CUCV - Rosscommon 24v to 12v Conversion

[Warthog]

There is a company called Rosscommon Equipment Center that has been retrofitting MVs to fire fighting usages for years.

http://www.roscommonequipmentcenter.com

One of their conversions was to change out the CUCVs (not the M1010s) 24v electrical system to a more standard 12v system. This allows easier access to starters and alternators. It also removed the 24v side of the equation for all the people that worked on the trucks that have absolutely no idea how the 12v/24v hybrid system works in the CUCVs

Their conversion just removed some parts and moved some electrical wiring. There is no need to hack up the wiring. Just disconnect connectors and tape them up out of the way. This allows someone in the future to restore the stock system without much trouble.

If you decide that the 12v conversion is for you then please do it the Rosscommon way as it is easy for fellow SS members to help out with any troubleshooting of the electrical system.

Here is the document that Rosscommon published

View attachment Rosscommon Equipment.pdf

 

[Warthog]

I will look into modifying the stock wiring diagram to show how the Rosscommon system works.

I have also added this thread to the CUCV Helpful Threads sticky

 

[M813rc]

Great idea, having a "standard" conversion method for those who must mess with it. Thanks Warthog!

Cheers

 

[Brogman]

Thanks!

 

[Refalgren]

As an very late addendum to this, since i've just finished successfully converting myself, i've modified the improved versions of the standard CUCV wiring diagrams to reflect the changes done by the Rosscommon 12v conversion. The diagrams also reflect the firewall resistor delete modification. I've forgotten where I got the improved wiring diagrams, so i'm not taking credit for any modifications except the above. The rest of it is due the original creator. Let me Know if there are any problems with the diagrams. Thanks.

 

[Inishturk9]

Fellow CUCv owners. I have an issue need help with. I have a 1984 M1009 CUCV Ex Florida Natl Guard. Before I picked her us she was changed into 12v don’t think perfectly. I had few problems first starter popping underdash starter relay. Cause was a starter wire had worn off insulation arc to frame blew fusable links etc. Fixed working fine. During these problems truck would start by jumping purple to purple white at relay. All fine for couple years other day started fine in morning but afternoon the famous click. Able to jump purple to purple white. It started. I decided to do doghead relay. Did it. But same click. Does start with jump purple to purple and white. MY QUESTION for 12 v not well done conversion should I do doghead differently. Should I just direct connect purple to purple white. Is the relaynededed. I haven’t checked power to new doghead solenoid yet. But it’s not working. Thanks in advance.

 

[Barrman]

As stated in both the threads you searched out and brought back to life. The under dash relay on a stock truck is where the 12 and 24 volt systems meet. It isn't needed in a pure 12 volt truck.

I would suggest tracing the wires you have. What ever wire is on the small terminal of the starter solenoid on the starter motor needs to be directly connected to the wire coming from the ignition switch on the column when it is in the start position. I'm not going to guess which color wires you have were. In theory, one of the 4 wires at the starter relay under the dash should be the wire coming from the ignition switch when it is in the start position. But it reads as if you have had a few wires cut out in the past so just trace the wires.

 

[Inishturk9]

Barrman. Thanks for the reply. If I could ask you another question. My belief is the blue white is from ignition smaller gauge and solid purple goes to starter solenoid. Larger gauge as well. I jump these and truck starts with ignition key. All wires are in the dog head relay as shown on this site as of yesterday. jShould I isolate tape off power wire old red heavy gauge in doghead and still use solenoid for purple blue/white and black wires appears to be ground. Or just connect purple and blue/white wires and what to do with ground or doghead solenoid I installed
Thanks in advance

 

[Barrman]

Technical Manual TM9-2320-289-20 appendix F diagram F-1 is the stock starting circuit. That diagram shows the purple/white wire is the signal from the ignition switch. The stock relay then sends power to the purple wire which is bolted to the starter solenoid. There is no blue wire.

I would suggest using a volt meter to verify the purple/white does have 12 volts when the key is turned to start. If so, then connect it to the solid purple wire. Which should get you 12 volts to the starter solenoid.

 

[Archetypo]

I'm in the process of doing the single alternator conversion, I'm a little confused about what to do with the alternator leads that hook to the 12v junction next to the glow plug relay. The instructions say the two leads end in a single ring terminal that's on the big lug of the junction, and to remove that terminal, insulate it, and tie it back to the harness. Since I'm removing the DS alternator, I wouldn't need that lead at all, but just leaving that terminal unhooked and insulated would mean the lead for the PS alternator isn't hooked to anything.

The diagrams posted by Refalgren show that lead connecting to the positive junction on the PS firewall. Is that the best/commonly accepted way to do it? Or should it go back to the 12v junction by the glow plug relay?