Hi Guys,

Hope you guys can help solve this mysterious electrical problem on my 1984 300TD that I've been trying to solve for over a week.

Thought the car just needed a new alternator as the Bosch Platinum Battery (1yr old) was not being charged. The following are results of the initial testing:

• Old alternator Failed AutoZone in car testing
• Yet, same alternator (old one) Passed AutoZone's Bench Test
• Battery Passed AutoZone's Bench Test
• New alternator that was installed also Failed AutoZone in car testing

Therefore, thought alternators are not faulty and it must be some type of wiring issue causing the battery not to charge. I inspected the following:

• Removed and Checked the main ground strap
• Remove and cleaned Negative Battery connector wire to frame
• Cleaned both battery posts and connectors
• "Charge Indicator Lamp" bulb installed(was missing & working now)
• Checked Alternator Connector: Key Off- Battery Voltage Blue wire slot no voltage when Key Off and slightly less than battery voltage when on

Still not charging at all or properly. With the car idling, the voltage at the connected plug on alternator measured only ~ 12 volts at both slots, but only around `11.8 volts across the battery. Doesn't seem to be charging even though though the voltage is higher at alternator than at the battery.

Why do you think the voltages are so low? Shouldn't the voltage readings at battery and alternator be at 13.0 to 13.5 volts? Does the lower voltage give a clue to what could be wrong with the charging system?

Now here's the mysterious or weird part. It's seems that my battery's voltage drastically drops after draw down of one start and idle a short while. A fully charged battery at 12.6V, drops to 11.8V after only 1 start and couple minutes of idling.

My Schumacher smart charger might be malfunctioning, but my multimeter confirms the voltage readings. After only two starts, the charger says that the battery is only at 10% charged at around 11.0V. It's taking about a hour to charge it back to 100%. The voltage while charging climbs slowly and only reaches over 12.5V only when over 80% charged. I suspect the charger's percent function might malfunctioning or acting up but not the voltage readings.

Does anybody have a idea of what's happening to the battery's voltage? How does this issue relate to my alternator not charging?

Possibly the glow plugs are staying on. Check for voltage at one of the glow plugs with the engine running. If you see battery voltage then they are still on and you will see low voltages at the battery and alternator.

PaulM

__________________
84 500 SEL (307,xxx miles)

What is the history of this problem?

The alternator cannot charge the battery if the bulb was removed, UNLESS someone has modified the wiring. If that is the case, then you need to inspect and correct all modifications to the wiring for the alternator/starter/ignition circuit.

What other bulbs were/are missing from the instrument panel????

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Yes Paul...you are absolutely right!. My glow plugs are staying on.

I've got think that only diesel enthusiasts would be able help me out that quickly. Thanks!

I am going to swap out my glow plug relay with a spare. Come to think of it, the relay is making some clicking sounds sometimes. I was told that it was due to low voltage when the battery not be charged .

My glow plug relay was only replaced a couple months ago. The previous one "fried" and/or burnt out while driving on the freeway.

Can you think of anything that might be causing my relays to malfunction so often?

Glad you notice that the charging bulb was missing before. After read former alternator/charging posts, I too was wondering how my charging system work before without the bulb.

I'm going to swap out the glow plug relay first. Perhaps without the drain of the glow plugs always on, the alternator can get the right voltage to start charging.

I think the only bulb missing now is my left arrow turn signal. It could be burnt out or missing, so I will try to check later. My brake light is always on and needs to address soon also. It seems that the previous owner did have a few wiring modifications under the hood. As of now, I haven't noticed anything on the instrument panel.

Good you found the missing bulb. The Owner's Manual says that if the charging bulb stays lit, to drive immediately to the closest M-B dealer for an emergency bulb change (bet they liked that business). Of course, you first need to know that the bulb works. It should come on in "run", with engine off, as do 2 other dash bulbs in my 84 & 85 cars (worn brake bads & low fuel). Think of that as a "bulb check". These "Motorola style" alternators require a "bootup current" to get their "field generator" section working, which they get via the dash bulbs. Older GM cars are the same. Was new to me since I came from Mopar world. BTW, I need to rev my engine slightly to get those bulbs to go out, since I put LED bulbs in the dash which don't conduct as much current.

To monitor charging while driving, I sometimes use a cigarette-lighter voltmeter. Correct that you should see ~14.3 V once the engine is running and the alternator is outputting. A fully-charged battery should show ~12.6 V (engine off).

This may be the source of your problem(s)....

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Thanks to Paul the voltage drain mystery has been solved. My glow plugs were staying on. Any damage I should be aware of driving around with constant glow plugs?

About a quarter of the circuit board of my Glow Plug Relay was corroded mostly with white powdery stuff and some spots of rust. The bottom of the black housing had a coat of the white and rust colored powder.

How did the circuit board corrode so quickly (about two months) when it is never humid here in Los Angeles? Is it mostly likely because anything is possible with a 30 year old circuit board?

The previous Glow Plug Relay burnt in the area the strip fuse. Do you guys think there might be a short and/or corroding wiring to the relay? How does one go about testing the wiring going to the relay?

