Mechanic's Truck : 1994 GMC Topkick with Dakota Body by Feterl

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If you are in the logging business, you have equipment--lots of it and it is expensive, and it breaks. For the most part, hauling the pieces into a shop for repairs is often not a very practical option. To some extent one may rely on dealer support to fix some things, but nearly every logging company ends up with a mechanic on duty. The nature of the work means that the mechanic must make house calls. The Mechanic can't stay in a steam heated shop building beside his Snapon Tool box and await the arrival of things to fix. If the operate could get the equipment to move as far as the shop, he would still be using it.

New: For a rundown on the tools this truck needs see: Mechanics Tools

This means that the shop goes to the equipment. The name varies, 'Shop truck', "Service truck", 'Mechanic's truck' -- but the meaning is about the same -- a support vehicle which will provide all one need's to fix what needs fixing.

These special purpose vehicles are generally designed to support a particular industry. If you look around you will see lots of trucks with 'service bodies' on them. This is often anything from a pickup on up with a bed on the back with side compartments where you can stow things.

Beyond that you will find special purpose equipment for the industry. If it is an electric utility you will find a bucket boom, or a digger-derrick facility added to the stowage body. Phone company trucks, and cable TV trucks are often have much smaller buckets --as they don't have to reach as high.

Railroad service trucks likely have no boom on them at all. Logging, mining, and agricultural service trucks usually don't need a man lift, but they do need a crane of some type, as the pieces of the equipment are often far too heavy to be 'man handled'.

One of the more popular brands of crane for a mechanic's truck is an "Autocrane".This manufacturer reputedly got its start building a folding crane that could live inside the trunk of an automobile and could be used for lifting drill bits out of the trunk for use in the oil patch.

They typical tool body made for a service truck is made of sheet metal, and a few are even fiberglass. It keeps the weight down (which is a problem) but when you start wanting to add a crane to the body, then things suddenly change. It is no small engineering feat to stick a crane up on the rear corner of a truck and have it lift heavy weights without prying the corner off the truck.

In the timber industry there is a strong tension between getting a truck big enough to do the job and small enough to be affordable or to not require a Commercial Drivers license (CDL).

By far and away the most common mechanic's trucks in the timber industry are Ford Superduty F350's. This is a one ton dually, and their popularity is not so much their usefulness, but their cost.

Crane issues

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The truck featured here is a cut above this minimalist truck, and is popular for serious mechanical work. The Dakota intermediate model calls for a 84" CA (Cab Axle) truck with at least a 24,000 lb rating. It is then equipped with a heavy steel service body, an 8,000 lb rated crane, and a hydraulic air compressor.

As with all cranes, the rating is good for comparison purposes but doesn't tell you much about the practical lifting ability of the crane as the rating is based on lifting capacity "at 85% of tipping" with the boom in its highest position (almost straight up. This implies that 8000 lbs is achievable on a 3 foot radius without tipping the truck over. Given that the right stabilizer extends out 3 feet or more this isn't surprising. The crane has only a 4000 lb cable in it which of course you can effectively enhance by using a block purchase.

My point here is not to insult the crane which is very credible and suitable, but rather to suggest that you should think of an 8000 lb rated crane as able to handle 4,000 lbs---this crane is rated for 4300 lbs at 11 foot radius (which is the length of the boom and therefore the radius with the boom horizontal. The boom, of course extends which will further reduce the lifting capacity.

The empty weight of the truck is just under 20,000 lbs before you add a welder or tools which is heavy for a truck of this type, but still,the limiting factor in what the crane will lift. You can't expect a 20,000 lb truck to stay rightside up if you try to lift 8,000 lbs 20 feet off to the side of it or behind it for that matter.

Cranes collectively as used on service trucks may be of several types. Some you will find are 'electric over hydraulic' which implies that a central hydraulic system is not used, rather, the crane uses electric motors for as many functions as possible and uses an electric on demand hydraulic pump for other functions. At least some of these units will use a cable gantry to lift the boom (with an electric winch).

