Our ML320 came to us with over 208,000 miles showing, and all the marks of a long life spent in faithful service. The trim is faded, the seat covers are split, and a quick check with an OBD-II code reader showed a cascade of complaints. Most alarmingly, the four-wheel-drive transfer case would not reliably shift into low-range. Before we head into the mountains to run the off-road Gambler event, we want to be more confident we will make it to our destination and home again.
Fixing the four-by-four
The W163 ML-Class has a full-time four-wheel drive system. There's a standard transmission and a transfer case, which splits power to two driveshafts for the front and rear wheels. The transfer case shifts between standard and low-range gears by means of a small electric motor mounted on the case. In our case, the motor wasn't engaging or disengaging low range all the way.
Taking the electric motor off is easy; it's just four 10mm bolts and a wiring connection, and it's right there on the transfer case. A "remanufactured" replacement motor is about $100, but the one we bought was completely dead right out of the box. This is a common occurrence, according to the W163 owners we asked.
Instead of ordering another replacement motor, we decided to try repairing the one we had. There's a plastic cover over the gear assembly, which we removed with three T15 Torx+ screws. We thoroughly lubricated the turning gears and then replaced the old motor and cycled it through a couple times - it's working perfectly now!
Swatting OBD-II codes
Our trusty Onboard Diagnostics 2 (OBD-II) code reader shows engine and transmission fault codes on any US passenger vehicle sold since 1996. It told us that we had a number of problems with our air-fuel mixture, starting with a faulty oxygen sensor. So we bought a replacement O2 sensor from amazon.com for $48. Then we learned the W163 actually has four of these sensors; two on each side. To be honest, we guessed which one to replace first.
Replacing an O2 sensor is easy, if you have a 22mm wrench handy. Luckily we do, so it was a matter of about 5 minutes to replace the passenger side sensor closest to the engine. Just unplug it and unscrew, then screw in the new one and plug it in. We got lucky because this did the trick and our engine codes went away. This will improve our fuel economy, and in the long run will save our catalytic converter. We'll replace the other three sensors if any of those codes come back.
Light up the night
Our ML320 came with the optional PIAA driving lights attached to the optional front brush guard. These lights look very sporty, but they have no discernable effect on road lighting. We fixed that with a 24-inch Sylvania LED light bar, available for about $100 at any auto parts store.
The wiring for the PIAA lights was right there behind the lower grille, so we clipped off the end from one of the PIAA assemblies and connected it to the Sylvania unit. Then we bolted the new LED bar to the brush guard and it lights up the road ahead like broad daylight, using the factory dash switch. We also have the second wiring connection to the other PIAA light if we want to add fog lights or rear-facing lights later. We also replaced a burned out headlight bulb.
Important tip: Remember that it's illegal to drive on public roads using a light bar, because it can blind oncoming drivers. This addition is for off-road use only.
Protecting our battery
Like many special-purpose vehicles, our ML320 doesn't get driven every day. With any such vehicle, slow drains on the battery are a concern. So we took this opportunity to install a couple of special devices that had been sent to The Star offices for evaluation.
The first is a smart battery maintainer made by Shell, the same company that has gas stations around the country. The battery in the ML320 is up by the passenger-side firewall, so we routed the connector cable out near the windshield. Now we can leave the vehicle plugged in and the battery will be fresh when we want to drive.
The Shell unit is special among battery maintainers because it automatically detects 6-Volt or 12-Volt batteries and charges both appropriately. It will handle vintage lead-acid batteries or the newer AGM or Lithium-Ion batteries just as well. The smart circuitry protects you from reverse polarity, over-charging, and short circuits. You can buy the Shell SBC400 in 1 Amp or 4 Amp versions for less than $50 at amazon.com.
Monitoring the charging system
The second item we installed on the battery is the Ultimate9 Bluetooth Battery Monitor. This little device installs alongside the battery maintainer, but connects with an app on our phone to give us a readout of the battery's overall health and the efficiency of our vehicle's charging system. With any 24 year-old vehicle, this kind of device is a good idea.
The screens on the app tell us the battery's voltage capability, and how much electricity is produced by our charging system, anytime we're in or near the vehicle. You can buy the Ultimate9 for $45 online at ultimate9.co (note the ".co" instead of ".com").
One more and we're ready to go
The last things we did to get ready for our adventure are simple. We bought a full-size spare tire because the W163 came from the factory with a little "convenience" spare. That won't do in the back country, so we found an ML320 wheel on eBay for $60 and invested almost $300 in a matching all-terrain tire. Before we head out, we'll also buy a $100 roof basket from Harbor Freight to carry our spare tire and an extra can of gas. Finally, we'll carry a full off-road rescue kit, including recovery straps, first aid, and fire extinguishers.
Altogether, our repairs and improvements to the ML320 amounted to about $700, not counting the bad shifting motor that we'll return. Add that to our initial investment and we're into this off-road adventure vehicle a little more than $3,300. We'll tell you how we did in the back country in the next issue.