Are there people who turn their glow plugs "on and off" manually so not to have to rely on these old electronic circuit boards?

The new Alternator is still questionable. Here the stats now that glow plugs are shutting off properly for now:

• Voltage when idling is around 13.5 volts
• Charging Lamp turning on and off as should
• Autozone's in car alternator test Failed, "Bad Regulator"

The voltage is not at 14.5 Volts, but how do I know the Autozone test is correct and the Alternator Regulator is bad. I tried to take a voltage measurement at the back of the alternator and did produce some sparks with the multimeter prob. So is it possible I fried the new regulator myself.

Should I swap in a good regulator and test the alternator in the car again at AutoZone so don't have to pull out the new alternator? What else can I do test the alternator before I pull it out for bench test?

The New Alternator just passed AutoZone's in car test.

I will check the my battery's voltage after a 30 mile drive tomorrow morning. If it measures 12.6 volts, I will assume the charging system is fine. Even though the alternator is giving me only 13.5 volts and not a 14.5 volts.

Do you think losing 2 Glow Plug Relays in two months are at all related? Even though this time it was corrosion and last time it burnt, could it be caused by faulty wiring?

What about Manual Glow Plug switches? (Maybe this should be asked in a new post)

Most European cars will charge below 14V and most domestic cars will be over 14V. My 500 with a 115 Amp alternator charges at 13.8 at idle with everything off except engine. If I turn on A/C w/fan on high, headlights, seat heaters, rear defroster, Aux cooling fan, etc the voltage at idle will go down to about 13.4. If the battery is low already, it may drop a little lower.
So I would say you are fine. I can't say why you have had 2 glow plug relays go bad other than they had 2 different problems so it is probably not a fault in the car.
Just for your information, a Mercedes will charge with the Battery light out in most cases. The brake pad warning light and low fuel light will still supply enough amperage to get the alternator started charging. Once it starts charging the lights are not needed and could be disconnected and the alternator will continue charging until the engine stops.

PaulM

__________________
84 500 SEL (307,xxx miles)

I suspect you really had only one problem.There is likely a solid chance the old alternator is still good. Cheap or free recovery of it if possible to have as a spare might be considered.

Especially if the replacement alternator was a Mexican type of rebuilt. Basically there is no way the normal charging system can keep up with the presence of a constant glow plug cycle drain.

Thirteen point five volts is acceptable. The battery may even last longer than being forced or overcharged up to fourteen point five volts. A fully charged battery is slightly less than thirteen volts. Constant overcharge cannot be all that good for them over time. higher cutoff voltage only allows faster replacement of lost charge if required. There may be a cost for this though.

I have to wonder if it may be one of the reasons that batteries in service today sometimes may not last as long as they did when I was younger. Our old diesels once started in the morning have no load on the electrical system other than the radio on most days. I suspect the lower cut off voltage of our alternators may have been a design consideration.

The Glow Plug Relay circuit board should not have any kind of corrosion on it. It sounds like someone has done some work on it. There should be a thin layer of conformal coating, available at electronics stores, to prevent this.

If someone has done some work on the board, they have to remove this coating to solder. They may have taken a shortcut and not restored the coating.

2108-12S Techspray | Mouser

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Glow plug relay problems are far too common. One thing I've learned is to never buy a good used glow plug relay. Manual glow is the way to go, you'll never have glow relay problems again and your glow plugs will last longer too.

I make it easy with my manual glow relay. I have 2 left. Very easy to instal. Wire up the mom push button to switched +12V, everything else is plug and play, including the glow light indicator.

http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/359811-sale-manual-mercedes-glow-plug-controller-w123-300d-240d.html

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks Guys!

My car is fixed and gain a tremendous amount of knowledge. That's great that all of the several questions we answered thoroughly in just a day. This community is a big reason you I am committed to driving my w123 as long as possible.

I got to try avoid asking this local mechanic who specializes in MB diesels for his diagnosis. His conclusions and solutions come about too hastily and because of it are not correct more often.

@Paul Great to know that the charging bulb is not mandatory as that solves the mystery of how my previous alternator worked. Also I try can put together the charging system on my other w123 without first having to get another bulb from the junkyard. Your opinion on the relays is much appreciated as you really seem to know you stuff.

@Barry Yes, really just one problem and the old alternated is still good. Hoping that installing the new was some type of preventive maintenance by replacing much older alternator that might have needed to replaced soon. At least the new one is 65 amps vs. 55 amps, if it even makes a noticable difference. Are you saying the Mexican rebuilds are of suspect quality?

@Sam I would like buy some Conformal Coating and thank for taking the time to post the link. It does seem kind of pricey at $24.22 for the can. Do you think that all 30 year old circuit board need to be re sprayed for insurance in protecting against corrosion?

@funola I will consider purchasing your manual glow plug solution. Any suggestions on how to turn on with a toggle or push button switch?

Run the wire through the fire wall grommet by the brake booster, connect wire to one side of a momentary push button switch, connect the other side of switch to +12 V (cig. lighter will work). Mount the switch in a convenient spot. With ign. key on, push and hold the switch to glow the plugs. Let go of the switch to turn off the glow.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked