Hi I'm in need of some of the expertise on this forum! It's a 1982 300cd that likes to kill the battery in under 15 min intermittently. I have tried looking for a phantom draw with a meter but everything checks ok. I have load tested the battery and it is good-used three different testers. The alternator is fine. The cables and connections seem good. It doesn't take much to start it-a quick boost and its fine. Then it will start again no problem. Can be weeks, then no start. Wtf can drain a battery so fast and not start a fire. When it does go dead I hear a motor moving behind the dash(guessing damper door driver resetting?) and the dash clock is loud. Now the warning buzzer that never worked buzzes when I open the door with ign off. And the chime module doesn't always chime when the ign is turned off. It's not the stereo either. Please help. It's driving me crazy
Cheers

Glow plug relay will drain it quickly.

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Eric, CPO, Submarines, retired.
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Mine: '68 Corvette LS1/4L65E, 83 240D, 2000 GMC 4x4, 08 FLSTC Anniv
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Thanks. Will the glow plug light not come on if the relay is sticking? Ill swap it out. Is the glow plug light not connected to the relay? Sorry I know I should have a manual but I dont

A hydrometer test is much better than a load test. Many of the newer testers don't test with a large load. They test with a medium load for a longer time which is not the same and can give false positives. We were actually having troubles with our Ford diesels for a while. One cell of one battery would get weak but only the hydrometer would show it.

You say the battery is suddenly being drained, but is there a chance the alternator is intermittently not charging and the battery is weak when you shut the engine off? Sometimes it's hard to test for a failure when a part is working. Maybe your starter is giving you trouble? Have you checked voltage at the battery when this happens? At the starter? Is it more likely to happen when the engine's hot? After a short or long drive? On humid days? When you buy pistachio ice cream?

If you suspect glow plug relay sticking you could try wiring a light to the glow plugs and leaving it outside the hood in a place where it's visible from outside the car. If you see the light on with the key off you know where to troubleshoot. You might also look around for an audible "low voltage" alarm which could alert you that the problem is occurring in order to do testing.

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When life gives you lemmings... make lemmingade.

Pick a part ... GP relays are $2 apeice here.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

I'd doesn't matter if the car is hot or cold. I bought a little led indicators that pops into the cigar lighter to watch for alt failure. $2 glow relays? Their listed at over $200 what am I missing here? It does happen more often after using the defrost fan. I'm still chasing it. Will keep you updated.
Cheers

He was referring to used relays at the boneyard. New run around 80 - 90 for 5 cyl w123.

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Eric, CPO, Submarines, retired.
Here's a sig line...
Mine: '68 Corvette LS1/4L65E, 83 240D, 2000 GMC 4x4, 08 FLSTC Anniv
Hers: '72 Corvette 454/4spd, '99 MB SLK, '93 Metro vert, 78 240D, '92 Silverado, '65 Fjord Rustang, '59 Fjord Fairlane, '17 Slingshot.

Just physically disconnect the glow plug circuit other than at times you are starting the car. After a time you will have a solid indication if the glow plug relay is the responsible culprit or not. It is pretty high on the list of possibilities but far from absolute certainty.

If you have auto wreckers in your area that have your model in inventory they can be a source of many good used parts . Glow plug relays for two dollars sounds like a pick and pull operations price though. That's where you go into the yard and extract the part yourself. Not every area has them still they have become more and more common with time.

Not only does my area not have those types of salvage yards. What yards we do have seldom get any of our older Mercedes cars in anymore. The rust in my geographical area has killed off the vast majority of those models some time ago.

For a time we had a fair supply of fairly low milage rusted out examples to select parts from. That time is consigned to history now.

When it is happening, dead / low batt, remove fuses one at a time & measure resistance on side feeding to (whichever) accessories. Only thing on all the time is the clock which would have negligable resistance & maybe radio, tho the europa radio is 'off', antenna is always fed with power but is off when down. 20-30 ohms will drain a battery quicker than you might think.

if you put an ammeter on the battery, then remove fuses one at a time, you will learn which circuit is leaking.

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'77 240D, 504H, OM617.952, etc.

