Intermittent power loss after overcharge incident

[potion]

After an alternator regulator failure, my '95 C280 (W202/M104) sometimes loses power and idles roughly. Sometimes the power suddenly returns. If not, turning the engine off and restarting it restores normality for a while.

Allowing it to idle roughly for a few minutes, and then turning off and removing all the spark plugs reveals that the No 6 plug is wet with unignited fuel. Injector, ignition or control? My local independant M-B agent says "control": he's not sure, but thinks that the ECU is cooked as a result of the over-voltage and needs to be replaced. He cannot verify this because his diagnostic computer has limited functionality.

Q. If I take the car to M-B, will they be able to pinpoint the problem?

Q. If it is the ECU, can I replace it with a good second-hand unit, and if so, must this unit be re-programmed before it will operate properly?

Q. If it's not the ECU, then what else?

Den

 

[C220GJS]

First I would suggest you check the fuse/s on the Overload Relay, this is a tall silver relay in the compartment at the passenger side rear of the engine bay. The purpose of this relay is to protect the ECUs from voltage spikes. It is possible the relay itself is faulty as the Seimens relays fitted to cars built in the early 90s came from a bad batch which are failing now.
If this is not your problem have a look at your engine wiring loom as they tend to degrade in high temperatures and the insulation starts to fall off causing all sorts of problems.

 

[124coupe]

Hi, this being an M104 and 95 vintage there are a few things to check:

wiring harness - has this been replaced?. If its old and with the rotting insulation problem then it will give endless problems and fry components because of random short circuits. Eliminate this first before wasting money elsewhere.

Best method is to (carefully) cut into the part of the standard loom that feeds into the coil circuits and check the insulation (cracks/powder/bare wire means a new loom is required.

A nailed loom almost always starts to cause trouble after a repair operation - bending the loom causes large areas of old insulation to crack and shorts out the wiring.

Ignition is of "wasted spark" design where one coil serves two cylinders and both spark together (only one is on ignition stroke so the other spark is "wasted"). Therefore if an ECU output stage is fried or a coil goes bad, TWO cyls are affected, not one.

Each coil sits on top of a rubber boot that provides the connection to the plug (therefore 3 boots in total). These boots are prone to failure from being in such a hot spot under the coil and should be replaced if over a couple of years old (they have a date code moulded into them).

The ECU tests the spark electrical characteristics and "senses" if there is a misfire - if there is a misfire, fuel to the affected injector is turned off (to protect the cat). A wet plug is therefore not a common occurrence! This process, however, can fail if the loom is bad....

If it was mine, I would carefully check the loom and then, assuming the loom is fine, get the fault codes read. We can then help interpret the codes and suggest answers.

If your Indie can't read the fault codes, find another! (or you will fall victim to "lets try changing x and see if that fixes it").

 

[124coupe]

Just seen the OVP reply.

If the OVP has fused, you would normally have consistent problems and the ABS (if you have it) would throw a warning light.

A malfunctioning OVP can usually be made to misbehave by tapping it (as the issue is bad solder joints inside). Replacement is about £50 or take it apart and re-do the dry joints.

The one wet plug would not be OVP related as far as I know.

 

[C220GJS]

Just seen the OVP reply.

If the OVP has fused, you would normally have consistent problems and the ABS (if you have it) would throw a warning light.

A malfunctioning OVP can usually be made to misbehave by tapping it (as the issue is bad solder joints inside).

The one wet plug would not be OVP related as far as I know.

Have to disagree here ,I had to replace my OVP relay a couple of years ago ,the problems (bad idle and stalling) were intermittent but increasing in frequency but at no time did the ABS light come on.
The OVP is actually a multi relay with separate circuits for ABS and engine module protection so it depends which part of the relay is failing.
Agree the wet plug is probably unrelated.

 

[television]

Thinking at the answers here, it would seem that the wiring harness around No 1 could have neen disturbed.

Just as a point of interest, If the alterator or reg pack fails it cant put out more than it did when it was working, its the opposite. These are just tales that come from people that do not know much about electrics.

Malcolm

 

[potion]

wiring harness - has this been replaced?.

By this I assume you mean the engine harness. That was replaced about 2 years ago at 130 000 km (free of charge by M-B, since there was an unblemished service history). There's now 190 000 km on the clock - could the new harness have failed so soon?

if an ECU output stage is fried or a coil goes bad, TWO cyls are affected, not one.

Only one is affected, so we won't bother with that, then.

rubber boot that provides the connection to the plug... ... are prone to failure from being in such a hot spot under the coil and should be replaced if over a couple of years old.

I don't think that they've ever been replaced. Probably not too pricey, so I might as well...

- if there is a misfire, fuel to the affected injector is turned off (to protect the cat). A wet plug is therefore not a common occurrence! This process, however, can fail if the loom is bad....

...or, perhaps, the ECU is bad? I think I will check the loom, and if thats fine will go to M-B proper and...

...get the fault codes read. We can then help interpret the codes and suggest answers.

Thanks everyone for the help...

 

[mattkh]

nailed loom

A nailed loom almost always starts to cause trouble after a repair operation -

Hi

What is a nailed loom ?

 

[potion]

...wiring harness around No 1 could have neen disturbed.

Just as a point of interest, If the alterator or reg pack fails it cant put out more than it did when it was working, its the opposite..

The original over-current problem manifest with the engine cutting out (obviously when the OVP relay energised), and then bursting to life just before stopping dead (when the OVP relay released). Of course, it continued to cycle like that until the ingition was switched off. It was fixed by replacing the alternator with a reconditioned unit (so my assumption that the reg pack was faulty may be incorrect), and the auto-electrician re-soldered the joints on the OVP Relay. The battery was a casualty though, having swelled out around each cell, and depositing a fair amount of acid around. Come to think of it, there's some wiring just beside the battery - there may be acid corrosion damage to that, but I don't think it would result in the symptoms I am describing - I'll check anyhow.

 

[potion]

What is a nailed loom?

Nailed = F*cked
Loom = Harness

When last were you nailed?

 

[124coupe]

Re the OVP, my comment re the ABS light was for a blown-fuse OVP and stands (I just went out and tested it).

A failing OVP does not necessarily activate the ABS light as the dry joint effect (the original quality control problem) does not electrically resemble the broken fuse.

What it does do, is destabilise the "reference" voltage put out by the ECU to be used to measure the feedback from various sensors in the engine - therefore it affects the ECU control of timing and fuel and therefore upsets the running of the car.

 

[television]

The original over-current problem manifest with the engine cutting out (obviously when the OVP relay energised), and then bursting to life just before stopping dead (when the OVP relay released). Of course, it continued to cycle like that until the ingition was switched off. The battery was a casualty, having swelled out around each cell, and depositing a fair amount of acid around. Come to think of it, there's some wiring just beside the battery - there may acid corrosion damage to that, but I don't think it would result in the symptoms I am describing - I'll check anyhow.

These things work like this. The engine runs on the alternator, around 30 amps,the consumers in the car can also draw up to 30 amp, whats left over goes to charge the battery, in the case of a 70 amp alternator you would have 10 amps min to charge the battery,and in the case of the 90 amp you have 30amp.

The OVP relay is the to protect the cars expensive ECU's Should the voltage rise over the 13.8 running voltage then the zenor diodes in the OVP relay will blow the fuse, dropping the voltage down to 7,5 volt limp home mode.

the only way that the voltage can rise above the 13.8 volt is if you have a duff cell in the battery that goes high resistance, sure the battery can get hot under this condition, as the plates are arcing inside With the OVP relay pin 30 is the 12volt in and pins 87 are the output, this can be measured by sticking a meter probe in both sides of the fuse holder.


Malcolm

 

[124coupe]

The new harness should be fine (different insulation materials).

