XM791 Recoveries

[Armada]

Outstanding rescue! This is one of those events where 'scrap trucks' fell into the right hands. Great save. Looking forward to your continued progress and success. Can't wait to hear their hearts beat again.

 

[cranetruck]

Outstanding rescue! This is one of those events where 'scrap trucks' fell into the right hands. Great save. Looking forward to your continued progress and success. Can't wait to hear their hearts beat again.

Agreed.
From personal experience with this series of trucks, I just know Oldfart and sons will fall in love with it (them).

 

[Oldfart]

Another wonderful afternoon in Colorado today. The good weather combined with a second free afternoon allowed Jollyroger and me to make more progress with our XM791's. We borrowed Ghostdriver's new batteries for her M43 and wired them into 0011 to test things. Again, no dead shorts. Fuel pump works (well enough to leak fuel from several flared fittings on the way to the engine.) Starter will engage as well. Head lights work, but not the horn. We had assumed the fuel lines to the injectors were removed from 0011 as well as 0001 because we found loose lines in the truck. We discovered today that those loose lines had to be from some other truck as 0011 engine seems intact except for a disconnected muffler. Additional treasures include a loose rifle holder, a two pin slave cable, a two pin to 4 pin adapter and an LDS-1a injector pump just to name a few. We have a complete set of cab top bows for both trucks and a nasty top canvas suitable to use for a pattern. That being said, we also notice some things missing like a heater fuel pump and some switches on one of the dashes.

We checked the fluids and the oil looked new. Since we had unplugged the fuel pump to avoid the leaks we decided to see if the engine would turn over. It does. Starter crank speed was enough to pump mouse nests out of the exhaust pipe and it sounded like the engine might want to fire if it had fuel.

I think I will pick up a set of proper batteries and terminal ends tomorrow so we can increase our starting reserve ~~ and keep Ghostdriver from skinning us alive for running off with the M43 batteries

.

 

[Stalwart]

I wouldn't have expected the horn to work, it's air powered and electrically controlled by the horn button.

 

[Oldfart]

I wouldn't have expected the horn to work, it's air powered and electrically controlled by the horn button.

Sad to say, both of THESE XM791's are set up with electric horns. 0001 horn has been removed and the power wire is hanging loose. 0011 has the horn installed. The horn and the horn set up looks a great deal like an M37 electric horn. The horns are mounted externally up along the right side the radiator. 0001 does have a bracket base where I have seen air horns mounted in photos of Cranetrucks XM, but no sign of ever having air plumbing or anything ever mounted there. I just hope the actual sound of the horn (when we get it workinig) has more presence than what I hear out of my M37.

I will be pouring over the TM again this evening looking for horn wiring and circuit breakers among other things.

 

[jollyroger]

I am less concerned with the electramatronical horn thingy and more concerned with getting the fuel system more together.

I sprayed some WD-40 in the air cleaner BEFORE we tried to turn the engine over. There was some resitance to the whole works turning over but suddenly it cranked over like there was no tomorrow. Seemed like it had starter drag. It tried to "hit" on a few cylinders with what was available for fuel and made cracking noises out of the loose exhaust pipe......

Unfortunately someone has changed out the Speedo and tach on the 0011 truck. The speedo reads 3400 miles and the tach reads all zero's on the hour meter. So no way to get a good indication of what and where the truck is for mileage. But I think there is hope for this one to be mobile again soon.

 

[jollyroger]

So my contribution:

 

[cranetruck]

Thanks for the update! Pls, send more tire pictures (PM perhaps) if you get a chance.
The date code on the one posted is probably 49th week of 1977 and it looks like a recap...

About the horns, the original was the electric and the air horn was added on an MWO in the early to mid seventies. I have both, with a separate switch for the air horn. The electric is good for getting someone's attention in the yard.

The horn button is pathetic on this as so many other MV's, not sealed at all and prone to corrosion. When I get a little extra money, I'll get the new sealed horn button installed. On these trucks, there is a sliding switch at the lower end of the steering column (just above the cab floor) that may also not be making contact.

 

[Oldfart]

Thanks for the update! Pls, send more tire pictures (PM perhaps) if you get a chance.
The date code on the one posted is probably 49th week of 1977 and it looks like a recap...

The tires on these things are unique. The one photographed had an FSN as well as a part number (why I took the picture). I could not track either of those at all. I will try harder when I get more time. It looks to me like the Goodrich's are noticably different from the Goodyears. I agree that some of the Goodriches look like recaps, but well done at the cap edge. Some of the Goodriches look like they might be just worn and never capped. The newer tread on the Goodyears is very obvious retread with the cap edge very noticable. Like I said, I snapped a cellphone photo of the one tire because it had readable dates as well as a part number and FSN. I can not find the date stamp on all the tires yet. There are a couple I have noticed on the inside of the tire at the bottom that are somewhat obscured because of the flatness of the tire. I will get to the tires in depth when we start to move the trucks around and I pump the tires up again.