The other approach is a full hydraulic system. This is in my opinion the better type. The truck has a major PTO driven hydraulic system, and the crane operates with a series of switches which control electric spool valves. On the crane I have there are but two hydraulic lines (a pressure line and a return line) that pass through the rotation circle. It is what they call a 370 degree rotation. This implies that you can turn it all the way around and there is no stop, but the electric wires have no swivel in them so if you keep going around and around you will twist up the wires.

While the electric cranes run off your battery (and can be used --to the extent of your battery capacity---with the truck shut off, obviously the hydraulic cranes require the truck engine to run, and run, and run. This can make for a lot of hours on the truck engine idling. This is a total disaster for gas engines, as idling the gas tends to wash the oil off the cylinder walls and engine life is really bad. Diesel engines handle idling better, though this service truck with only 120,000 miles shows quite a bit of blowy suggesting that even diesel engines are not immune from wear while idling for extended periods of time.

Compressor issues

In times past it was common to install a belt driven compressor under the hood and run it as an accessory off the engine. This has fallen into disfavor for a couple of reasons. First, the compressors require more power than the accessory drives are designed for (which leads to slipping belts and the like) and second, in the newer vehicles there is so much other garbage under the hood that there simply isn't physical space to stuff one more thing in there. This means that service trucks now usually have either a gas drive (separate gas engine) compressor or a hydraulic drive compressor. The Dakota system shown here has a hydraulic driven 20 FM compressor in the front of the service body with the air tank directly above it. Some trucks will place the compressor either in a compartment or up on top of a side compartment. This keeps the bed space open, but at the price of other space.

There is also the problem of the air tank placement. Small air tanks can sometimes be stuffed under the truck or put in some out of the way place, but bigger air tanks are better, and since the preferred 60 gallon air tank is larger than a barrel, it needs a lot of space. Placement of the compressor (and hydraulic tank on the floor in the front of the box with the air tank above it is about as good as you can do.

Welder Placement

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Miller Bobcat 225G

Welder placement presents a similar problem. The welder doubles as the generator for electric tools and is a must for a complete service truck. Really small welders can be mounted 'on the rail' (usually on the left side as they won't fit under the crane boom which goes up the right side. This works for the smallest of the gas drive welders, however a perennial favorite welder is the "Miller Bobcat' a 225 amp 3600 RPM welder (with a robust 6 kg AC capacity) and it is a bit robust for such an installation. It is 4 feet long and just under 2 feet wide and will technically fit up on top the box, but it is really too big and heavy for such an installation. We mounted ours crossways and a couple feet off the box floor directly behind the compressor. This way you can get to the compressor and can slip stuff under it that you may be hauling.

What you really don't have space for in a truck of this size (with a I'D foot box) is big welder such as Lincoln commander or a Miller "Big 40". These welders are of the 400 Amp capacity and often have a 4 cylinder water cooled engine turning at 1800 RPM. They are often 5 or 6 feet long and weigh in around 1500 lbs (Miller Big 40). They are so heavy that to keep the truck balanced you have to put them in the middle of the truck, and they are too long to fit inside the body crossways and too heavy to put above the body so if you put one of them in your truck, it is full.

If you can't live without a 400 (or larger) welder, then this truck is not suitable for your needs. There is, of course, a solution, and that is the next size larger truck. Instead of using a relatively short 84" Cab-Axle truck, you go to a longer wheelbase truck, and put either a welder deck or a welder box in front of the service box. The welder deck is cheaper but leaves your welder out in the weather. Some people will use a 4 foot welder deck and stick both a large welder and a gas drive compressor on it. This gets the compressor out of/off of the service body. The extra 4 feet of wheelbase makes the mechanics truck a barge which may or may not be an issue, but the ultimate mechanics truck has a 400 amp welder. There is plenty of reason to have one if you are doing heavy work. You need the extra amps for heavy welding (as with 3/myths" and 1/4" rod) The big welder is also much more proficient with an Air Arc (for scarfing out old welds) and some models (particularly the Lincoln Commander) will produce enough AC current to run a really large plasma cutter.

Tool Stowage

A Mechanics truck isn't complete without tools. The problem is how to haul them. There are a couple different solutions. Some truck bodies just have lots of 24" doors and you can put shelves in, or a drawer kit for your tools. The other design has a couple large double doors on the left which allow you to stuff classic mechanics red tool boxes in the compartments. The compartments are more than 20 inches deep, so the 18 inch deep tool boxes fit nicely. The previous owner of this truck had a 40 inch long box in it, however we settled for a 36 inch long Proto roller box (with the wheels removed) in the front compartment, and a 'top box' with the lid removed over the wheel well. We then constructed shelving beside it for oversized items. Large and heavy tools are the order of the day in the logging business. One of the features of the truck is a 1" drive impact socket set from 7/8" to 3 inches by 1/16"s with some of the sockets actually being 1-1/2" drive. I made a special shelf in top of the center compartment out of a lightweight 8 inch I-beam to hold this set as it was far too heavy to go in the tool box.

I carry both an 1-1/2" impact wrench and a 1-1/2" drive X4 compound which facilitate the use of the 1-1/2" sockets that are blended into the 'set'. A Proto 1" drive rachet, and a Proto 1" T-Bar, and a couple chunks of 1-1/4" water pipe about 4 feet long round out the collection. For serious hard turning, I use double X-4 compounds and stack them. The big one is an X4 model 1500 with a 4:1 ratio and a 3/4" drive in and a 1-1/2" drive out. On top of that goes a smaller compound with 1/2" in and 3/4" out. Normally I then run the mess with a half inch ratchet, but when push comes to shove and something won't turn, I use a half inch flex handle, and have been known to enhance that with a short pipe, which is why I started buying 1-1/2" sockets when I could find them on the second hand market. I just flat twisted off a Proto 1-1/2"-1" adapter one day, and they don't warranty those adapters.

Beyond the big stuff, you need the 1/4", 3/8", 1/2" and 3/4" drive socket sets in both SAE and metric, and a good assortment of end wrenches in both metric and SAE up to about 2-1/2". Brand name wrenches become somewhat unaffordable beyond 1-1/4" so we visit the cheapy mail order tool places for the larger sizes.

The left rear door of a tool truck traditionally has the hanging things, and my truck is no exception. The hammers and punches get the upper shelf and the bottom is dominated by a 24V battery charger. Those things take up a lot of space, but are a necessity when you have 24 volt equipment to service. The Miller welder also provides a convenient AC power supply to run the charger.

On the right side of a mechanics truck you will usually find the gas bottles in the front compartment, and the welding supplies nearby. Sometimes the welding leads are housed with the gas bottles, but we elected to put a 25 gallon propane bottle in the Bottle compartment leaving no space for the arc welding leads. Since the welder was 'up top', we put a rack under the crane boom for the leads.

The second door back gets the welding supplies, various grinders, an electric drill and drill bits, etc. Parts and shop supplies go above the wheel well on the right. The section under the crane has the crane remote control, a self retracting reel with 50 feet of half inch air line, and space for sling chains and shackles for the crane.

The back Bumper

A mechanics truck is not complete without a wide back bumper/workbench. Feterl graciously provides a back bumper about 30" deep at a convenient working height. There is stowage underneath it for long items. Some Wooden blocks, bars, sledge hammers, chunks of pipe are in order here. The previous owner also carried an inventory of iron here for making field patches to broken equipment, "bandaids" which he would weld over cracks and breaks if you will.

The Back

With the compressor and air tank taking up a couple feet in the front, you have about 8 feet left inside the bed to haul whatever else is needed. The Welder cuts into this space, but with it elevated 3 feet other things will slide under it. We have chosen to use a part of this space for a 'spare tire'. Since the Tire and wheel size is the same onthe mechanics truck as it is on our highway trucks (10x22.5" tubless), the spare serves double duty as a spare for the highway trucks that don't carry a spare.

Selection criteria for Mechanics tool truck or Service Truck

Service trucks tend to be spendy things. If you have owned one or used one you likely already have some pretty strong ideas about what you want or need. If you have never had one, any truck seems better than what you have, and the differences are likely just a blur.

There are, however, some very critical decisions that you have to make in considering these trucks. The first thing to consider is the type of truck (chassis) that you want. If you look around, you will see all sizes of trucks. You will see some pickups, and you will see a good number of 1 1/2 ton trucks(Usually a Ford Superduty), and then you will see the mid range trucks which are further subdivided into the 26000 lb class and the over 26,000 lb class which, of course, is the breaking point for the need for a CDL driver's license.

