Medium Duty Ambulance FAQ (Freightliner, International, GMC, Chev)


Expedition Leader
There is a lot of valuable information particular to medium duty ambulances being buried in various build threads. So I am proposing that we have an FAQ (Frequently Asked Questions) specific to the medium duty chassis ambulances. By this I suggest the C4500, C5500, International and Freightliner chassis vehicles.

I will dedicate this top post to links to various build threads. Read this thread as the "master thread for Ambulance to Camper conversions FAQ"


Medium Duty

Ozrockrat FL60 Freightliner-ambulance-conversion-project

rirenz FL60 Buying-amp-Building-a-Medium-Ambulance-into-an-RV-

rossvtaylor FL60 From-Freightliner-ambulance-to-RV-we-hope!

scooter421 International 4700 1994-International-4700-Conversion-Begins

Dirtfarmer515 C4500 New-here-just-got-an-Ambulance

MNtal FL50 The-Rambulance-FL50-Freightliner-Ambulance-Camper-Tow-Rig-Conversion

Mattersnots 2009-chevy-medium-duty-4x4-kodiak-ambulance-conversion

Pennersm 2000-aev-traumahawk-on-a-freightliner-fl-60-conversion

Jambulance80 2009-kodiak-5500-duramax-crew-cab-ambulance-conversion-project

If anyone has a thread we need to link to send me a PM and I will include it in here.
Last edited:


Expedition Leader
Tires and Rims

This is the one I get most questions about.

There are basically 3 types of hubs used on the medium duty rigs:
8 on 275mm (these seem to be the most common. Especially if you want hydraulic disk brakes. The rear axle will probably be rated up to 15,000 lbs.
10 lug hub pilot . Same as most later model commercial trucks.
10 lug budd. Older trucks and you can generally tell by the tapered lug nuts.

If you are still looking for a rig I would suggest you put a priority on the 10 lug hub pilot type hubs. This will give you infinitely more choices for wheels/tires.

What size wheel can fit? (these are a couple of handy calculators: Everything, Tire Size Comparison)
Without a lift and minor trimming:
MNtal has fitted 275/70R22.5 (37") duallys to his rig with minor trimming.

Sawzall engineering and 4" lift
I have 365/80R20 (43" x 14.5") on mine with very minor rubbing on the steering arm at full left lock (polish not break type rubbing)

8 on 275:
-I run 20 x 11 military surplus beadlock rims. These originally came on the RG-31 MRAP. There are a lot of them about but they are hard to find. If you go with these you need to run an MPT type tire such as the 365/80R20. (MPT tires have a bead wall that is shorter than normal). I also had to drill out the stud holes to fit my current stud size.
-20x8.5 alloy military surplus. Probably OK to run the G177 11R20 but you will need to check the bead size on both the tire and the rim.
-Another option is the 20x9 alloy rims off the MXT International. These are about as easy to find a rocking horse poop. Tires are basically limited to the Continental MPT's in the USA.
-You can get 22.5 dually rims with the 8 lug pattern. Check the truck wreckers and also look at the RV wreckers as a number of RV's had them and got an easier life.
-Another option if you want to go singles is the duplex disc wheels they use on the front of cement trucks etc. These are also hard to find now.
- There are also the European rims that they use on MAN trucks. They run a locking ring and can be used with the 365/80R20 tires. About 400 euros each plus shipping.

10 lug hub pilot:
Spoiled by choices
Accuride Duplex Disc Rims for singles (i.e. Cement truck) 29378SP, 29683SP, 29374SP are the 12.25". 29380SP, 29376SP, 40036SP are the 13". 41140SP, 41142SP are the 14". My SWAG (scientific wild arsed guess) is that the 29378SP would be the best option as it is basically a rim with the mounting surface very close to the outside of the rim. That is assuming your rim mount surface is about 20" difference between front and back like mine is. Back 71” face to face, Front 91” face to face.
Last edited:


Expedition Leader


Medium duty trucks will nearly all run Allison transmissions. Most I have seen are the 3000 series (3060 or 3000EMS). Be careful when checking the vehicles out as some are configured as 5 speed even though the box is a 6 speed. They can be reprogrammed but it costs $$ and Allison is wary about doing it without a letter from the dealer.

In some of the Chevy/GMC (1999 onwards) trucks they have the smaller 2000 series Allison. I have no experience with this particular box.

