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  #1  
Old 03-22-2010, 06:05 PM
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Voltage Drop at Glow plug Relay

So I have been messing around a little bit trying to figure out what's with a rough idle and varying amounts of smoke at startup. During the winter months I figured no big deal--at least it was starting--but now as its getting warmer the issue almost seems to be getting worse.

I did the normal glow plug checking procedure and they all check-out fine (they were replaced last fall) at about .4 ohms. The peculiar thing is that with the ignition off, the relay is getting full battery volts (~12.4) but when I turn the key to start the glow plug cycle the voltage at the relay drops to 10.4 volts, slowly climbs to about 10.9 V during the cycle and that after the relay shuts off hops back up to 12 V.

It seems to me like a bad ground somewhere, but I don't really know where to start. I tried replacing the battery ground cable but that didn't make any difference. I'm hoping someone who has also experience with this issue on an om603 knows some other grounds to check. Where does the relay ground to? The wires all just disappear back into the fuse box area.
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Old 03-22-2010, 06:11 PM
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Quote:
Originally Posted by esloc View Post
The peculiar thing is that with the ignition off, the relay is getting full battery volts (~12.4) but when I turn the key to start the glow plug cycle the voltage at the relay drops to 10.4 volts, slowly climbs to about 10.9 V during the cycle and that after the relay shuts off hops back up to 12 V.
Except for the battery voltage (12.4 volts) being a bit low, there is nothing peculiar taking place.
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Old 03-22-2010, 06:13 PM
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Originally Posted by tangofox007 View Post
Except for the battery voltage (12.4 volts) being a bit low, there is nothing peculiar taking place.
Shouldn't the glow plugs be getting the full 12 volts from the battery?
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  #4  
Old 03-22-2010, 06:33 PM
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http://www.xantrex.com/web/id/329/DocServe.aspx
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Old 03-22-2010, 06:40 PM
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That article says a loaded battery will still put out 11.9 Volts. The 10.4 that I'm getting is quite a bit lower than that.
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Old 03-22-2010, 07:48 PM
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Voltage drop through the wires and relay contacts can account for the drop at the glow plugs. Your best point of refference is comparison numbers with another simular car. The manufactured does save some money by not really making some wires large enough to deal with the heavy currents quite often.

Even with a fully charged battery eleven volts at the glowlugs is about the highest reported I ever remember hearing. Yes the glow plugs are rated at 12 volts. They just never see it unless perhaps after the engine is running and they are still on. You may only have 8 or nine volts at the glow plugs. That is a little too low.

A good battery should be resting after several hours at 12.5 volts or slightly higher. An older battery will have more internal resistance that lowers the output voltage even further at the battery under heavy load. Mind at the glow plugs.

As a first test I would start the car and read the voltage at the battery at say 1/3 rpm. The charging voltage from the alternator may be low. That could be reflected in the battery being undercharged. If it is 13.8 volts or higher and the car is in proper use the next stop would be to test the battery at a quality free checking battery place. It could be getting tired instead..

Last edited by barry123400; 03-22-2010 at 07:57 PM.
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Old 03-22-2010, 07:59 PM
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On the 617s with Pencil Glow Plugs you always have Battery Voltage at the Strip Fuse and the Flat Copper Strip/Wire.

Some say that the only 100% check of the Glow Plugs is to pull them out and use a Jumper Cable to actually Glow them using the Battery. Ounce in a while the Ohm out OK but are still not working correctly.

While the Glolw Plugs are out you could Ream or otherwise clean the Carbon out of the Glow Plug Holes.

You did not list the milage on your Engine.
If the Glow Plugs check out OK and the rough Idle is not being caused by an Air Leak (or another Fuel delivery issue) what is left is Injectors or a issue with the Compression.
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Last edited by Diesel911; 03-22-2010 at 08:10 PM.
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Old 03-22-2010, 09:14 PM
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It's obvious. The voltage will always drop alot under load. The fact that the voltage is "fine" with the ignition off shows that you have no connection problems.

Thusly you just have a weak battery that drops too much when glowing.

If you want to be sure go ahead and check the voltage at the battery itself, then turn on the glow plugs and check it.
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Old 03-22-2010, 10:35 PM
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The weak Battery idea sound good. You could also take the Battery to the local Autoparts for a free Charging and Load Test.
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Old 03-22-2010, 11:37 PM
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The mileage on the car is only 104,000, which I believe to be original. The injectors were just rebuilt by Sean Watts with Monark nozzles. I haven't done a compression test but haven't had any reason to suspect low compression--I would expect symptoms once the car is warmed up, which there are not. There seem to be many ways for a bad glow plug to hide itself and if things don't figure themselves out I might pull them all again sometime in the future. As for the battery, I had it tested a couple months ago and it was fine, but I'll bring it by autozone to be tested.

No one suspects a bad ground somewhere or a faulty relay?
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Old 03-22-2010, 11:42 PM
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Again make sure the alternator is putting out enough voltage to charge the battery fully. Many old mercedes have sub standard voltage outputs from their alternators. If you have the six cylinder 603 engine they can be a little cold blooded when starting. You did not post what you have I believe. Our moderator recommends two glow plug cycles for those engines.

If you have the five or four cylinder 616 or 617 engine check your valve clearances if they have not been done as well. This is almost a mandatory thing to do every fifteen thousand miles. Many engines out there cannot remember the last time they were done. This can effect start ups.

Last edited by barry123400; 03-22-2010 at 11:51 PM.
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Old 03-22-2010, 11:50 PM
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if you are reading 12.4 on a battery it is not fully charged, or it's damaged.
a fully charged battery should read 12.7 or higher. odds are good that your alternator is not fully charging the battery, or the battery is in need of replacement. have it thoroughly checked by a chain store.
before having it checked, I'd disconnect the neg terminal, and connect a trickle charger 2amps or so for 24 hours. then check the battery voltage. it should read 13.2-13.8 then connect the Neg terminal and turn on the headlights for 5 minutes. then disconnect the neg terminal again and test the voltage. if it's not reading 12.7 the battery is suspect.
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Old 03-23-2010, 03:48 AM
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Answer:

Quote:
Originally Posted by esloc View Post
So I have been messing around a little bit trying to figure out what's with a rough idle and varying amounts of smoke at startup. During the winter months I figured no big deal--at least it was starting--but now as its getting warmer the issue almost seems to be getting worse.

I did the normal glow plug checking procedure and they all check-out fine (they were replaced last fall) at about .4 ohms. The peculiar thing is that with the ignition off, the relay is getting full battery volts (~12.4) but when I turn the key to start the glow plug cycle the voltage at the relay drops to 10.4 volts, slowly climbs to about 10.9 V during the cycle and that after the relay shuts off hops back up to 12 V.

It seems to me like a bad ground somewhere, but I don't really know where to start. I tried replacing the battery ground cable but that didn't make any difference. I'm hoping someone who has also experience with this issue on an om603 knows some other grounds to check. Where does the relay ground to? The wires all just disappear back into the fuse box area.
300DT
W124.133
1987
OM603

My best guess:
Your startup smoke is in order of cost:
* Contaminated fuel, and/or possible Cavitation damage.
* Bad or failing glow plugs.
* Damaged injector.
* Injection pump timing issue.
* Out of calibration injection pump.


Glow plugs link thread All diesel models
http://www.peachparts.com/shopforum/diesel-discussion/137732-glow-plugs-link-thread.html#post1019018

Fuel Delivery:
Diesel Fuel Delivery:
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