I suppose there is a place for all of them. On the light side, the pickup with a small Autocrane and a service body---It works for light maintenance. If you are changing filters, and jump starting dead batteries, and maybe changing a truck tire on the road now and then, a pickup will handle the job.

On the other end of the spectrum, If you are doing full field service work on heavy machinery, there is no substitute for the over 26,000 lb CDL required field service truck. Trying to use a truck under the 26,000 rating to dodge the CDL requirement just doesn't cut it. If a mechanic is to be trusted to perform field service work on machines that may be worth half a million dollars or more, and use a $100,000 truck to do it, he ought to be able to pass the drug test to qualify for a CDL license. You simply can't take a truck and add a service body, a crane, a welder, gas bottles, a compressor and every took known to mankind and have the result weigh less than 26,000 lbs, and still have reserve capacity enough to haul some heavy parts to the job site--or broken parts back to the shop.

The most regretted field service trucks, however, are the Ford Superduty 1 1/2 ton trucks---- usually rated in the 16,000 lb range. Quite a few of them get sold to penny pinchers who think they can buy one of these trucks and make it function as a heavy equipment field service truck. They are a whole bunch cheaper, and on the second hand market are real cheap, and for a reason. They have died. They are uniformly overloaded, overweight, and equipped with welders, compressors, and cranes inadequate for the job. When you find their remnants in a used equipment auction, they have clearly paid the price. You simply can't do field service work on heavy equipment with half a truck. It takes several tons of tools to do the job, and the ability to lift thousands of pounds with the crane. After all, the field service truck can't do field service unless it can reasonably lift the engine out of the largest machine that it supports. Note the word 'reasonably'. Remember, cranes are rated with the boom pointed straight up. What they will handle with the boom sticking straight up, and what they will handle with a radius sufficient to pick an engine are two different things. For a rule of thumb, I would divide the 'rating in half to get a feeling of what you can reasonably do. This implies than an '8,000 lb' crane will, for example handle a contemporary diesel engine up to around 4,000 lb models---which these days means most that are less than 500 HP. Indeed the question in my mind is whether the now sort of industry standard 8,000 lb crane is really large enough. No field service truck ever had a crane that reached too far or lifted too much. Just remember in thinking about what needs lifted, that you will we using it at a 10-14 foot radius, not the 3 foot radius at which they are rated.

Think about the size of air compressor needed. Running a blow gun or a paint sprayer is one thing. Big impact wrenches are another. A little gas drive compressor doesn't make the cut, nor does tapping off the trucks air brake system. IN the days when service trucks had gas engines 'running the truck' was a real problem as it burned prodigious amounts of gas and cut out the rings due to poor lubrication when run at low load/speed conditions. Diesel engines are far less sensitive to these problems. Do expect to size a compressor large enough to sustain your impact wrench. Otherwise you have wildly changing air pressures which produce wildly inconsistent results in your work. Inasmuch as 1 1/2 inch drive impact wrenches are a practical necessity in heavy field service work compressors in the 50-70 CFM range are appropriate. To be sure, it is a lot cheaper to try to skin by with a 1 inch drive system, but those wrenches max out at around 1200 foot lbs of torque on a good day. This just doesn't make it if you encounter a string of rusted down 7/8" or 1" bolts---or even non-rusted down larger ones. I think someone has a rule of thumb that the wrench drive size should normally be as large as the bolt size, unless it turns hard, and then a size bigger.

Anyhow, I am a great fan of the truck driven compressors. If you have a hydraulic system for the crane anyway, a hydraulic driven compressor is a no-brainer. Autocrane has a line of electric cranes which are quite popular but I see them as a holdover from the days of gas powered service trucks as the electric cranes can work off a battery saving the need to have the truck running forever. Another consideration about whether you want a truck that needs the truck engine for stationary work is whether you have air brakes or not. The smaller trucks likely won't have air brakes which in turn implies that they probably won't have a reliable parking brake which makes them a bit of a menace to society unless every where you work is very flat.