 

[cranetruck]

Bob, the tires were specifically made for these vehicles and discontinued in the '80s AFAIK. You won't find the NSN #s on the internet as a rule, for the tires or a number of other items.

In the image above (post #6, the front tires are Goodyears and the rear two BF Goodrich.

When you start filling them with air, remember that they are "low pressure" tires and only need 30 psi for highway driving...

 

[Oldfart]

Ghostdriver and I swapped in some higher CCA batteries so Ghostdriver could install the ones we borrowed from the M43. We got more CCA and supposedly the same AH capacity as the original U6TL's, but I wonder. It's hard for me to believe less plates and acid (less chemistry available) can have the same AH. I can understand more surface area out of the same amount of lead will increase the cranking amps, but total capacity?

Anyhow, in the process of testing our connections, we continued to turn the engine over with the fuel shut off in an effort to pump oil through the system.

Short video of that cranking.

 

[cranetruck]

In the second image, you have what looks like two different batteries. If you run the starter motor with different batteries in series, the cranking amps will be limited by the battery with higher internal resistance.
I have to say that it sounds like the cranking rpm is on the low side.

 

[Oldfart]

In the second image, you have what looks like two different batteries. If you run the starter motor with different batteries in series, the cranking amps will be limited by the battery with higher internal resistance.
I have to say that it sounds like the cranking rpm is on the low side.

They are a matched set of batteries. 850 Cold Cranking Amps which is as good as the U6TL's top rating. Some of the U6TL's were as low as 600 CCA. As an Electric Car dealer from the late 70's I got awfully good with battery differential problems. One bad cell in a series would just mess up everything and if in parallel, the higher resistance would send all the current to the other side. Currently, there are no U6TL's available in the Denver area. I bought a pair for our M220 when we got it back in town, but everyone is out of stock and they mumble about slow sellers and having to buy a pallet to get some. The batteries I got are claimed to have the same Amp Hour rating as the U6TL's, but given the smaller size, there just has to be less available chemistry. Meaning they will not whip as long as the larger battery, but the punch behind the whip should be the same as long as it lasts.

The engine crank speed is getting better each time we turn it over. We shot some lube in the intake and disconnected the tank fuel pump. It was painfully slow the first revolution and we made several bumps to keep from overheating the starter. It actually made an effort to start on the lube we shot in the intake. We are "exercising" the engine trying to get oil pumped around before we actually make an effort to run the engine. We are consiering on attaching the slave cable to one of our other MV's when we go for the actual start.

In between exercising the engine, we are reconnecting the fuel pump and testing/repairing leaks in the fuel line. So far it has been loose couplings. We are taking the lines apart at each leak and checking for flaws or something amiss and then we reassemble. I did discover the neutral safety switch was removed and on the floor of the cab in an unserviceable condition (stem rusted closed.) It appears the switch wires have been jumpered.

Clearly we have many things to check before we attempt our first start. It would be nice to find a neutral safety switch so we don't have to rob one out of 0001. I have found several TM's for XM791's et. al. on line in PDF form. Those are downloaded to my computer and I spend until the wee hours reading. I found instructions on how to insatall and adjust the neutral safety switch, but no part no. or FSN listed in the ORD manual. I can't believe that switch is unique to that truck. Although many things are unique and need to be hand fabricated according to the TM for Repair and Parts.

 

[cranetruck]

Bob, the neutral safety switch is the same as the one used for the "flame heater", 2920-00-781-1953 (8389470), a normally open push button switch.

Kind of figured that you had the batteries right, it was only a cautious observation. Don't you just love how all the filter canisters are top loaded...Is the original primary fuel filter still on your trucks, the scraper type (see image)?

Second image shows some of the discoveries made by removing top layers of paint.

 

[Oldfart]

Bjorn,
Thanks for the switch info. I figured it should have a wider application than just this series of trucks.
Yes, both of these trucks have the wiper primary filter. 0001 is missing the second heater fuel pump and 0011 is missing the lines and wiring to the pump. 0011 has a crimped fuel return line. It will pass fuel back to the tank, but it got flattened over the top of the tank (just guessing it might have happened when the van body got removed.
We have noticed differences between both trucks. Besides the bug like headlights on 0001, it had a larger battery (4) box on the frame under the van body, but still has the 2 battery box under the passenger seat minus all the cabling. 0001 also uses a short negative battery wire to the frame rather than a long wire to the engine block as depicted in some of the TM stuff. The wiper motors on 0001 are all metal while 0011 has lots of plastic parts to the motors (which has disintegrated due to sun rot).
I found a statement in one of the TM's that indicated the bug like headlights were used with the winch application, but obviously that wound up not being the case. It looks like side reflectors got used instead of marker lights as well.
So much to learn about one of these trucks let alone the changes as they produced the series.
We see you made some inquires of the Pershing Group. Jollyroger attempted to participate so we could ask some questions about our trucks, but the admin would not allow him to join. Since you have access, we will gather the names we found on the trucks and get them to you. Maybe some of the guys who manned the trucks are still around.

