SL500 re-starting problem

BiKenG

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Couldn't think of a better section in which to post this. It's not limited to engine and involves some electrics (which has no dedicated section), but it may not be electronics related. So I posted it here.

I've had my SL (see below) for some years and starting has never normally been a problem. When not in use for any extended period I have 2 CTEKs to keep the batteries in tip top condition. But twice I have now experienced a problem trying to re-start the car.

Most recently this came after it started perfectly in the garage (CTEKs had been connected), then a 30 minute drive after dark and switched it off. Within a couple of minutes I tried to re-start the car and the tortuously slow cranking of the engine gave all the appearances of a low battery. I was just using the shift lever Start button and don't believe I tried the actual keyfob in the switch. I tried a few times, but it was turning over so slow it wouldn't start. Somewhat perplexed as I thought this had never happened before, I arranged for an emergency break-down call out to come and help. After a few more minutes, having got out and locked the car, on the off-chance I unlocked it, got back in and tried to start it again. Cranking slightly faster it eventually caught and started. Without trying to diagnose it further, I cancelled the call-out and drove home, carefully avoiding having to stop the engine and parked it in my garage (without connecting CTEKs), using a different car to continue the evening.

The next day I tried the car again and it started perfectly. Several times and showed a decent 12+v when off and 14+v when running. So no obvious problem.

I am aware that the battery is not in exactly youthful, but the car has only done 20K miles and is kept garaged. Although a failed cell can cause a battery to exhibit erratic behaviour, this seems different. In particular, after starting perfectly, a short drive rendered it incapable of cranking the engine fast enough to start. Then I just managed to get it started, the same short drive home and left for about 20 hours and all appears good again. None of which is consistent with a failing battery. So I began to think.

And remembered that this had occurred before, albeit with such minor consequences I had forgotten.

This involved a longish (100ml + drive) and stopping in a hotel car park and then a few minutes later trying to re-start the car to move it. At that time, as in the more recent episode, the engine wouldn't crank fast enough to start. I decided I was quite happy with where it was parked and that I'd worry about it the next day. And then promptly forgot all about it until having walked to the car the next morning and started it easily, without any problem, spinning the engine over 'with gusto' and immediately starting and running perfectly. I then forgot all about it again.

Until it happened again a few months later, as described above. Which puzzles me as I just don't think a failing battery would behave like that. To be so perfect at all starts, except for these 2, separated by months and in each case, having been left for a while, problem has miraculously vanished.

I believe that the big rear battery can in some circumstances charge the small front (starting) battery, which would explain how after sitting a while, it was able to start. But it does not explain why the starter battery became so low. Both times after a drive long enough for a fully functional charging system to bring it completely to full charge. But if the charging system is at fault, why only those 2 isolated incidents in the several years I've had the car.

Which makes me think the complex electronics in this car may be the culprit. Is it possible that in some circumstances, trying to re-start after only 2 or 3 minutes would somehow confuse the system into not providing full power to the starter. Is it even possible to be restricted in this way?

One other similarity in the 2 incidents, that may or may not be relevant - in each case, when trying to re-start the car, I fumbled it. Thinking about other things I pressed the start button before placing my foot on the brake pedal. Realising my mistake I pressed the brake, then thought that wasn't the best way, took my foot off, then back on and pressed the Start button (or some such similar sequence of fumbles) in quick succession. Could that have 'confused' the car's electronics in some way that resulted in its inability to fast crank the engine?

If I have to replace the (what is now ageing) front battery, then I will, but I'm loath to try and diagnose a problem simply by buying new parts until the problem goes away. So if the problem is actually related to some electronic control unit and not the battery at all, I don't want to find out after (needlessly) replacing said battery.

I have read a brief mention of something vaguely similar in another thread, but it was quite old and so better to start a new one specifically about this problem.
Anyone any clues as to what is actually going on?
 

BTB 500

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I would check the earth strap from the engine. Slow cranking must either be lower voltage than normal, or higher resistance.
 

wongl

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I wonder if you have a faulty battery which is getting warm/hot as it is being charged during your drive? The internal resistance will raise with the temp to the point the battery cannot deliver enough amperes to crank the starter motor properly. Leaving it overnight allowed the battery to cool down sufficiently for the internal resistance to drop to a more respectable value perhaps?

Have you got your battery tested?
 

E55BOF

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I would check the earth strap from the engine. Slow cranking must either be lower voltage than normal, or higher resistance.

+1, and also clean up all the power cable connections at the battery terminals and the starter motor.
 

ernie

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+1, and also clean up all the power cable connections at the battery terminals and the starter motor.
I had a similar situation many years ago following a run in a car that had virtually no electronics. In this case, the car wouldn't start on the battery when hot but started easily using starter when cold and the starting handle when hot. ...for those of you who remember these things! It turned out to be a faulty battery.