In the earlier trucks there was a couple of variations. Contact Scooter to get specifics but the earlier box did not have lockup on the higher gears and therefore tended to run hotter and less efficiently. I think it was the AT series that had the 4 speed and no lockup. The MT series had lockup.

MD 3000 series. 6 speed with 2 overdrives. Some came with the ability to check the fluid level from the keypad. This is a major advantage as the oil is very thin (plus expensive) and the dipstick is usually about 5' long. So it is very easy to misread the levels. Allison also is very specific about the fluid to be used. They recommend a TS285 synthetic (preferably the Allison brand) and it takes about 3 flushes to get it into a box with unknown fluid. At 27 litres (29 qts) per fill and about $12 a qt it gets very expensive. But we have had great success using this oil and genuine filters on mine sites. I personally would not run any other fluid in the MD/MH series boxes.

Just as another caveat: Dealers will try to charge you to "reflash" the transmission controller when you change to big wheels. What they are actually doing is resetting the adaptive learning in the controller. This will happen over time anyway if it is not reset, it will just take a lot longer.

Biggest killer of these boxes is contamination from coolant (need to flush and replace clutch packs $$$$). In my heaviest usage such as being bogged in the silt in Baja I have never seen the transmission temperature on any of my trucks approach overtemp. But do keep an eye on the temps if you are crawling in 1st gear as it may not have the torque converter locked and they will heat up quicker.
Last edited:


Expedition Leader

All of my trucks have run the lo ride Bostrum seats. These are a fair quality air ride seat. But what I have found is that they have taken a beating in ambo use. Expect to at least replace the shock absorber. In 4 out of 6 seats I have also had air leaks from the air adjusting switch. These are not that expensive to buy on ebay etc but fairly expensive and not easily available from the dealers. These days I take 1 or 2 with me in my spares box and especially when picking up a new truck.

Bostrum also make a refurbish kit for these seats. Its called FRED. Checkout my M2 ambo build thread for details.

For the rear of the rig some people have been removing the attendants chair and putting in the recliners from a Toyota Sienna. Once again keep an eye on my M2 build thread as I have one in the garage and will be installing it on a swivel base before long.

One advantage of the airride seat bases is that it is not that difficult to fit just about any car seat onto the base. I have used the 2nd row seats out of mini vans on a couple of occasions as they are cheap here in Phoenix. (there is a guy that buys them straight from the handicapped van upfitters). If you checkout the photos of the FL60 I built you will see those seats.

One word of warning. It is just about impossible to fit the full size seats you buy from the truck dealers if you try to use the air ride setups that come with them. Better off saving your money. Or if you insist on trying please buy the one that is sitting in my garage (the princess chair) rather than a new one. Sienna seats will fit but you are wasting $$ as you cannot use the recline feature in the front of our trucks.
Last edited:


Expedition Leader

Send me any links/info you want included. My input will be Rockwell Mertior and hopefully Axletech as a separate thread.

Take your title down to Freightliner and get signed up for DTNAconnect. It will give you access to your parts including the actual diff carrier code. Then you can use this chart to define if you have a limited slip or no spin diff. Basically if you have a "F" for the 9th character it is an open diff. If you have anything else you are lucky. You should also be able to read this straight off you diff tag.

View attachment 347170

My axles are RS 15-120 rear and FC941 front. The width is Back 71” face to face and Front 91” face to face.
Last edited:


Expedition Leader

MNtal's excellent summary.

Personal Opinion / Preference Ranking
8.3L Cummins - 1/2 million mile motor, and as reliable as death and taxes. More power than you can shake a stick at. They put them in 60-80,000lb city busses and rod the poo out of them. In-frames are fairly easy to do and can be done for $2-3000.
5.9L Cummins - Baby brother of the 8.3L. They are very capable of moving 25-35k lbs all day long but slightly more susceptible to heat issues in large vehicles, or vehicles pulling a lot of weight. They are the probably the most common Diesel Engine in the US other than the Ford 7.3L.
DT466 International – Reliable as they are loud, also a 1/2 million mile motor. The only other motor I would consider but definitely not my first choice.
CAT 3126 – Respectable engine, slightly more expensive, Only engine I do not have personal experience with.
T444E International – DT466’s little brother, meh
Mercedes – I wouldn’t touch them *****.


Expedition Leader
Cruise Control Switches. Part Numbers

Freightliner FL60 1999. These are supposed to be the part numbers for the cruise control switches for one running the 24v ISB Cummins. (not sure how cruise works on the mechanical injection ones)


In theory the wiring is already in place including the links to the Cummins ECM.

More to follow if I get it working:Wow1:


Expedition Leader
Cummins 5.9 Freightliner FL60 1999 Oil Pressure Sender options

This is more for record keeping and helping those searching for Cummins oil pressure problems.

I have been having an ongoing intermittent check engine light. After getting a couple of tools to run diagnostics it came down to oil pressure low. Now my truck has a good old manual gauge so I could confirm there was nothing wrong with the oil pressure so it was just the feed that the ECM was seeing. Not a big deal I thought. New sender and check the wiring and I am all sorted. But Cummins threw a wrench into the works. It seems like the old sensor/sensor ( cummins 3408428 ) is now replaced by a switch ( 4076930 ) and a wiring harness ( 3164755 ) and it needs an ECM re flash to work. It also turns the variable reading into a set 55psi effectively creating a dummy gauge.

Some some more google analysis I found a couple of possible alternatives. 1 was an International part number and when I checked it out it would require rewiring. The other ( Cummins 4921511 ) has been superseded a number of times ( 4326849 ) is a straight replacement and does work.

So to summarize for the inter web searches. Cummins ISB 24v
Oil pressure sender sensor 3408428 can be directly replaced with a 4921511 or 4326849 sensor directly from Cummins. At least it worked OK on my 1999 Freightliner.
Egad! Another ambulance thread! Wonderful!!

Here's a list of all the ambulance manufacturers I could find - there may be others, but these are at least out there:

• American Emergency Vehicles (AEV)
• Braun
• Crestline
• Demers
• E-1
• Excellance
• Frazer
• Frontline
• Horton
• Leader
• Lifeline
• Marque
• McCoy Miller
• Medic Master
• Medix
• Med-Tec
• National
• Osage
• PL Custom
• Road Rescue
• Taylor Made
• Wheeled Coach
At $41,900 it's way more expensive than the ones I've read about on this forum. When I asked for opinions on different engines the Duramax got no mentions.

Sent from my iPhone somewhere on the face of the earth using Tapatalk
If it's more than 15 years old, the Fire Chief's Association states that it should be only used for stand by or reserve. If they want $42,000, it's probably being sold as an operational, usable ambulance instead of a retired ambulance. Retired units usually run less than $25,000
It's a 2008 so you are probably correct. It does still have stretcher , o2 and other emergency equipment. 72k miles.
It's the 4x4 that interested me.
I have not seen many MDA's with 4x4. Right now my small 4x4 Ambo suits me but if I hit a point where I can do more travel in the future I feel an MDA would be more appropriate. I really want 4x4 even though it is not necessary most of the time.
Maybe those hydraulic front drive axels will come down in price by the time I am ready to step up.

I'm interested in knowing about the Duramax as well! I did not see it on the engine list.
Last edited:
OK - 2008 is still within the 15 year window, and it's less than 10 years old, so it is definitely an ambulance that could be put into service, particularly also since it is 4wd.

Many departments retire ambulances at 100,000 - 150,000 miles, and usually when they are about 10 years old. Volunteer operations may run them longer.

Also, a fire department could keep it even after the 15 year window if it was a backup unit - as long as they had at least one primary ambulance, they could probably retain that unit (especially with 4wd) for use during blizzards, etc.
From Wikipedia:

The Duramax is a General Motors diesel engine family for light to medium duty trucks, designed by GM and Isuzu. The 6.6-liter Duramax is produced by DMAX, a joint venture between GM and Isuzu in Moraine, Ohio. The Duramax block and heads are poured at The Defiance GM Powertrain foundry in Defiance, Ohio. This engine was initially installed in 2001 model year Chevy and GMC trucks and has been an option since then in pickups, vans, and medium-duty trucks. In 2006, production at Moraine was reportedly limited to approximately 200,000 engines per year.[1] On May 9, 2007, DMAX announced the production of the 1,000,000th Duramax V-8 diesel at its Moraine facility.[1]

So, it's basically an Isuzu design, but it sounds like there are plenty of them out there. Me? I'll stick to my Cummins