UPDATE: March 2004

I just returned from a large Ritchie Bros. Auction at Maytown, Washington. Ritchie Bros. is a big auction house that sells used machinery often in fixed locations around the country via periodic auction sales. It is the common dumping ground for equipment whose owner no longer loves it or wants it. Tired old trucks get redistributed by the hundreds in a couple of days, and at least this year half a dozen or more service trucks from the logging and construction business were in evidence.

There weren't any two of the trucks alike. What I would class as the not quite big enough trucks dominated the list with most not big enough to require a CDL, and having a 4,000 or 6,000 lb crane instead of my preferred choice of a much larger crane. A vast majority of the cranes were 'Autocranes' which are unique in the business in that many are electric, which implies that they can run off of a couple batteries so you don't need to leave the truck engine running to use the crane once in a while. The Autocranes are easy to recognize, as they have a boom that is held up via a multipart gantry cable, instead of a hydraulic cylinder. This is not all bad, but I think the hydraulic telescoping boom has something to offer, particularly if they offer feathered controls.

My take is that the advent of the diesel engine for mechanic's trucks sort of shifted the balance here. Gasoline truck engines really do bad things if they idle for hours at a time. They burn lots of fuel, wear out the piston rings, foul the spark plugs, etc. By contrast, the diesels idle fairly efficiently, and don't suffer as badly from cylinder wall lubrication failures as to the gas engines during long idles.

The newer electronic diesels have the 'hooks' in them for remote starting and stopping of the engine, and I would expect as the service crane makers integrate those features into their systems, the bugaboo of extended idle times will finally be put to rest.

On the same lines, Most of the compressors on these trucks I saw today were gas drives---fairly small compressors, often sitting up on the top of the left tool box, either in front or behind the similarly placed welder. Once again, if you are going to run heavy impact equipment (such as a 1 1/2" impact wrench--or even a busy 1" wrench), you need a big compressor and a large receiver as the 1 1/2" drive impact wrench takes a LOT of air.


Another issue that needs addressed if one is building a 'real service truck' is the wheel base. It has long been chic to build a service truck on a very short wheelbase truck. Some got that way because they just got a stronger pickup and put a service body on it, Some got that way because the 84" cab-axle trucks are realy cheap on the second hand market. Other people select them because they are very maneuverable.

Well, if you are going to bother to have a real service truck that busts the 26,000 lb limit you better bust the 84" cab-Axle number as well. My service truck is one of those. (Shorties). The truck is rated for 32,000 lbs, with a 12,000 lb front axle and a legal max 20,000 lb rear axle, and the 84" cab axle will take about a 12 foot tool body (7 feet in front of the axle and 5 feet behind).

Load the truck up with tools to my standard load and it weighs 23,000 lbs. --- about 6,000 lbs on the front, and about 17,000 lbs on the rear. Now, bearing in mind that the front 2 feet of the bed are filled with an air compressor and receiver, I have 10 usable feet of bed exactly balanced over the rear wheels into which I can load how much more in order to get up to the maximum potential legal weight of this truck????? Tell me how to load parts and equipment in the 10 feet of bed that is balanced over the rear axle so that it adds 6,000 lbs to the front axle and 3,000 lbs to the rear axle. Lots of luck!

Clearly, if you are going to need the hauling capacity of a beyond 26,000 lb truck, having an 84" (7') cab axle design is pointless. To be sure the strong truck holds up better and that is a plus, but there is simply no way to load it legally beyond 26,000 lbs. because of balance problems.

It makes a lot more sense to go to a next longer size truck (110") or so cab axle. Even if you use the same tool body, put a welder deck in front of the tool body. A 30" or 36" wide welder deck between the tool body and the cab will provide a place to set the welder forward----and allow for the installation of any sized welder needed--- even the large 1500 lb 400-600 amp jumbo welders while moving weight forward. Alternatively, if you are not into a big welder, you can share the welder deck with the compressor. You still won't get the truck loaded to 32,000 lbs. before it goes overweight on the back, but getting the welder and compressor in front of the service body, and cleaning out the clutter in the front of the body will make a big difference.

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