It is so intermittent in nature he will not catch it by methods we use for constant drain issues.. Just my opinion though. A suspicious system or component has to be left disconnected for some period of time to get some proof it is the issue unless he is just plain lucky.

For the antenna circuit for example put the antenna up and pull it's power plug. Disconnect the plug at the unit when it is up.

One question of his not answered was will the glow plug indicator always come on in a situation like this if the glow relay is intermittently coming on? The answer is no.

Doing this yourself in running down the problem is both cheap and effective. Intermittent problems in professional hands are not always the best ideal.

For example if the car where taken to a mechanic and he knew these cars well. About the only thing I could see him trying first was subing of the glow relay at your cost of course as a start point. Not knowing these models well could result in very high costs ultimately on occasion. Substitution of parts for a problem can add up too fast. Best to figure out what is needed first and this is cheaper overall as well.

You can effectively mimic this substitution by disconnecting between uses. These cars have reached the age of self applied logic instead of throwing money at in many ways. It is getting pretty expensive just to keep a much more modern car on the road properly in general paid for maintenance costs. I am seventy one and still will not pay 75.00 for an oil and filter change at my local dealership for our newer cars. This is four litres of synthetic oil at say 25.00 plus a five dollar filter. What bothers me is that most people do not understand the cost is not 75.00. You have to earn the money first as it is taxed. Plus there are unrecognized costs in earning the money.So it is really costing you at least a hundred dollars. Is five minutes time really worth 75.00 in your mind at that skill level?

Say I take the difference between this and me doing the oil change myself and by and large the money not spent gets into a well run investment fund along the road. Ultimatly I am well paid to change my oil. It is not how much money you make it is more how you utilize it. This does not make you tight but helps make you much better off from a financial perspaective over time was my experience.

I know this everything paid for system makes the economy go round at the same time. We do not endlessly damage it by our own interventions at very lower numbers. I live in eastern Canada but grew up not that far from your location. Americans are perhaps less familiar with our garage repair setup. To say they can be punative at times is an understatement though.

Part of the allure of these models is the owner can with a little thought handle most situations with this site helping. I feel it is like having twenty people looking over your shoulder and everyone means well plus most of them will have some level of experience. To paraphrase I see it as a table full of food. You learn to go with the opinions and experiences suggested you think are the most sensible in your situation. Or you want.

That is not only free but increases your judgement and skills along the road. Sorry about the long post but there is little posted of why some of us fix our own cars. My base income level at 71 makes it still possible to pay for everything easily with a surplus every month. Using alternative methods and learning how to apply them in many areas has made this canadian and his family pretty comforatable over the years. The knowledge may even be responsible for a pretty high income level by Canadian standards still at 71. It could have been otherwise. I also get a lift from the enjoyment and knowledge that my costs are under control. The most positive thing though is I really enjoy the hands on part as well.

Most arcane source for an electrical draw I have read is one where minor coolant leak was dripping onto the alternator. Took the OP crazy time to figure out.
If your car sits outside, check your exposed wires for mice bites.

If you need a new glow plug relay PM me with the part no. so I can cross reference the one I have.

Since the 1982 300CD does not have the KLIMA module, that is one source of common electrical draws to strike out.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

Here's a quick update. It seems to happen after I use the blower fan! Getting cold so ive been using it. How much current do these suckers draw? Wiring all checks out good but I don't have an amp clamp so I will pick one up tommorow. Crazy how fast the batt goes dead. . As for the alternator I know it gets wet when driving through large puddles but I have no constant leaks on to it. Will keep you posted and thank you for all the tips.
Cheers

Just pull the blower fuse when it is not actually in use. You do not need an amp draw meter yet in my opinion. Although you might want to periodically measure the battery voltage with the engine running.

It might be there but too low to keep up with needs. Your looking for at least 13.5 volts at idle. Forget water splashing on the alternator as the direct cause. Almost all cars experience this with no issues generally.

There is also a possibility of confusion here. Can you reach down and turn the alternator pulley by the connected cooling blades? A simple loose belt is all too common. When it gets too loose it usually gives a squeal when the alternator is trying to high charge just after starting. Not always though. You should not be able to turn the alternator by hand.

Do an archive search for "blower fan".

maybe it does have something going on.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD