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  #1  
Old 08-16-2017, 11:33 PM
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W124 300E alternator question

Hi everyone,

This is my first forum post on this website and before I ask my question, I just want to thank everyone here for their time and effort in trying to solve people's problems.

I have a 1991 Mercedes 300E, it is very lightly used and currently has around 55,000 miles on it. Recently, I have been experiencing a problem with my vehicle. Hopefully, somebody on this forum has experienced the same issue with their car.

So here goes: while in traffic for longer periods of time - with the gear in drive - the tach will start to oscillate up and down very fractionally around the idle RPM. The tach will reach 650RPM then drop to 550RPM and then start bouncing back and forth. A small amount of time later, the speakers begin to cut out (this has no effect on the actual stereo display) and then the air conditioning blower motor begins to cut out. Just to note, I refilled the AC system and had a thermometer in the vent. While experiencing these issues the vent temperature rose significantly (obviously meaning the components of the AC system were not receiving proper power.) After this, the SRS warning light comes on, and then so does the ABS antilock break light.

However, once the car begins to drive again, all the symptoms instantly disappear. My first thought after reading extensively on this forum was that the alternator brushes were to blame. (According to some people, 50,000 miles is where the brushes tend to wear out. My car being at 55,000 miles this made perfect sense to me.) Obviously it's worth trying new brushes before replacing the entire alternator, as it brings the cost of repair down significantly.

Unfortunately, after taking it to a garage, they said the only thing they could do to help me was replace the alternator. "They needed to warranty their work." At this point, I let them replace the alternator with a Bosch premium rebuilt 70A alternator.

A few days later, I experienced the exact same issue with the car. I brought it back to the garage, and they told me the alternator was failing when hot. They replaced the alternator once again.

Now, I am still experiencing the exact same issue - with one stranger quirk. While the tach is oscillating up and down, it also spikes for moments to 1000 RPM. I did not notice the engine revving or running roughly whatsoever.

This problem does not happen constantly and cannot always be reproduced. I took the car to a Mercedes dealer, which managed to replicate the problem. The battery voltage was normal when there was no load on the system. However, when the AC system and radio was turned on, the voltage actually reached a low of 11.81V. When I took it back to the garage I was originally at, they have been unable to reproduce the problem and have told me they were unable to reach a voltage of below 13V.

A couple more things to note in case they matter:
- This is a Canadian car and therefore has daytime running lights
- I have recently replaced the NA headlights with a pair of used Bosch European headlights - this is the only modification that has been done to the car.
- The stereo and all components of the stereo system are completely original.

I am leaning towards thinking my solution at this point is to install a larger alternator, as many people have done on this forum. (Like the 143A and 150A alternators found on some S Class models from 1996 to 1999.)

However, I'm not completely sure obviously. Again, I really appreciate everyone's time and effort in helping me solve this situation. It means a lot to know there are people who care about these cars and how to fix them when things go wrong!

Thank you everyone for your help,

Winston

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  #2  
Old 08-17-2017, 06:27 AM
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Trevor Hadlington
 
Join Date: May 2014
Location: Worcestershire in England
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I would go along the lines of a failed OVP relay .
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  #3  
Old 08-17-2017, 06:28 AM
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Trevor Hadlington
 
Join Date: May 2014
Location: Worcestershire in England
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I would go along the lines of a failed OVP relay .
To check it you remove the plastic cover that sits at the rear of your battery .It only pushes in there , so its easy to remove it.
Now your looking for a relay with a fuse sitting on the top .It will have a clear cover that hinges up and will let you replace the fuse if its blown .If the fuse id good ,then pull the relay out ,now give it a shake .Noise of something loose inside will tell you its
shot . Also check the pins on the bottom , thats if all other checks are fine .
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  #4  
Old 08-17-2017, 10:17 AM
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I agree that the OVP relay is suspect. But I disagree on diagnostic procedures. There is NO WAY to test an OVP, at least no way using readily available tools. That includes the procedure in the shop manual, as well as any silly stuff you read here. If the fuse is blown, you can assume the relay is done. If the fuse isn't blown, you can't assume anything....the relay may still be bad. The reason is that the zener or MOV that provides overvoltage protection can be failed, even as the relay continues to appear OK. The "shake method" is a new one, I'm chuckling.

The thing to do is to substitute another OVP. If that isn't the problem, then I'd suspect the crank position sensor.
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Old 08-17-2017, 12:10 PM
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The OVP relay can be a source of problems as suggested above. They're not expensive to replace and are a VERY common failure item that will affect several parts of the car. If the car really is idling down around 500-550 RPM, it can cause low alternator output. The normal idle speed when in gear should be somewhere around 650 RPM.

If you want to diagnose low voltage, get a cheap voltmeter that plugs into the cigarette lighter and watch the voltage as you sit. If it heads down and keeps going, you have a charging system problem of some sort. At idle with everything in the car on, you should be 13V+ if the charging system is working correctly.

How old is your battery? Occasionally an old battery can misrepresent an alternator issue.

Has anyone checked the engine ground strap to the chassis and the chassis connection to the battery? Poor contact can trick the alternator into thinking that it is producing adequate voltage due to low load conditions caused by poor cable connections.
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  #6  
Old 08-18-2017, 03:52 AM
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Quote:
Originally Posted by optimusprime View Post
I would go along the lines of a failed OVP relay .
To check it you remove the plastic cover that sits at the rear of your battery .It only pushes in there , so its easy to remove it.
Now your looking for a relay with a fuse sitting on the top .It will have a clear cover that hinges up and will let you replace the fuse if its blown .If the fuse id good ,then pull the relay out ,now give it a shake .Noise of something loose inside will tell you its
shot . Also check the pins on the bottom , thats if all other checks are fine .
Quote:
Originally Posted by Mxfrank View Post
I agree that the OVP relay is suspect. But I disagree on diagnostic procedures. There is NO WAY to test an OVP, at least no way using readily available tools. That includes the procedure in the shop manual, as well as any silly stuff you read here. If the fuse is blown, you can assume the relay is done. If the fuse isn't blown, you can't assume anything....the relay may still be bad. The reason is that the zener or MOV that provides overvoltage protection can be failed, even as the relay continues to appear OK. The "shake method" is a new one, I'm chuckling.

The thing to do is to substitute another OVP. If that isn't the problem, then I'd suspect the crank position sensor.
Really appreciate both of your quick replies to my thread! I was a bit tired when I put my question together and left a really important bit out. The dealer actually sourced an OVP from Mercedes Benz Classic in California and trial fitted it on the car. They ended up finding no change - the car still exhibited the symptoms. I really thought it was that too after checking on these forums.
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  #7  
Old 08-18-2017, 04:15 AM
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Quote:
Originally Posted by Diseasel300 View Post
The OVP relay can be a source of problems as suggested above. They're not expensive to replace and are a VERY common failure item that will affect several parts of the car. If the car really is idling down around 500-550 RPM, it can cause low alternator output. The normal idle speed when in gear should be somewhere around 650 RPM.

If you want to diagnose low voltage, get a cheap voltmeter that plugs into the cigarette lighter and watch the voltage as you sit. If it heads down and keeps going, you have a charging system problem of some sort. At idle with everything in the car on, you should be 13V+ if the charging system is working correctly.

How old is your battery? Occasionally an old battery can misrepresent an alternator issue.

Has anyone checked the engine ground strap to the chassis and the chassis connection to the battery? Poor contact can trick the alternator into thinking that it is producing adequate voltage due to low load conditions caused by poor cable connections.
Hi Diseasel, thanks for the input. As I just posted, I made a silly mistake not to include the fact that the dealer sourced an OVP from Mercedes Benz Classic in California to trial fit on the vehicle. They ended up experiencing no change in symptoms.

As for my idle speed, the tach needle usually stays stable just above the 500RPM mark. To me, it looks more like around 600RPM, but I could be wrong. If it helps, here's a picture of the tach around 10 seconds before I begin to experience those symptoms.



As for the cigarette lighter voltmeter, that seems like a very good thing to try (especially because it's so cheap!) When I get a change I will give that a shot.

As for my battery, I am 99% sure it's not to blame. I bought it in December of 2016 (it was actually dated January 2017) so the battery had not been sitting in a store for any time whatsoever. I dropped it off at the batteries plus I purchased it from for them to do a 24 hour test. The results came back stating that the battery was in excellent condition and actually producing 100 more CCA than intended.

The engine ground strap to the chassis and chassis connection to the battery are things over my head. I haven't heard anything about them at all, neither from the dealer or from the garage. I will definitely bring this question up to both of them the next time I am in contact with them.

Do you think I am on the wrong track with replacing the alternator with a higher amperage one?

ALSO - the garage revealed to me today that the alternator they have installed on the car is not actually a 70A, but an 80A one. I wasn't aware of this at all - probably should have took my phone camera down there to try and see the model number of the alternator they installed!

Again, I really appreciate your effort in helping me solve this strange problem!

Winston
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  #8  
Old 08-18-2017, 05:26 AM
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Trevor Hadlington
 
Join Date: May 2014
Location: Worcestershire in England
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Check output from alternator . Check all earth points from engine to body and that the battery conections are clean , tight and greased up. For the radio to go off this points to earth problems. Is the battery fully charged up . Another thing is the regulator in the alternator the brushes could be worn down . After these checks .Remove and clean the air control valve .Just remove the plug on the back remove 2 screws and clean it ou.t with white spirits .Dont put it in to the fluid as it electrical parts inside will get damaged
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  #9  
Old 08-18-2017, 08:28 AM
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Are you sure the serpentine belt is good and snug? Belt slipping - belt needs adjusting or maybe tensioner worn out.
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  #10  
Old 08-18-2017, 09:42 AM
Diesel Preferred
 
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Location: Charleston SC
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Has anyone checked the condition of the battery? Load test the battery, replace if not up to snuff.

I'm not sure which alternator your car has, but if the connector has blade-type push-on connector, the female side can get weak and not grip tightly enough to keep good contact. I had this problem on my '87 124. My '95 124 is different and has stud/nut connectors on the alternator.
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  #11  
Old 08-19-2017, 05:49 AM
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Trevor Hadlington
 
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Location: Worcestershire in England
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Do this after you find other help fails . Pull the speedo console out and remove the conector from the rear of the rev counter .Then see if problems still there . .But i say again for the other items to go off means earth problems ..Another thing you can look in to is the blade fuse that sits in a box fixed to the front spring turret .

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