Cyl 6 is under the coil at the rear and so has the boot which gets hottest of all. It is worth eliminating the 3 boots as a cause of the problem. They are about £5-60 each. They need a firm push to allow the coil to seat properly.

Also, make sure the car has the correct spark plugs as it runs best only on the correct dirt cheap ones (under £3 each).

Re the wiring around the battery, there are additional 12v feeds taken to various components so worth a good check to make sure.

You could also swap 2 coils over and see if the problem moves - they do have two separate outputs so still possible for a coil problem to be present. Again, the rearmost coil gets the hottest.....

 

[potion]

Reprt back after the M-B service centre put their diagnostic system on the car.

Good news: ECU's fine. They also found that the MAF Sensor settings had to be 'adapted', whatever that means. The engine loom and the coil boots were fine.

Bad news: an incident-free journey home, but as I was pulling into my driveway, the rough idling problem returned (commensurate with the loss of power problem).

The saga continues...

 

[television]

Reprt back after the M-B service centre put their diagnostic system on the car.

Good news: ECU's fine. They also found that the MAF Sensor settings had to be 'adapted', whatever that means. The engine loom and the coil boots were fine.

Bad news: an incident-free journey home, but as I was pulling into my driveway, the rough idling problem returned (commensurate with the loss of power problem).

The saga continues...

Let me have the first 6 numbers of VIN

Malcolm

 

[eric242340]

intermittant power loss

MAF sensor adaptation? This is commonly done with Star D, we have to do it a lot, but I believe they have missed the real problem on this one. I would first try swapping the coils to see if the problem moves to another cylinder.

 

[potion]

Let me have the first 6 numbers of VIN

Here y'go Malcolm: 202028

Denis

 

[Hibbo]

Just as a point of interest, If the alterator or reg pack fails it cant put out more than it did when it was working, its the opposite. These are just tales that come from people that do not know much about electrics.

Malcolm

Sorry to disagree Malcolm, but surely it is possible for an alternator to put out a much higher voltage if the regulator goes.
The reg controls the field current, decreasing the field current as the revs increase to keep the output voltage steady. If maximum field current produces an output of (say) 12.5v at idle, then a duff reg that's putting out full field current all the time would cause the output voltage to rise in proportion the engine speed.

 

[124coupe]

What did the "M-B service centre" say about your wet plug symptom?

As long as you are sure that was real, it remains the problem to be found. Its a strange one and I would swap the coils (it may fail only when thoroughly hot) around if the "service centre" have tested and proven good the under-coil boot (how did they do that??).

The ECU uses the spark characteristics to detect misfire and turn injector(s) off so, in theory, a coil issue might stop that process working.

I would have expected a stored fault code related to the misfire (the MAF can't affect just one cylinder as it just measures the total volume of air entering the mainfold to fine-tune the mixture).

There would have been a MAF code stored assuming it was disconnected at some point and then the ignition turned on.

If there's a chance that "wet plug 6" is a red herring, I would still suspect the OVP - see if tapping it affects running of the engine - the relay components may be bad.

I have found that Bosch fuel injection centres are best equipped to diagnose these engines - they have the data to check the various sensors (possibly as a second best to MB dealers - although many dealers struggle with the older cars and are, of course, twice as expensive per hour so need to be twice as good!).

 

[television]

Sorry to disagree Malcolm, but surely it is possible for an alternator to put out a much higher voltage if the regulator goes.
The reg controls the field current, decreasing the field current as the revs increase to keep the output voltage steady. If maximum field current produces an output of (say) 12.5v at idle, then a duff reg that's putting out full field current all the time would cause the output voltage to rise in proportion the engine speed.

The max voltage that can be put out is 13.8v, with the battery in place. if a diode goes down they always go short circuit, this effectivly shuts the thing down, of the five diodes in the pack,two pairs will give one sympton and the other pair a different sympton,this can be observed by looking at the charge lamp. the 5th one in the exciter would simply cut off the charge.

malcolm