M715 Bounces Back

Back around the turn of the century, or shortly thereafter, I picked up a nice straight rust-free M715 from the GSA in Arkansas. I got the truck back to OD from fire department red with the exception of the dash. I ran the truck a little but it soon turned into a non-runner with low compression on two adjoining cylinders. The truck went into storage for quite a while I pondered an engine rebuild or some other direction.

Four or five years ago I scored a HMMWV 6.2 take out through GL at Fort Polk which got me to thinking about doing a straight 24V diesel conversion on the M715 using a CUCV trans and transfer case. I had the 24V injection pump on the take out to which I added a CUCV oil pan and oil filter mount. I got a HMMWV wiring harness off EBAY and proceeded to strip out only those circuit pieces I needed. A CUCV terminal block went on the firewall for a 24V feed source, a 24V golf cart relay was purchased for the glow plug circuit and a NAPA diesel filter to replace the CUCV version. I planned on using the HMMWV alternator mount and mount the M715 alternator but there was fender conflict. A local electrical shop did me a 24V rebuild on one of the CUCV alternators and that solved that problem.

I've been piddling with all of the above for several years, stopping/starting, loosing interest and getting re engaged. Finally I decided this year I needed to wrap this up. I buttoned up all the electricals and everything checked out good on the meter. I had installed a new GM fuel pump on the engine before it went in. I got to cranking point last week and found out I had a bum fuel pump so I dumped the mechanical in favor of a Walbro 24V pump installed on a separate circuit with a CUCV black out switch to activate. The stock main power switch, via the old coil wire, now feeds the injection pump. The original foot starter switch was wired to operate through the park position safety switch on a Lockar auto floor switch.

Today it fired and ran for the first time. I still have to get the custom radiator, a process which got screwed up because I didn't pay close enough attention to the draft drawing dimensions from the radiator builder. I had to do a 3" body lift to get my drive train in using my motor mount choices on the original horns and doesn't look like the mechanical fan will clear or line up just right so the final add will be a 24V Spal electric cooling fan.

There's still a lot of detail to finish such as linkage for the transfer case, not wanting to use the stock CUCV in cab assembly. It's been a trip for me but I am happy that it's ending up like I wanted.

nice, just remember the kickdown switch in the transmission is 12v, i dont know if it would take very kindly to having 24v send to it.

Its the only wire going to the trans, its on the passenger side. It gets power when you have the pedal to the floor and makes the trans downshift. It will function perfectly fine without hooking that wire up, it just wont downshift for you.

Also you are going to need to either use the 24v hmmwv glow plugs, or get the dropping resistor that is on the cucv's. (you'd be better off to get the 24v glow plugs.)

The CUCV uses a vacuum activated switch which is why I converted the HMMWV injection pump from an electric kick down to the vacuum switch in the picture. I also transferred the CUCV vacuum pump to the HMMWV engine.

Whew I've got some late night typos in this epic, such as "...Novak motor pumps...". How about motor mounts.

PS on the last I also converted to the HMMWV glow plugs so everything cooks on 24V. Only used the CUCV parts necessary for the install like oil pan/oil pump/filter change over and vacuum kick down.

MILRET45 said:

The CUCV uses a vacuum activated switch which is why I converted the HMMWV injection pump from an electric kick down to the vacuum switch in the picture. I also transferred the CUCV vacuum pump to the HMMWV engine.

Whew I've got some late night typos in this epic, such as "...Novak motor pumps...". How about motor mounts.

Click to expand...

the vac on the cucv controls the shift points and firmness to some degree. the "full throttle" switch is an electric one operated by a 12v signal that comes from a simple switch when the pedal is mashed to the floor. (this is the same operation as any civilian diesel with the th400 trans.)

I wouldn't mind to see an overall of the underhood...
And by the way, that looks like a nice rig with a really clean install. Keep up the good work.

Sweet truck! Sounds like you paid attention to detail. VERY nice.3

Re: CCATLETT1984 comment on the electrical downshift connection.

Sorry I challenged you on that one. It has been a long since I parted out a CUCV and I totally missed the issue of the electrical kickdown when I pondered my combination of CUCV and HMMWV parts. I only thought about the backup light connection on the trans which was not a consideration. Looks like I will have to be content with manual downshifts.

Thanks for the heads up. This is what happens when there is too much elapsed time between the start and finish lines! As I drift further into old age I'll probably need a pilot style check list to start this thing:

1. Fuel pump switch on, check.
2. Main power switch on, check.
3. Depress and activate glow plug switch for 5 count, check.
4. Depress foot starter till fires, check.

Nice truck I had the same thought go through my mind with on of my m715 that has a bad egine. If you still have the old engine and want to get rid of it let me know.

Charles, pm sent your way with phone contact.

For the underhood question from dispatcher1011 here's my circuit layout (less the addition of the electric fuel pump). Used HMMWV 24V sending units on block. HMMWV glow plug leads and IP pump connector wires stripped out of a full wiring harness. Original M715 wiring connectors (oil pressure, water temp, alt) plugged into new senders.

Very nice looking truck! About the re engaged, umm, well, never mind.


DJ

I also converted my M715 to the 6.2 l Detroit Diesel engine, but on the original transmission. Driving performance is close to perfect, like automatic trans from 15 to 60 mph. What you can expect more from an engine! Engine now has 336 hours, the truck is at 103200 miles I onwn the truck for only 28 years! First replacement engine was a Hanomag 65 HP engine which served me until the injection pump failed due to its age.

Come on more pics of your baby there. She is real purtty

I have some pics from the meeting in Septfontaine / Lux. The cardbord under the truck is due to enviremental requests by the authorities.
Also an updated engine room pic.
And some other impressions, too.