Prior to purchase you should either inspect the truck yourself, or have it inspected by a mechanic or inspection company. Try http://www.truck-remark.com/ or National Truck Protection for inspection companies.
You can perform a decent inspection yourself on many of the trucks systems and components. At a minimum you must have a dyno done. This will tell you the blowby numbers, which gives you the most basic condition of the engine. The dyno will also give you the horsepower at the rear wheels. Compare this to the rated hp it should be 80% of the rated power, more or less. Much less than 80% and I would not buy the truck unless it was priced such that in in-frame could be done and still have a decent price for the truck. Another test that can be done is a coolant compression test. This is done under load and checks for air in the coolant system. There should be none. If there are bubbles in the coolant, it is most likely a bad head gasket.
Make sure the truck has a current DOT inspection. This will not catch everything, but at least blatant safety items are covered. A dyno and DOT are normal pre-purchase events. Walk from any deal that does not allow them. If there is a VIS Check facility in the area, it is highly recommended. It will check brake and suspension operation. It catches things that a DOT can not catch. As an example, new kingpins can be over $1200 a VIS will catch this nothing else will. You will almost always have to cover the cost of the dyno and VIS. Sometimes you can negotiate a pass/fail deal with the seller but not usually.
Have the ECM data dumped (printed out). Check the ECM miles against the odometer miles. They should be close or there should be a good explanation. Check the engine serial number against the actual number. Check the average speed driven. Check if the truck was governed to a limited speed. Check for any fault codes. You are looking for anomalies, and things that do not make sense. There should be no charge to dump the ECM.
On the exterior, pop the hood and check for fiberglass repairs on the inside. You are likely to find some make sure they are reinforced good, and that they are not cracking. Note if the bumpers and any bumper extensions are damaged they often are. You can negotiate for new bumpers, later. You will often find minor fiberglass crazing it is not a real problem, but something to note for negotiations. Check the vertical cab fairings they often have damage at the tops. Check the vertical cab fairing brackets they should be in decent shape. On a Volvo look at the tubular brace that ties the fairings to the back of the cab. These are often rusted through. Make sure the doors close correctly, and without an extreme amount of effort. Ensure that the tow hooks (two of them on a Volvo) are in their places in the drivers compartment(s). Verify the fire extinguisher is present and charged and that the warning triangles are there.
Check the engine over for leaks and for general external condition look at the belts and hoses. If you have coolant test strips, check the DCA level and pH of the coolant if it is not extended use coolant. When having the engine checked it is best to be there when they hook up with the laptop so you can see the inactive fault codes before they clear them. Go through the engine, transmission, ABS, gauges, etc. If the engine is a C12 make sure it has had all the updates done. If it is a Cummins truck start it right after the key is turned on. Don’t give the fuel transfer pump a chance to pump all the bubbles out and you’ll know if you have air leaking troubles this will cause rough running.
If it is a T2000 or Pete 387 make sure to test drive it in a 15mph crosswind. Do the doors leak air? Does it have the secondary seal? Push on the corners of all the sleeper panels. Are they still bonded? Play with the wipers. Do they park in the right spot every time? Or are they erratic? ($300 part) If it is a T2, does it have the complex reflector headlights? Have them installed if not (huge improvement). Also tell them you won’t buy it until they install the drivers mirror glass with temperature display (it is a 2 second install) If it has the AG200 suspension have them check the pins & bushings for play. Check for composite front springs (they are rare but are a KW highway tractor option). Have they been beat up with highway debris? Do they have any deep gouges or feathered areas? How are the sleeper side fairings? Are they bent or beat up? ($600 range each piece). Check the clutch rod heim joints and bellcrank for wear. Make sure the clutch pedal has about 1″ of freeplay.
Make sure the radiator ground wire is connected and not torn up. Listen to the fan hub at high idle. Does it rattle and sound rough? How much friction material is left? Check the sleeper leveling valve rod. Is it worn out with loose joints? If it has a cab or sleeper mounted tailpipe listen to it real close during the test drive. Is the drone acceptable to you? Or are you going to want to rip it off and run it over 10 times at the first truck stop you see?
Take a look at the batteries if the box is accessible. In almost all trucks, there are four of them. They should be clean and the terminals in good condition. Other than that, you wont be able to tell much.
Make sure the rear tires have the same tread pattern and wear level. Note if they are virgins, or retreads. Note the tread depth you want 50%. Note the condition of aluminum wheels. It is not likely they will be polished, but check them for deep scratches and curb damage. Wheels can be sanded to get out deep scratches note for negotiations. The frame will be rusty in most cases, but note how bad it is. Sometimes it has been sandblasted and painted. Note the quality of the job there should be no flakey rust left. If the hitch is an air slider, the rails can be used in mounting an RV air hitch on a plate. A regular hitch may require additional bracing to mount the RV air hitch.
Crawl under the truck and look for loose wires, and oil. There will almost always be some oil and grease. That is normal. Look for leaks around gaskets. Check for air leaks where you can (make sure the air is up on the truck).