Ernie
 
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BiKenG

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I wonder if you have a faulty battery which is getting warm/hot as it is being charged during your drive? The internal resistance will raise with the temp to the point the battery cannot deliver enough amperes to crank the starter motor properly. Leaving it overnight allowed the battery to cool down sufficiently for the internal resistance to drop to a more respectable value perhaps?

Have you got your battery tested?

No, not tested yet, but it just seems an unlikely battery symptom. I agree with the overheating bit, except why has it only done that twice. All other times there has been no problem.

Likewise, corroded battery connections would not miraculously fix themselves the next time I tried to start it. Also, the car is very clean, almost like new. Only 20K miles, always garaged and for at least half its life, not used in the winter. I realise corrosion can be hidden, but it just doesn't seem feasible. In fact I'm sure I checked for voltage drop across the main connections and found none, but it's a few months ago (car's not been used) and I may remember that wrong. So I'll check it again.
 

Ted

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As @BTB 500 has said, slower cranking is a lower voltage (across the starter motor) unless the engine is tight (I’ve come across a seized engine and a road roller with gear oil or some other super thick oil in the engine).
So there are a number of tests to go through to find the low voltage - usually starting at the battery, but as stated it is definitely worth checking the engine/ chassis earth connection.
As a rule of thumb, there should be a max 0.5 volt drop in the supply or return circuit with the max volt drop also being 0.5 - i.e. if you have 0.5 volt drop in the supply, that is acceptable as long as there is 0 volt drop in the return.
May be worth getting a competent auto electrician to look at this as it should be pretty simple to track down.
 

E55BOF

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The auxiliary battery is there to provide extra urge when starting, but if the main battery is healthy and fully-charged, it is perfectly capable of starting the car on its own.

Intermittent faults can be a bugger to trace, but go back to first principles. If the engine is cranking slowly, but cranking, pretty much the only thing that can realistically cause that is not enough power reaching the starter motor. That points to either a faulty battery, or a poor connection. If it was a faulty battery, it would normally be worse on a cold start. Get those connections checked, including those at the starter motor if the battery and engine earth lead connections are clean and tight.

If you find nothing amiss, look to the battery. If you still find nothing, if it happens again try connecting a jump lead between a good earth on the engine and a good earth on the body; if that solves the problem, you have your answer.
 

wongl

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The auxiliary battery is there to provide extra urge when starting, but if the main battery is healthy and fully-charged, it is perfectly capable of starting the car on its own.

My understanding is that the Aux Battery is there to maintain the voltage to the ECU (and other critical systems) during starting as the voltage from the main battery will drop during cranking. The Aux Battery on my car is connected via a relay and I pretty sure it do NOT support the main battery to crank the car during starting or a ECO re-start.
 

MikeInWimbledon

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The starter battery in the front only runs the starter.

The Consumer battery in the back ONLY runs consumer items. It is designed to go flat if consumer items are left on. (R230's routinely spend long periods unused in garages)

The Consumer battery does NOT "assist" the big starter motor in the front.

Going back to the exam question, this "sounds like" an electrical fault in the wiring to the starter, rather than the battery. You've had good advice for sure but, as a above, I'd suggest that an auto-electrician is needed to quickly diagnose the problem.

.
 

wongl

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The starter battery in the front only runs the starter.

The Consumer battery in the back ONLY runs consumer items. It is designed to go flat if consumer items are left on. (R230's routinely spend long periods unused in garages)

The Consumer battery does NOT "assist" the big starter motor in the front.

Going back to the exam question, this "sounds like" an electrical fault in the wiring to the starter, rather than the battery. You've had good advice for sure but, as a above, I'd suggest that an auto-electrician is needed to quickly diagnose the problem.

.
I don't think this is how the Aux Battery is setup in my SLK R172 nor on my W246 and W205.
 

MikeInWimbledon

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I don't think this is how the Aux Battery is setup in my SLK R172 nor on my W246 and W205.

Aye, my comment is about Ken's R230 SL500.

I'm no expert on the R172, but I thought the R172's smaller battery, behind the seats, was only to run consumer items when the car had activated "stop start," to guarantee that the big battery could start the engine when required, on demand.
 

Jobsworth

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I don't think this is how the Aux Battery is setup in my SLK R172 nor on my W246 and W205.

It isnt. The R230 SL works in a different way. It has a smaller battery in the front that operates the starter motor, but it isn’t as small as you are imagining.

To the OP, I think I’d be inclined to look at replacing the starter battery first, but this fault could be to do with the battery control module in the boot. It is a split charging unit. It charges both batteries, but I don’t know to what level it prioritises which battery as being more important. If it is not recharging the front battery enough to replenish after the first start it could feasibly give the scenario you described
 
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E55BOF

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Interesting how the R230's battery setup is; I'd assumed it was similar to the W211, with a large battery in the boot and a much smaller, motorcycle-type one, in the engine compartment.

