NEED HELP. 1986 300SDL Electrical Issues

[Fiebs]

Here’s a doozy for you. This morning I drove my 1986 300SDL on a four hour round trip to St Louis. On the way back we ran into some rain. After driving through the rain several things began to happen:

1: immediately, my air conditioning quit working(was fully functioning) only getting hot air out of the defrost vents. Pressing the fan button increases flow slightly. None of the push buttons for the AC system do anything.

Upon getting home I parked the car for a while, thinking everything’s fine. I come out to start it and the battery is drained. I jump it and then:

2: My ac works for a split second before cutting off and doing the same thing again. Only now, when I press the AC buttons, my radio starts losing power intermittently, along with the same issues presented previously. Along with this, now the vehicles turn signals don’t work.

3: I parked the car, turned it off, and now when I turn the ignition the starter won’t even try to engage. All fuses look fine.

Can someone help me out? This is my main transportation right now

[jdulle]

I would check the battery and grounds first. Check that the battery has correct voltage and cold cranking amps. If all that checks out, I would look for water entry and corrosion in the drivers footwell where many positive wires come together in a block. Also check the blocks near the battery, you may have to remove battery to access. My car is a w124 but w126 should be similar. Also could be a diode in alternator went bad.

[Diesel911]

Originally Posted by jdulle

I would check the battery and grounds first. Check that the battery has correct voltage and cold cranking amps. If all that checks out, I would look for water entry and corrosion in the drivers footwell where many positive wires come together in a block. Also check the blocks near the battery, you may have to remove battery to access. My car is a w124 but w126 should be similar. Also could be a diode in alternator went bad.

And recharge the battery.

[Diesel911]

Originally Posted by Fiebs

Here’s a doozy for you. This morning I drove my 1986 300SDL on a four hour round trip to St Louis. On the way back we ran into some rain. After driving through the rain several things began to happen:

1: immediately, my air conditioning quit working(was fully functioning) only getting hot air out of the defrost vents. Pressing the fan button increases flow slightly. None of the push buttons for the AC system do anything.

Upon getting home I parked the car for a while, thinking everything’s fine. I come out to start it and the battery is drained. I jump it and then:

2: My ac works for a split second before cutting off and doing the same thing again. Only now, when I press the AC buttons, my radio starts losing power intermittently, along with the same issues presented previously. Along with this, now the vehicles turn signals don’t work.

3: I parked the car, turned it off, and now when I turn the ignition the starter won’t even try to engage. All fuses look fine.

Can someone help me out? This is my main transportation right now

Oddly you did not mention if your charging light came on or if your vehicle has a voltmeter.

Jumping the battery is not the same as recharging the battery. You can't really see what is going on till the battery is fully charged.

Get an inexpensive voltmeter from somewhere. Harbor Freight has them.

When the battery is fully charged apply the voltmeter to the battery terminals and have someone crank and hourly start the car.

What you are looking for is the batter voltage before you start cranking. After you start cranking you are looking for how much the voltage drops during the cranking.

If it starts you are looking for the charging voltage.

Back to the last 2 paragraphs of your post. You jumped it, it started but the battery was actually still dead (not recharged or simply the batter is no good).

If you bought a new battery just to get you on the road you still need the voltmeter to tell you if you are charging or not.

You need to have a changed batter for this. If you have a charging indicator light when you first turn the key all of your warning lights should light up. If the charging indicator does not light up it could mean the bulb is dead. I don't know about on yours but on the older models you need that bit if resistance that the charging light provides for the alternator to charge normally.

If you know the batter is good and fully charged but the voltage is dropping, you need to find what is doing that.

With the engine off safety disconnect the negative battery terminal clamp. Remove the positive wire from the alternator and reconnect the negative battery clamp and wait and see if that stops the battery drain.

If that stopped the battery drain you have a diode issue inside of the alternator.

What will drain your battery fast is if the glow plug relay is stuck on. Use that voltmeter and see if you are getting voltage at the connector pins on the glow plug relay that go to the glow plugs while the key is out of the ignition.

[Benzadream]

^^^ This is an excellent post with great advice! Very logical and methodical. The only way to fly^^^

[JHZR2]

Need to know battery (car off) and alternator (car running) voltage.

Things can get funky with bad fuses and ovp relay. They can get really funky if you have a shorted battery since the voltages will put things out of range.

[Fiebs]

As of now Ive all but confirmed it is the alternator. Charging the battery and starting the car runs all my electronics for a while but over time drains the battery, its not getting any charge or very minimal charge from the alternator. Is there any sure-fire way to make sure its the alternator itself and not a connection between the battery and it? If not im just going to go ahead and replace the alternator.

[Sugar Bear]

No charge situation is frequently a bad voltage regulator on these. It is easy to change.

Has the voltage output been tested?