1985 300SD//Charging System Fault - PeachParts Mercedes-Benz Forum

eelsfoot · #1 10-16-2011, 11:26 AM

Working a couple different issues on this one right now, but the charging has me lost.....

New Bosch alt, belts, new battery, and recently changed glow plugs. Cleaned all the leads coming off the alternator to the junction block. Battery reads 12.5v. Then reads less after startup, so not enough getting put back in for a charge.....but now I'm lost.

Have read through a lot of threads about the subject, but I just can't seem to put it all together in my head. Pulled the lead off the glow plug relay and took readings off each pin.....they all read the same, so glow plugs all good?

I'm not that up on the ins-and-outs of the electrical system, but can take multimeter readings and somewhat understand the basics.

Anyone have advice on how to help pinpoint my culprit?

Thanks,

Lance-

t walgamuth · #2 10-16-2011, 11:30 AM

I am not that good with electrical issues. I do know to suggest checking your grounds on both ends of each cable. Bad grounds often cause goofy things to happen.

barry123400 · #3 10-16-2011, 08:11 PM

Make sure your alternator belt is not too loose. Generally as a rule if you can reach down and turn the alternator cooling fan blades with your fingers.Thats with the engine off of course the belt is too loose. Second though if all connections seem secure is to have the new alternator bench tested. Could be a dud. If not your last alternator may have been fine. Alternator light on? Both before turning engine on and when running?

layback40 · #4 10-16-2011, 08:20 PM

Does the red idiot light come on when you turn the key & go out when the car is started?

When you answer this we can move on to the next step.

shadetree77 · #5 10-16-2011, 08:23 PM

do you have the upgraded "fast glow plug system", if so it will stay on after you start the engine and will keep putting a draw on the battery till it's done (depends on temperature for some, other's have a set timeframe (90 sec. i believe). i have a built-in volt meter and it reads about 11-12v while the fast glow is active after starting

eelsfoot · #6 10-16-2011, 08:52 PM

My daughter just changed the battery out again, and is dropping the car off for me to tinker with.

I'll check the battery light indicator tonight......think it does light, and then go out on start. But I'll confirm.

The car does not have the fast-glow upgrade.

eelsfoot · #7 10-17-2011, 05:16 PM

Had a look at the car this afternoon. The red idiot light illuminates with the key turn....and then extinguishes upon starting.

The glow plug light "flashes" on for an instant, and then extinguishes.

My daughter had AutoZone trickle charge the battery for 2 days, put in back in the car and drove it over to mine. After approx 15 mile trip.....with I'm sure a couple stops thrown in for Red Bull ect.....I tested the battery at 12.1v. After startup, and a slow increase to 3000 rpm, battery reads 12.04v

Next step?

barry123400 · #8 10-17-2011, 06:18 PM

Well if the belt is not too loose the alternator is not puting out much. The light on the dash going out indicates some output. You have to have the rebuilt alternator checked out. Rebuilts are questionable.

Certainly it might be something else. But to procceed without checking out the rebuilt may consume a lot of time. The dash light acting as it is indicates the exciter circuit is being activated and by going out when the engine starts some alternator output is indicated.

The alternator output voltage balances against the voltage supplying the lamp in the dash and it goes out. The battery voltage you found does indicate if there is any current output from that alternator it is insignifigant.Or is being used in something else instead of charging the battery. Like the glowplug circuit.. You can have the identical symptoms from a loose belt. Do not be suprised if the alternator is a dud either.

I just re read your post.Disconect your glow plug relay. It may be on all the time. There is no way a charged battery could sag as low as this one is unless a lot was going on during that 15 mile trip. Then read the charge voltage. The glow plug relay remaining constantly on may not be indicated by the glow plug light. Not all that unusual of a fault and fools many.

I am actually suprised the car would even start with the battery voltage that low. It has almost no charge in it or has a shorted cell. Twelve point one volts to me is almost a totally discharged battery. Or the remaining five cells are well charged.

If you own a battery charger hook it up and see if the voltage rises. If not a cell is shorted. Most store clerks will diagnose this as a discharged battery. Another alternative is to sub that battery with a known good one. Make sure to observe battery polarity if you do.

eelsfoot · #9 10-17-2011, 08:21 PM

Yep....now battery is too low to start. I've hooked up the trickle charger and will continue on with the checks tomorrow after work.

As far as the glow plug relay; will it suffice to just pull the 80A fuse?

Thanks Barry.

Lance-

eelsfoot · #10 10-23-2011, 01:55 PM

Took me awhile to get to it, but learned a couple things I think....

With the glow plug relay out of the loop, there is no change to the charging. So it appears the relay is fine, and now I need to remove the rebuilt Bosch alternator and have it bench tested?

So my next step is testing the alternator, or is there anything else to look at before?

Cheers.

Stretch · #11 10-23-2011, 02:20 PM

If I were you I'd just go and bung in a new voltage regulator instead of removing the alternator and then getting it tested.

There's a picture here of the voltage regulator / alternator brush assembly (as I called it then) in Assembly (3) here =>

PeachPartsWiki: Refreshing the Alternator

A voltage regulator swap should be a quick and dirty way of seeing if that is the problem (which I suspect it is with a charging voltage of 12.04V)

eelsfoot · #12 10-23-2011, 05:46 PM

Thanks Army....I'll give it a go. Just ordered a Bosch replacement....$23. The rebuilt alternator was just installed, and hoping the Bosch quality is worth something. So if anything, I'll have a replacement in the glovebox.

I'm out all week working, so I'll report back on the weekend with what the results are.

Thanks,

Lance-

eelsfoot · #13 10-29-2011, 05:04 PM

Yeah.....so put in a new voltage regulator. Put the battery from my SDL in, and no go. Won't even turn over. I'm about at my wits end with this thing.....

Last week I could jump start the car. Had a chance to try and work on it today, and now nothing. Battery dash light comes on....along with all the other normal lights....glow plug light does not illuminate, but it's been flakey before. Temp. oil, fuel gauges reset. I can hear the glow plug relay click off after about 30 seconds. Turn the key------nothing.

Thought all I was working on was finding a charge fault......any ideas on this situation?

Thanks,

Lance-

Stretch · #14 10-30-2011, 05:24 AM

Quote:

Originally Posted by eelsfoot

Yeah.....so put in a new voltage regulator. Put the battery from my SDL in, and no go. Won't even turn over. I'm about at my wits end with this thing.....

Last week I could jump start the car. Had a chance to try and work on it today, and now nothing. Battery dash light comes on....along with all the other normal lights....glow plug light does not illuminate, but it's been flakey before. Temp. oil, fuel gauges reset. I can hear the glow plug relay click off after about 30 seconds. Turn the key------nothing.

Thought all I was working on was finding a charge fault......any ideas on this situation?

Thanks,

Lance-

Oh man it isn't meant to get worse!

Starting with the simple stuff - Have you by any chance left something disconnected? Blown fuse? Terminal clamps on the battery nice and tight?

Does the solenoid on the starter motor click?

vstech · #15 10-30-2011, 09:31 AM

if you put in a good battery, and the terminals are clean and tight, the next thing to inspect is the starter itself...