 

[cranetruck]

As you can see in the "horns" picture above a couple of posts ago, I have taken some liberties when it comes to the head light placements. The original, as delivered from Ford, had the lights in the cab with or without the winch except for the blackout driving light, which was positioned higher up and to the outside for winch models.
The bumper mounted light clusters all came later as an MWO and some were not moved for years, judging from some later pictures I have seen.
The only variant "qualified" to swim was the 656 cargo version and the space between the cab and bumper was filled with polyurethane for buoyancy. For my xm757, I removed that, IMHO, ugly decking, covering the space between the cab and bumper.
If you calculate the buoyancy created by the boxy cab, you'll find that it is just enough to keep the 12,000 lb front end above water. The 656 also had the space between the windshield stowage box and cab filled with polyurethane.

The larger battery box also came later, as well as a muffler. Don't know if all changes came on the same MWO, there is only one MWO# on a tag by the glove compartment on my truck.
It's more efficient to keep the batteries next to the starter than having them 5-6 ft away. On the 757, the new battery box was placed on the dromedary platform behind the cab. I'm sticking with the original design and two batteries easily crank the engine. The cranking amperage under normal conditions is about 250 amps by a measurement I made using a shunt, however, the engine starts so easily (within a couple of seconds) that the peak amperage may have been higher. Also, the -2 engine has a higher capacity oil pump and there is good oil pressure after about 7-8 seconds even after the engine has been sitting for a week or so.
Think of any problem or drawback found on the deuce or 5-ton from the sixties and you'll find an improvement on these trucks.

The Pershing people seems to be an exclusive bunch and they rarely talk about the trucks, but I'll pass on questions if you like...

 

[Oldfart]

I was struck with the battery wiring on 0001. My years of electric car experiences tells me the frame ground could be a problem. Too many places for a loose connection to develop. Just better to run a heavy cable all the way to the engine block or starter itself for a ground. When I put the two 8D batteries in 0001 I kept eying the rusty bolt where the ground cable attached to the frame. When we get to working on 0001 again, we will likely run a 00 cable from the battery to the engine block.
It is hard to catch peak amps on starting. Most analogue meters are still reacting while the event is over. We sold one brand of electric car that used a parallel switching system for motor acceleration. Most customers would just love to hammer the accelerator pedal bypassing any cushion effect from the lower voltage. We got a digital meter/recorder on one and found almost 1400 amps at 48 volts at motor start up. As the motor started to turn it would drop down to a couple of hundred pretty quickly. We had warranty problems with the solenoids that shifted the batteries from series to parallel as the high peak loads would arc the contacts. The factory would only stand behind one replacement. The other brand of electric car we sold were made in Canada and they used Lucas electrical component's (you know Lucas, the Prince of Darkness). Their emergency shut off switch was a one time thing if you used it under load. Those cars were 72 and 96 volts which really stepped up the volume when a contact blew.
Given the amount of missing and broken stuff we are facing, there will be no doubt changes from the original vehicle conditions. There was something mounted on both vehicle front bumper rock guard panels. The mounting holes are on both the left and right sides like an additional light or something. No residual wiring though. I suppose the chances of finding a windshield stowage container are slim and none. And ah, yes the tires. Much to overcome, but what is life without a few challenges?

 

[cranetruck]

The original battery-to-starter wiring did not go via the frame, there was/is one pos and one neg cable directly to the starter. The frame and other ground returns go to/comes from the starter neg terminal.
Also, there is one dedicated ground return to the instrument panel and the batt/gen gauge. The instrument panel itself is shock mounted if you look closely. Other interesting improvements include no less than three ground wires to the fuel tank(s), the pump, the level sending unit and a "static" ground.
Not clear on you bumper rock guard...there were side panels on the cargo version (see image below).

PS. Bob, I appreciate you taking an interest in the details of the basics of these vehicles.

 

[cranetruck]

Is there enough material left of the radiator cover to make a pattern for a new one?

 

[Oldfart]

Don't know. I was just to the farm to photo my daughters M43 missing lift shackle bracket. I will head back later this afternoon and take a hard look. I think the hanging down flap of material blew off on the trip home.