Tech Q&A

Richard Simonds

Updating seat belts

Q: You wrote an article about the new better seat belt and shoulder harness and even gave the part number in a past issue of The Star. Do you have that part number? All the original seat belts and shoulder harnesses lock you in so tight; you can barely move. I want to replace the original Kangol (magnetic latch) seat belts with the design that has a retractable shoulder belt so that I can move forward to reach the emergency-brake handle and generally have more flexibility in the seat.

A: I just confirmed that the MBUSA Classic Center in Irvine, California, has the new harnesses in stock, with a retractor assembly at the base and a standard latch mechanism like modern seat belts. These belts are for the front seat of Mercedes-Benz models from the 1960s and 1970s. Contact the MBUSA Classic Center as listed below and ask for part No. 114-860-23-85 to verify that this is correct for your vehicle. The retail price for the new, more comfortable seat belts is $454 each.

Key-fob issue

Q: I recently bought a 1992 500SL with 75,000 miles – in show condition –  with everything working correctly except the door-locking system. The manual key works just fine in both doors and trunk, but I have trouble with the key-fob system on both keys. I can open and lock the car from the trunk with the key-fob button just about all the time. However, when I try to do the same near either door, most of the time nothing happens. The key fobs work the locks in the dark and sometimes after I have driven awhile. I have replaced the batteries in both fobs several times, with no difference. I have cleaned the door handles where the red, green and pickup lens are, but that hasn't changed anything.

A: The problem is most likely the infrared receivers on the doors; these are the components with the red/green lights. They will need to be replaced for your key-fob transmitters to actuate the door locks. These are known to stop working after approximately 20 years. Contact your dealer to have this repair done.

Cold-starting an older diesel

Q: With the temperature already hitting record lows here in the Northeast, I’ve been told that in colder weather, I can start my car when the preglow indicator light goes out, that I should then cycle the key several times to power the glow plugs longer before starting, and that I shouldn’t crank the engine too long when attempting to start. Is this true?

A: Mercedes-Benz specialist Jaime Kopchinski responded: All three of these suggestions are myths – at least regarding Mercedes-Benz diesels from the 1970s through the 1990s.

Let’s look at the example of a 1983 300D. Regarding the preglow indicator, the owner’s manual does say to wait until the preglow indicator goes out before starting, but this isn’t completely true for very cold temperatures and older engines.  The glow plugs need 30 seconds to reach their maximum temperature of 1,180C. 

Unfortunately, the glow plug relay is designed to only show a preglow light of 30 seconds if the ambient temperature is -22F!  If you want to maximize the amount of heat your glow plugs are producing for a cold start, they need to glow longer than the light suggests before you start the car. The glow plug relay continues to power the glow plugs for 20-35 seconds after the lights goes out on the dashboard until a safety shutoff finally cuts the power. There is no need to cycle the key; just wait until you hear the relay switch off with an audible “clunk” noise.  By then, the glow plugs will have heated to their maximum temperature.

Regarding how long to crank your car before it starts, sometimes it’s useful to look at the owner’s manual for even the very basics.  In the section, “Starting and Turning off the Engine,” you will find two very important statements:  first, “Release key only when the engine is firing regularly …” and second, “Do not interrupt the starting process.” Now, I know it may sound like abuse of your starter, but they really mean it. Keep cranking as long as it takes and don’t stop, even for as long as 30-60 seconds. You will find that as you crank, the engine will begin to turn faster and faster until it eventually is turning fast enough to run. It takes time to build up the heat necessary for the engine to run on its own. The starter was designed for this abuse – it’s why these cars have such big batteries.

Now you know how to start your car in winter.

What about preparing it beforehand?

Adjust your valves to get the greatest possible compression. I typically do this each fall.

Install a correct-sized battery with clean high-quality cables. Periodically clean the battery terminals, both cables and the ground strap from engine to chassis. 

Use a thinner oil. Synthetic oil, which flows better than conventional oil when it’s cold, allows the engine to turn more easily when starting. Mercedes recommended synthetics for winter use as early as 1979, but modern synthetics are now popular for use in these older vehicles year-round.

Use a block heater, which greatly reduces engine starting effort. Plug it in at least an hour before starting; longer if possible.

Use winterized diesel fuel appropriate for your state’s climate. Use a fuel additive or add kerosene if you’re starting your diesel in cold weather with summer diesel fuel in it. Never use ether or starting fluid; this could cause engine damage.

And of course your engine should be in good condition in the first place, with all of the injectors and glow plugs, and so on, working properly,