I have been a long time lurker of these forums and finally joined because I think I have something that isn't covered yet.

I just bought a 1986 300SDL with 308k. My grandparents had the same car a while ago so I figured I would try to get one since I have worked in it before.

I bought this car and more was wrong with it that originally divulged.

The radio does not work, but the illumination of the on/off switch works when the lights are on. The power antenna does not work either. The sunroof does not work I was told, but from how he described it I don't think it was electrical. The tachometer does not work, but I have not tried the cigarette butt trick yet. The analog clock is broken. The accessory power jack/ cigarette lighter does not work.

I have checked out the fuses and all of those all look ok.

The rest of the vehicle is in good condition except a flare in the transmission when shifting, but I am going to replace the modulator cap and if that does not work, tighten it a little.

Another problem might be the glow plugs or diesel heater. When the car is cold, the idle is very rough, but it gets better after 2 minutes of sitting. Also, when I plug the car in at night, it does not make any noise, like anything is happening.

Thanks for any help and if anybody thinks these problems are correlated.

Welcome.



FIRST, replace all the fuses in the box. Yes, even if they appear fine. Also clean the contacts while in there. Many issues arise from corroded contacts. Also check the 10A fuse in the Over Volt Protection relay, this is by the master cylinder and is a rectangular box.

Radio could have a popped fuse behind it. Antenna only works when the radio is on. Are there dashes on the screen such as "- - - -" or is the screen simply blank? Blank is good, dashed out is bad.

There is no tachometer amplifier for the 603 Diesel. So the cigarette butt will not work. From memory the tach runs off the EGR controller down in the passenger footwell area. I forget where the pick up for it is at.

These clocks fail often. How to repair is beyond me. Probably something simple.

Purchase a MityVac and set the transmission vacuum properly. My guess the shift pressure was turned down to soften the shifts. This tool is invaluable in PROPERLY repairing one of these vehicles.

Pull glow plugs and test them properly. If they glow, they are good. If they do not glow, they are bad. This is the definitive test for them as simply Ohming them out will not always work. Chances are you have an out plug or so. Does your squiggly line light up when you glow the plugs? If not, you have one or more plugs burned out.

How long is the block heater pluged in for? And you should once again, Ohm out the cable and verify there are no shorts in the cable.

Sunroof is probably stuck, typical. Leave it shut until you are ready to deal with it properly.



Now, get to work on your problems.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Welcome to the forum.

Do the turn signals and the hazard lamps work?

Thank you for the quick replies.

Yes, the hazards and turn signals work correctly.


I ordered a bunch of fuses that will be here sometime this week, I will replace them and clean the contacts.

The radio does not turn on at all, but the illumination lights up the the on/off button when the headlights are on. There is a button to put the antenna up and down, does that only work when the radio is on also?

I left the block heater on for around 2 hours.

The glow plug light does not turn on but it makes the buzzing noise for about 10-15 secs and then I turn on the car when that is done. I will look into testing and replacing them.

Is this what you are talking about for a MityVac?
http://cgi.ebay.com/ebaymotors/MityVac-4000-Automotive-Vacuum-Pump-Brake-Bleeding-Kit_W0QQcmdZViewItemQQhashZitem518dd6f9deQQitemZ350272027102QQptZMotorsQ5fAutomotiveQ5fTools

The transmission shifts hard from first to 2nd but then misses from there. Might be a sign of things to come.

Well, unfortunately, you don't have a fuse issue as best as I can determine.

Therefore, the radio and the antenna must be diagnosed individually. The first thing to do is to confirm that you have 12V at these devices. If so, the problem lies with the device itself.

With regard to the power antenna..............it won't work unless the radio also works...........typically. However, you can attempt to operate it independently of the radio via the use of the switch on the panel above the radio. Have another person press this switch to the top position and listen for the motor to operate in the trunk. If the motor operates and stops, the problem is with the mast. Nobody has cleaned or lubricated the mast in ages and the motor's clutch is just not strong enough to raise it.

On my 85 300D I have a Becker radio. I'm not sure how the SDLs work

When I first powered up my car the #2 fuse immediately blew. I replaced the fuse and soon enough my radio died and the power antenna stopped working.

I pulled my radio and sure enough I discovered a fuse that blew on the back of the radio. I remember it was like 11:30 at night when I fixed this so I went down to my stash of christmas lights and pulled a similar looking fuse. I put it in and sure enough I had my radio working again.

The antenna was a bit harder battle. I pulled the entire unit out of my trunk and opened it up. There was this thick gunky junk at the bottom which I realized was silicon lubricate. I cleaned it all out and jumped the little motor for the antenna.

Fortunately it worked! I was able to control the antenna by switching polarity. It was the daughter board on top of the motor that was dead.

I emailed hirschman's US office and the email response I got back was just so funny. I'll post it here because i still get a kick out of it! The email might be of some use to you since it has some part numbers and a gent in Burbank who can help you.

I believe 79mercy sent me a replacement one.

The capacitors on my clock had died a long time ago. He also sent me a replacement one before I bought my soldering iron. Go and pull your cluster and see if there is anything obviously wrong with it before you call post mortem on the capacitors.

* I hope this is useful. I know the 86 300 SDLs are a lot different then my w123 but the troubleshooting methods are kinda similar

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-Typos courtesy of my mobile phone.

Well I guess things working isn't always good news.

I will pull out the radio and check on the antenna.

I called an independent Benz shop and they said it would be in the low to mid 2 digits for them to replace a faulty glow plug, and hopefully it is just a burned out glow plug.

Assuming the '87 shares some wiring design with earlier models, you've got three (or four, counting the one on the back of the radio) fuses. One, which apparently works, is for the lights only. Another provides power for the clock and is hot all the time, a third provides power for the radio to work. The radio routes a signal out through the antenna switch to the antenna, which gets power directly, too, so troubleshooting a bad antenna usually needs a good radio first (possibilities include bad radio, bad switch, bad motor, bad fuse). I think the fuses for the radio are #1, #2, and #4. I'm not sure about the antenna fuse, but yes, the switch normally only works if the radio is on and working.

Check your owner's manual and check the fuse box and read the fine print, you might see 'radio' listed for the three fuses. You might also want to check if your cigarette lighter works - if that has power, then it's likely the main (switched) power to the radio is also okay.

A BTDT on the glow plug test - I'd verify a source for replacements before testing if this is your main mode of transport. One or more is probably bad, and you don't want to run around looking for a new one in a borrowed ride.

For the block heater: check the other end of the cord where it plugs in to the heater at the block. Since you haven't popped a breaker in the house/garage, it's probably safe to assume the cord isn't completely corroded, but there's also no guarantee it's plugged into the heater.