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Onboard air compressor powered by Cummins

81536 Views 91 Replies 44 Participants Last post by  WorkBody
When I bought this truck back in November, my girlfriend Lauren asked if I was going to put an engine-driven air compressor on it. Why would someone ask that? Well, I have onboard air on three of my Scouts, and I installed a system on her diesel Scout Terra, too. So she knows that it’s a pretty handy thing to have. But when she asked I laughed and said “No, there’s no way to do it on this truck; just not enough room.” But that got me thinking: Maybe it is possible?
So, after many hours of R & D, I was able to install a Sanden compressor that not only fits, but runs off a single belt. No second belt, no hybrid pulley, and no relocating any of the stock accessories. I can reinstall the stock belt without changing anything. The alternator is in its original stock position.

Three of my previous OBA setups use York compressors, and one uses a Sanden compressor. Traditionally, most OBA setups have used the York compressors, but they are becoming more scarce, and the Sandens are more common (and more compact). The main difference, besides overall size, is that the York compressor is a two piston reciprocating design, and the Sanden has seven pistons connected to a wobble plate. The two types are lubricated differently: the York compressor has an oil sump like an engine, and the Sanden is lubricated by the oil circulating along with the refrigerant in an A/C system. So the Sanden doesn’t hold lubricating oil inside, but blows it out with the compressed air. Oil has to be added to the Sanden to keep it lubricated. Some folks have set up lubricators in the suction side, and others have used grease, but I just check and add oil when needed. There is a special dipstick for Sanden compressors for checking the oil level. I’ve been doing it this way on one of my Scouts for 15 years now, and it works great. The compact size of the Sanden, and the fact that an extra compressor came with the truck, made it an easy choice to go with the Sanden for my Cummins.

Compressor mounting and belt setup:
Removing the engine lifting bracket and upper alternator bracket leaves a nice flat boss with seven tapped holes. There are three 10mm-1.5 and four 8mm-1.25.

I transferred the hole pattern to a piece of 3/8” X 5” flat bar which serves as the compressor mounting bracket. A second piece of 3/8” flat bar is used as a spacer to allow the compressor mounting bracket to clear the water neck.

With the bracket bolted to the engine, I could then line up the compressor and determine the hole locations for the compressor mounting bolts. I drilled and tapped the holes in the bracket to accept 8mm-1.25 bolts. 1 inch long spacers were welded to the bracket. Another bracket was made to replace the upper alternator mount.

Compressor installed. Again, this compressor is identical to the A/C compressor used on the truck.

Idler pulley installed between the fan pulley and the new compressor. This allows the belt to clear the water neck, and also gives the belt more wrap on the fan pulley. The bracket for the idler attaches to two existing 8mm-1.25 tapped holes in the front of the block.

New belt routing.

With the new compressor and idler pulley installed, I used a string to measure for the new belt. I measured a total length of 112.5 inches which is 31.5 inches longer than the stock belt. A Gates K081124 belt is just right.

A toggle switch inside the cab serves as the main ON/OFF switch for the system. The toggle switch gets power from the cigarette lighter circuit which is only hot with the key on. This prevents the compressor clutch from draining the batteries in case the system is left on accidentally. The switch activates a relay which sends power to the pressure switch. The pressure switch is connected to the compressor clutch coil. When power is present at the switch, it will engage the compressor clutch whenever the pressure falls below the cut-in point. When pressure exceeds the cut-out point, the pressure switch opens and the compressor clutch is disengaged. This cycle will continue according to air usage and keep the system between 80-100 PSI as long as the toggle switch is on and the engine is running.


I found these fittings (this is an assembly of two fittings) which are field repair fittings for the A/C compressors used on the Dodge CTD. They are listed for Freightliner trucks, but it’s the same compressor. These fittings bolt to the compressor head and accept common #8 and #10 A/C beadlock fittings.

The inside of the #8 discharge fitting just happened to be the right size to tap for 1/4” NPT. After tapping the hole, I screwed in a 1/4” NPT to -8 AN adapter.

A small filter is attached with a #10 beadlock fitting to the suction side of the compressor for clean intake air.

I used stainless steel braided PTFE (Teflon) hose with AN fittings for the first part of the system between the compressor and control components. Compressed air can be quite hot, so the hose has to be able to withstand high temperatures. This hose is rated for 400° F. and 2000 PSI. I could have used hydraulic hose, but this stuff looks trick!

For the rest of the system , 1/2” air brake line was used with push-to-connect fittings to plumb the air tank and quick connects.

A coalescing filter is installed between the compressor and the check valve. This filter traps any liquids (oil and water) from the compressed air. The liquids drop into the bowl which has an automatic drain valve. Whenever the pressure drops to zero, the drain valve opens and drains out any trapped liquids. The baby bottle connected to the drain lets me see how much oil has blown out of the compressor.

Edit: After operating the system for the first few cycles, I found that excessive oil was being released along with the air through the unloader valve. For some reason the oil was getting past the coalescing filter, perhaps because the pressure switch is below the filter in this setup. This has never been a problem in the system on my Scout. I had read about blocking a port inside the compressor body to prevent oil from being discharged along with the compressed air, so I decided to try it. Removing the head and valve plate allows access to this port in the compressor body. The port lets oil pass between the lower and upper ends of the cylinders. Blocking the port keeps oil from mixing with the discharge air. After doing this mod, I’m no longer seeing any oil at the unloader valve when it opens.

This is the hole to be blocked. It’s the only passage between the lower and upper ends of the cylinders other than the cylinder bores. The hole is at the 12:00 o’clock position; the compressor is upside down in this pic. I tapped this hole to 1/16”-27 NPT and installed a pipe plug.

Aluminum manifold for the pressure switch and related components, tapped with 1/4” NPT holes.

Manifold with components installed: safety pop-off valve, pressure gauge, 80-100 PSI pressure switch with unloader, and check valve with unloader port.
When the pressure switch opens and cuts current to the compressor clutch, it also opens the unloader valve. This releases all air pressure in the system upstream of the check valve through its unloader port. This lets the automatic drain in the coalescing filter open, and also prevents the compressor from starting up against pressure the next time it engages.

Brackets for air tank. The curved pieces are a mounting kit I bought with the tank, and the flat bars w/studs are homemade.

Brackets bolted up.

Air tank bolted to outside of passenger side frame rail.

Quick connects for air hose at front and rear of truck.

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Wow! Awesome write up! Good job!

Now sell a bolt on kit like you have made.
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That's amazing. I've been waiting for years for someone to post exactly this! You could/should put this together as a kit for sale on here. I'd buy.
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+1 for a kit
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Excellent job! Would like to see some more pictures of clearances around the compressor.
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great write up, do you think is can work on a 03
I've seen the bolt on York kits for these trucks, but the are expensive and bulky. The Yorks just look out of place.

This I like though...

Good job. Thanks for sharing!
I'd be game for just making templates and part numbers available for a price.....
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^^^^^^ I would be game for something like that also... Very nice workmanship on this install.

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It would be so easy to draw something up in CAD, find some one with a laser table or plasma table and bust out 10-20 kits at a time. Buyer needs to do the welding...
or if you felt really swell, you could share all the part numbers and dimensions with us.....
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Very nice writeup, and the pictures really bring it all together. Like Off-Road said, some part numbers would be great (I'm interested in the specific Sanden compressor used).
I wanna know how much oil you use on the Sanden. The oil loss through the air was a no-go for me as I don't want to have to keep pumping oil into the darn thing.
x2. I wouldn't mind converting the Sanden pump on my crawler for the same reason.
Thanks for all the props! I spent lots of time on this, and I'm very proud of the result. I'm not going to make a kit, though; I've spent enough hours of my life on this project already! I will however be happy to share part numbers. I'll make a list soon and post it here.

I wanna know how much oil you use on the Sanden.
Every Sanden compressor has the initial oil charge amount on the label. It was 210 cc on the compressor I used, which is the same unit as the A/C compressor, at least on my 2001.

jerrybbc said:
great write up, do you think is can work on a 03
Member jmcmurray did a very similar setup on his 03 using a York compressor.
Every Sanden compressor has the initial oil charge amount on the label. It was 210 cc on the compressor I used, which is the same unit as the A/C compressor, at least on my 2001.
No. I meant, how much leaves the compressor? How often do you have to refill it? (I guess that also depends on how often you run your compressor.)
No. I meant, how much leaves the compressor?
After plugging the oil hole as described in my original post, the amount of oil leaving the compressor is insignificant.
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After plugging the oil hole as described in my original post, the amount of oil leaving the compressor is insignificant.
I see. Sounds promising. Guess I can use the York that I have sitting in my shed for something else.
Parts list/ sources
Compressor Sanden model SD7H15, part number 4775
Compressor and alternator brackets custom made
Intake filter K&N 62-1050

Idler pulley Dayco 89169
Idler pulley bracket custom made
Belt Gates K081124
Compressor field repair fittings Airsource 4243S & 4245S
I bought these from Fleetpride; they have many locations nationwide

MSC Industrial Supply
MSC Industrial Supply Co. | Find Power Tools, Hand Tools, Machine Tools & More
Coalescing filter MSC 80072457/Coilhose 26C2-DM

Grainger Industrial Supply - MRO Products, Equipment & Tools
Pressure switch Grainger 5B417/ Square D 9013FHG2J27X
Check valve with unloader port Grainger 5X780/ CDI Control Devices CB25

Manifold custom made

-8 AN PTFE hose and fittings
Intake filter K&N 62-1050

Eastern Marine/ Trailer Parts Superstore
Trailer Parts Superstore® - Shopping Index
Air tank Stock # 1583065
Air tank mounting bracket Stock # 1583176
Push to connect fittings 1/4” pipe X 1/2” tubing Stock # 1566253
Push to connect fittings 1/2” pipe X 1/2” tubing Stock # 1566255

Other air tank brackets custom made

Air brake line 5097T71, various colors and lengths available
Pressure gauge 3846K8
Metric fasteners and various spacers
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any way to template the brackets with a 1"square in the corner so I could print them and coppy them?
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