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This should help you to determine the overall conditions of your particular Unimog. However, there is no guarantee that every vehicle shows the same symptoms. Most of the info presented here is nothing new about regular wear and tear on older trucks. You can start with Gas-Engine-Diagnostics or Diesel-Engine-Diagnostics and slowly move on to the Transmission-Tests. The Drive-Train-Checklist is very important on Unimogs because of the hub-reduction-gears, sometimes also called final drives, can be very costly if warn out beyond repair or simply broken.
If you have the luxury upon buying a vehicle, that you can test drive it for a longer distance or time, please check and note down the engine and transmission oil level. Get on a freeway (to warm up oils) and go for a hundred miles or so and check the oil level again. The transmission should not show any signs of loss of oil. The gas engine's oil consumption should not exceed 1/2 pint of oil. The OM352 oil consumption should be less than a pint of oil as well. The consumption on a real good engine should be way less in both of the cases.
There is always the big question : How About Rust on your lovely Unimog ???

Gas-Engine-Diagnostics : This paragraph applies only to the M180 or M130 6-cylinder single overhead cam engines used in the models 404.1 and 404.0 Unimogs.

1. Check the motor oil and water/antifreeze in the radiator for the appropriate level and eventual contaminations (make sure that the engine oil does not contain water (could look somehow like mustard) or gasoline (you can smell it), the cooling liquid should not show oil floating on top it either).
2. Take off the valve cover and check the clearance on all valves. If some of them are too loose, it's not as bad as if they are too tight. If they are too tight, chances are that they could get burned out on their seats which is not good for the compression on the individual cylinder. Since the valve cover is open you can check for worn timing chain guides which are an indication of quite some hours on the engine. Make sure the timing chain is tight on both sides of the cam shaft sprocket and try to lift the chain on the top of the sprocket. If you can lift it just a bit, the chain is worn out.
3. Warm up the engine, drive the truck and listen for any noise coming from the engine, like knocking of any bearings or wrist pins.
4. Take the spark plugs out (look at the color of the insulator, should be brown except if the engine idled for a long time or other compression or oil leaking failures, wrong spark plug, etc...) and perform a compression check on all cylinders with the throttle at least half open. It will take about 3 to 4 power strokes to get the final result on each cylinder (make sure that the battery cranks the engine on the last cylinder as well as on the first, or the results will not be accurate with a weak battery. The readings can be somewhere in the range of 100-150psi depending upon the compression ratio of your engine (stock Unimog engine or cylinder head from 220/280S passenger car). The important part is that the readings on all cylinders should be about the same (+/- 5psi). The bigger the deviations, the more wear on the engine. If the readings are not close together, you can squirt about 1/2 cubic inch of motor oil into the cylinders (through the spark plug holes) crank the engine a couple of turns (with the starter) and do the compression check again. If the readings go up more than 5%, then the piston rings are already allowing some blow-by.
5. Take off the distributor cap and try to turn the distributor finger a few degrees. You should feel the spring load of the centrifugal advance mechanism and the finger should return to the initial position upon release.

Diesel-Engine-Diagnostics : Here we talk about the OM352 engine used in the 406/416 and some later models. To a certain degree it an also be applied to other Diesel engines as well.

1. Check the motor oil and water/antifreeze similar to the description in the gas-engine-diagnostics.
2. Take off the valve cover and check the clearance on all valves, similar to the description in the gas-engine-diagnostics. The timing chain test does not apply to the OM 352 engine, since it's not an overhead cam shaft design.
3. Warm up the engine, drive the truck and listen for any bad noise coming from the engine, like knocking of any bearings or wrist pins. It will be a lot harder to distinguish between the Diesel knock and rod knock or main bearing knock. Sometimes a worn injector can cause the engine to exhibit loud knocking as well (it's not as easy as with a gas engine)
4. Take the injectors out and perform a compression check on all cylinders (there is no throttle valve on most Diesel engines). The procedure is similar to the gas engines above, except the readings should be in the 230psi range for the OM352 engine.

Transmission-Tests : One of the worst damages here are caused by a very simple problem, lubrication failure, as indicated in item 1.

1. The drive trains (drive shafts) are completely enclosed, which is a very good design. However, if the transmission leaks oil to the drive shafts, you will not see any leakage on the outside or on the ground. All the oil will fill up the torque tubes, encapsulating the drive shafts and it will finally leak into the differentials. What this means is, check the transmission oil level along with the differential oil levels. If excess oil (1/2 pint or more) pours out of any differential it can be an indication of transmission leakage.
2. Try to shift into 5th gear from both sides, 4th as well as 6th and make sure you don't grind gears. The synchro on 5th gear is usually stressed the most on Unimogs for the following reasons. The gap between 5th and 6th is quite large, so the engine will run out of power on uphill slopes and you have to shift down into 5th gear. On the 404 model this might even force you to shift down to 4th gear, which makes you to go through the 5th gear twice.
3. Try to find a very steep uphill slope (45 degree or more, but don't flip your Unimog over) with soft ground and try to climb it with the differentials locked (don't lock them as you climb). It should be so steep that all 4 wheels will start digging. That's a very good way to verify with the help of an outside observer that all the wheels are locked.

Drive-Train-Tests : This wouldn't be a big deal on any normal vehicle, but our Unimog is equipped with hub reduction gears which give it the exceptional ground clearance and relatively small differential size (less torque on the differentials and drive shafts).

1. Check the oil level on the hub reduction gears by opening one of the upper 2 fill screws. If oil drips out, the level is ok. If it pours out, it has either been overfilled or the differentials are leaking into the rear axle hub reduction gears (the differential leaking does not apply to the front axles, because of the exposed CV-joints on the front axles). If you have to refill or add, open both fill plugs and start filling from one until it starts flowing out the other one.
2. A very important test is to look for metallic contamination or water emulsion or rust in the oil. If the oil has symptoms of metallic flitter (looks a bit like metallic paint), there has been some wear inside. Most of the time it can be cured by changing the oil a few times, but check for wheel bearing play on this particular axle end. There is also a chance that the oil could be contaminated with water. The symptoms would be either mustard colored oil or rusty looking oil, depending on how long this contamination has been present. In either case the wheel bearing play or excessive gear play could be a judge about eventual damage to the hub reduction gears.
3. Look on the wheels for any leakage for oil, to find out if the seals are still working properly. Don't get it mixed up with leaking wheel brake cylinders or calipers. The oils seals are relatively easy to replace on the 406/416 model, however on the 404 model you will need a hub puller to do the job.
4. The wheel bearing play and some of the ball joints can be checked with the wheel jacked up. Also look for gear play as well. The steering related ball joints are best checked with the wheels on the ground and one person moving the steering wheel left and right while the other inspects the play on all of the steering ball joints.
5. Off course, there should be no oil or grease leaking out of the steering box as well.
6. The brakes deserve some attention to leakage and wear on the pads or shoes as well. Don't forget to inspect the oil lines on the axles for pinch spots from a previous trailer tie down. No one of the rubber brake lines should exhibit any cracks or other symptoms of aging and abuse.

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