on stationary equipment as protection against unattended oil pressure loss. I could be wrong but thought I'd ask.

 

[Rebrown]

My wiring diagram does show the external diode, but not the position. I found that in another post and will go look for it today. The oil pressure switch is new but I need to check that it is actually closing when it should. They don't make these things easy to work on and I don't twist and bend like I used to. The maintenance manual has good troubleshooting steps I will do this morning.
Being diesel, there is no ignition and the fuel shutoff is manual.

 

[Rebrown]

I took the wire mold apart and there is no diode. I measured resistance both directions and it was the same, so definitely not there.
When I crank it over the oil pressure switch closes and I have 12V at the alternator so it appears they are using the switch to control the excitation on the alternator.
I think I will just leave it alone and put the dash back together as soon as I get a new tach cable installed.
Thanks all for your help and suggestions. I am just surprised that a 60 year old MF40B still runs.

 

[Jim.ME]

(quoted from post at 13:46:05 08/24/21) I took the wire mold apart and there is no diode. I measured resistance both directions and it was the same, so definitely not there.
When I crank it over the oil pressure switch closes and I have 12V at the alternator so it appears they are using the switch to control the excitation on the alternator.
I think I will just leave it alone and put the dash back together as soon as I get a new tach cable installed.
Thanks all for your help and suggestions. I am just surprised that a 60 year old MF40B still runs.

Is your wiring diagram a factory one or one someone had for a conversion to an internally regulated alternator? It seems strange it would be in the factory wiring and not in the parts catalog.

 

[Rebrown]

(quoted from post at 12:47:14 08/24/21)

(quoted from post at 13:46:05 08/24/21) I took the wire mold apart and there is no diode. I measured resistance both directions and it was the same, so definitely not there.
When I crank it over the oil pressure switch closes and I have 12V at the alternator so it appears they are using the switch to control the excitation on the alternator.
I think I will just leave it alone and put the dash back together as soon as I get a new tach cable installed.
Thanks all for your help and suggestions. I am just surprised that a 60 year old MF40B still runs.

Is your wiring diagram a factory one or one someone had for a conversion to an internally regulated alternator? It seems strange it would be in the factory wiring and not in the parts catalog.

Factory maintenance manual, but I am finding differences with old and not so old models.

 

[JMOR]

(quoted from post at 14:49:59 08/24/21)

(quoted from post at 12:47:14 08/24/21)

(quoted from post at 13:46:05 08/24/21) I took the wire mold apart and there is no diode. I measured resistance both directions and it was the same, so definitely not there.
When I crank it over the oil pressure switch closes and I have 12V at the alternator so it appears they are using the switch to control the excitation on the alternator.
I think I will just leave it alone and put the dash back together as soon as I get a new tach cable installed.
Thanks all for your help and suggestions. I am just surprised that a 60 year old MF40B still runs.

Is your wiring diagram a factory one or one someone had for a conversion to an internally regulated alternator? It seems strange it would be in the factory wiring and not in the parts catalog.

Factory maintenance manual, but I am finding differences with old and not so old models.

or what it might be worth?, I have diagram for MF175 that used the Delco 10DN with separate VR which shows no diode. So, maybe the diode came about with the 10SI, as is does show in MF275 with 10SI?

 

[Jim.ME]

Sounds like you are all set now but for the record (hopefully this may help someone in the future).

After going through the three 265 parts books on line, harness diode 273986M1 does appear in the 265 parts book for units prior to serial number 9A349239, not in the 9A349239 and after, or the 265S parts books. For a gas engine it shows in the harness loom close to the alternator. For diesels it appears it would be just a bit into the loom where the instrument panel wiring is brought together. So it looks like the older ones had it not the newer ones.

You are right the service manual may cover all the 265s (except maybe the S series) and if they didn't give the serial number break, one can end up hunting for something that isn't there.

 

[DirtMover]

Rebrown,

I need to do some wiring work (soldering, etc) on the dash of the MF40B. I tried to access it through the side panel where the transmission filter is located, but there isn't enough visibility. Just wondering if you had removed the dash during your repair and if so, how easy was the process. Also, have you seen any documentation in a service manual that describes this process. Thanks.

 

[Jim.ME]

(quoted from post at 10:53:01 07/10/22) Rebrown,

I need to do some wiring work (soldering, etc) on the dash of the MF40B. I tried to access it through the side panel where the transmission filter is located, but there isn't enough visibility. Just wondering if you had removed the dash during your repair and if so, how easy was the process. Also, have you seen any documentation in a service manual that describes this process. Thanks.

Rebrown may be around, but he hasn't made a post since his last one in this thread.