Check the condition of the headlights. If the covers are milky they need replacing, or polishing. Generally, they need replacing. Check that the headlight high beam switch is operable (the truck needs to be running, with the park brake off). This often goes on Volvos. Also verify that the daytime running lights work correctly, and that the other running lights and turn signals are operable. You would be surprised what does not work even with a current DOT.
On the inside, turn the key on and make sure all the diagnostic lights are functional especially the ABS light. Sometimes bulbs are removed to mask problems. Once the truck is running all lights should extinguish. There is no acceptable explanation for a fault light remaining on it needs to be fixed. Make sure that the instruments all work. They should not constantly jump around. Depending on the instruments you may not be able to verify correct operation for example, rear axle temps can only be verified if you drive the truck some. Check that all switches are functional (to the extent you can without driving). In particular, verify that heat and air works properly dont forget the sleeper controls. Verify that the instrument lights work and adjust up/down. Make sure any powered windows and mirrors work smoothly. The drivers display should work once the truck is running. Go through every screen and verify the LCD display is not damaged, and note the status of the screen. If there is a Road Relay (or comparable display) in the truck, do the same with it. These two displays are very expensive to replace. Check the radio and CD player for correct operation. Make sure all the speakers work (on Volvos, sometimes the ceiling speakers are not connected this is normal). Verify the wipers work, that the intermittent feature works, and that the washer works.
Check that the seat air controls work, and that the seats maintain their settings. Examine the seats for condition of material and foam condition. Seats can be rebuilt, or easily replaced. Figure on spending $2-300 to rebuild a seat, and $600+ to replace a seat. This is typically negotiated as part of a sale. Same with the mattress they are almost always replaced as part of the sale. Check that all interior lights work. If there is a TV, verify it is functional. Verify the refrigerator is operable (check for a dash switch), and that any inverter works. Make sure any doors on cabinets work, and that the latches are functional. Doors often break and are just sitting there. If there is an upper bunk, operate it.
Before starting the truck, check the level of the air system. If it has at least 60psi, bleed the three tanks first (you may need pliers). If it has no air in the system, you will have to air it up first. Check for water and oil. A little is within the normal range, but a lot indicates a bad air dryer, a saturated desiccant cartridge, or another air system problem. A truck with a good air dryer will normally have no fluids in the air system. Air system problems can be expensive and aggravating to resolve. Start the truck and air it up. It should build air at a good rate, and fairly evenly in the primary and secondary systems. Perform an air brake test (if you do not know how to do this, consult a CLD manual, or look elsewhere on this website). Shut the truck off with full air (120-130 psi). Verify that the truck holds air crawl around and check for obvious leaks. Check the air system again after an hour or so. It should not have bled down more than about 5 psi (at the most). If it has, you need to figure out why.
If you have a CDL they will let you drive the truck (and you probably do not need to read this). If you do not have a CDL they may not sometimes they will. If you do not know how to drive a manual, or have never driven an HDT before, now is probably NOT the time to learn. Have the salesman drive it. If they do not let you drive it, go for a ride with the salesman. Check the alignment the truck should track true with your hands off the wheel for 10 seconds or more. There should be no vibration in the driveline, and the wheel balance should be true. Find a place without traffic and check the braking. The truck should stop fast and true from various speeds sometimes it will pull a little, but it should not pull a lot. On an autoshift truck, ensure the truck operates properly in Hold. Verify what gear you can start out with in an autoshift. If the truck is not programmed to start in a higher gear than 2 you can have that changed (in most cases, up to 5). Check the clutch brake operation the brake should engage within a half second or so of fully depressing the clutch (at a stop). Otherwise it may need adjustment or replacement. Obviously, the truck should accelerate smoothly without misses or surges. Verify the cruise control works. Verify the Jake works in all positions.
Everything is negotiable. You may not be able to get the price down, but you may be able to get lots of work done for that price. If you are buying from a dealer then you can get lots of work done as part of the deal. If buying from a broker (without repair facilities), you may not be able to get any work done, but you can negotiate price, based on needed work. Do not sign the delivery papers or take delivery until ALL the work is done. A deposit is normal and expected. Make sure it is refundable without penalty. Get this in writing. Things to negotiate:
Parts and accessories you want at dealer cost. They will likely restrict this to stuff specified at time of sale. All future parts and service at fleet rates. Clarify what that rate is on labor. New rubber for free, or at reduced cost. The tread on the front should match if not they should correct for free. The treads on the rear should match if not, correct for free. I would negotiate for new virgins on the back at asking price, or at reduced price for the tires as part of the deal.
For sure, there should be a new mattress.
New seats at dealer cost, or reduced cost. Sometimes included in the deal.
Complete service all fluids changed. Synthetic in rear and trans. Oil changed. Filters changed, including the air filter. Spare fuel filter(s) included.
If you dont know when the last overhead was, negotiate a rate for an overhead.
Body work for asking price, or a reduced rate. The bumper should be in good shape, or negotiate a price now. If you want to upgrade the bumper you may be able to get dealer cost. Now is the time.
If in doubt about the batteries, negotiate to replace them. There should be four batteries if not, have them add one and negotiate to upgrade the others to new ones.
Alignment should be included if needed. Same on tire balance.
Negotiate with the dealer to reset the ECM parameters as part of the deal. Almost all dealers will do this.