Electrical Short? in 300SD - PeachParts Mercedes-Benz Forum

300sd2000 · #1 07-03-2002, 01:04 AM

Hi,

I have a 84 300SD. I have had some problems with the electrical system with the battery showing low voltage. Naturally, I tried replacing the battery (Diehard). Again, after several trips...difficulty starting. Seeing that the alternator hadn't been changed eve and the voltage was reading around 11-12 volts with the car on, i changed the alternator. Now with the car on running about 13 volts at idle with no load. However, I am still having intermittent problems where car just won't turn over and the battery voltage will be like 8 volts or something. Jump-starting gets it going. Sears seems to think the battery is okay according to their tests. New starter about 3 years ago (BOSCH).

Just right before the problems appeared, I had changed the glow plug relay and replaced it with one ordered the one from fastlane here. The box says that the new circuit design keeps it on for first 5 minutes or something. What I didn't do is change any of the older glow plugs (as the resistance seemed to be okay). Preglow seems to light up fine and start the car.

I also tried checking for a short in the car with a test lamp between the negative cable and negative battery post and didn't detect any problem there.

It seems something is causing a massive power drain in the car and only intermittently.

Has anyone seen a problem with these new glow plug timers by BOSCH?

300sd2000 · #2 07-03-2002, 01:50 AM

in thinking about this some more, I was wondering if the newer glow plug relay, along with my old "possibly bad" glow plugs could be causing an increased drain during the 3 minute warmup period. The car runs into problems on short (city) drives and does fine when taken on the highway?

Any ideas?

300sd2000 · #3 07-08-2002, 12:27 AM

In reading some other posts, could a bad voltage regulator be causing this? I do not get more than 13.4-13.5 at idle with accessories off and 13.6-13.7 at full acceleration in park.

Also, I seem to have problems with the car discharging at warm-hot temperatures iwth a lot of accessories on.

Since I replaced the alternator with a reman bosch, I'm wondering if I should have changed the regulator?

How much do they cost and how much of apain would it be to change now that the alternator is back on the car

Thanks

(300SD 126 series)

300sd2000 · #4 07-12-2002, 08:56 PM

ok I still can't figure this problem out@!

I've exchanged the battery and the glow plug timer with another 300SD and that one's running fine and this one keeps getting stuck. A new alternator (with voltage reg) did not help either. Seems as though the car just doesn't charge up enough. Jump Starting will ALWAYS work. This car is always around town (i.e. limited highway driving). I did the test-light current leak with a multimeter test too and have only 40-50 mAmps Draw.
Q)
1. Can a bad glow plug cause an excessive draw.
2. In reading about others, it sounds like around town driving can cause this...should I mess with the idle speed and up it a notch? RIght now it idles right at 500-750 in the dash.
3. To wire in an ammeter, you would need to place in series with the hot wire coming from the altenator?

Ridge · #5 07-12-2002, 10:19 PM

gentlemen,

When I assembled my MBZ from a complete pile of spare parts, the mechanic guiding me through the whole operation which took me five months, time and again said. "If you have any electrical problems with these cars check your ground wires first!"

There can be rust buildup under the ground wire near the battery and there is an entirely separate ground cable on the bottom of my engine that he specifically pointed out that should be there and must be rust free at both ends and tightly connected.

Since it's a simple thing to check and entirely free of costs, that's where I'd start first!!

When you hear hoofbeats, don't think of zebras.

Ben

300sd2000 · #6 07-15-2002, 09:23 PM

Ground wires look fine, and the problem is still persisting (getting very annoying)..

Question: 1) What is the small rectangular box with one wire sticking out (I transferred this from old alternator). Could this be the cause? Is it a diode? (NOTE: it is NOT the voltage regulator--I have chagned that already).

300sd2000 · #7 07-15-2002, 11:34 PM

Possible reason for problem: Hooked up a multimeter to the cigarette ligther and drove car....found with AC, Lights and Aux Fan on, voltage could drop to as low as 11.3 V in idle and no higher than 13.2 with all accessories off at high speeds

Voltage regulator problem? It's on back of alternator right? (I actually bought a rebuilt MBZ alternator so you'd think it wouldn't be defective?