It makes my point even more valid, though; check those connections, and if no joy, look at the starter battery.
 

BTB 500

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It isnt. The R230 SL works in a different way. It has a smaller battery in the front that operates the starter motor, but it isn’t as small as you are imagining.

Yup the R230's front battery must still be a decent size to crank a big V8 on a cold day. Of course the previous generation R129 SL (with less electronics) makes do with just a big battery in the boot (IIRC mine is 100 Ah).
 

wongl

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Aye, my comment is about Ken's R230 SL500.

I'm no expert on the R172, but I thought the R172's smaller battery, behind the seats, was only to run consumer items when the car had activated "stop start," to guarantee that the big battery could start the engine when required, on demand.
Yes this is correct - the point I was making is that as far as I am aware, the aux battery on the MB cars I have owned, none of them is intended on boosting the main battery to crank the starter. In fact all the aux batteries (ranging from a 80ah motorcycle battery to a capacitor) is intended to isolate and preserve the voltage to the electronics from the nasty starter motor!

I am very surprised to find that this is not the case in the R230, but then again perhaps the Aux battery in the R230 is more meaty than a mere motorcycle battery?
 

renault12ts

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V8s should be very easy to start. I hadn't driven my car two weeks recently and when I did try to start it it barely turned over...but one cylinder fired and off she went. So, even slow cranking should have started it easily.
 

MikeInWimbledon

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Just to outline this again, the R230 is usually a garage queen that is often left parked up for long periods of time. It's more typically a third, or fourth car rather than a daily or winter car, so it runs the risk of running flat, particularly over the Winter months.

Secondly, it has some bonkers electrical demands, due to that retractable roof, and all its luxury gizmos.

So those very clever chaps at Stuttgart said, we mustn't compromise starting by letting "Consumer electrical demands" use up all the juice.

And they may have been moving some weight out of the front into the boot, to balance out that V8.

As E550BOF says, the best first step is to check those electrical connections.

As a BTW - why Mercedes didn't fit some kind of on-board trickle charger is anyone's guess. Probably just reluctant to recognise the need for additional charging.

.
 
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BiKenG

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First of all, there are obvious tests that can be done to figure out voltage drops and poor connections, but that is not actually the main problem as I'm perfectly capable of doing all that (having spent years instructing on Motorcycle mechanicals and electrics). My big problem is how the hell MB have cobbled together this system with 2 batteries. How are they connected, when and what is the result of that? Also the specific symptoms do not point to bad connection or battery failure in a 'normal' (i.e. single battery) vehicle. In those cases it is more likely to have a problem after being left, but be fine a few minutes after a journey. Hence it points to the problem being with MB's battery control system on the R230. About which I have now read totally contradictory information. But first, a definition of terms:-

• Starter battery. This is the smaller of the 2, located under the bonnet/hood and is basically just used for turning over the engine.

• System battery. This is the larger battery, in the boot and is used to power everything else.

This system ensures (or tries to) that even if the car's on-board electronics drain the System battery, the Starter battery (with no drain when off) will be able to start the car, after which the System battery can be replenished from the engine/generator output. But…

The Battery Control system interconnects these 2 batteries in ways that are unknown to me and apparently unclear to many others also.

As I mentioned above, the symptoms I experienced are almost the reverse of what would be expected in a normal car with failing battery. I am 99% sure it is not bad terminal connections and let's face it, unlikely to be a starter or relay problem. Because it has only occurred specifically when trying to re-start after being off for only a few minutes and it cranks too slowly to start (I mean really slowly, hardly turning over at all). At all other times the engine spins fine and starts perfectly. This either does not indicate a failing battery (and/or other hardware), or MB's Battery Control system is confusing the symptoms.

Here's what occurs to me:-

If the systems battery can 'assist' the starter battery, that could explain the symptoms I experienced. With a failing starter battery unable to start the car by itself, the systems battery helps and the car starts. But it may take being off for some time to allow the battery control system to recognise that the starter battery has a problem - it could never do this while the engine is running. So after stopping the car, it takes longer for the control system to figure out the problem than the few minutes I allowed it in the cases I described. So when I tried to re-start, it was having to rely on the starter battery alone and that has now actually failed. So the engine cranks too slowly and the car is unable to start. After a period of time not running, the battery control system again recognises the problem and the System battery kicks in when required and the car starts next time and the next time etc. Only when the control system has not been allowed sufficient time to recognise the Starter battery problem does this issue arise.

This of course relies on the predicate that the System battery can 'assist' the Starter battery. I have seen this denied, but I have also seen it specifically stated to be the case and also that the Battery Control system includes a DC-DC converter for exactly this purpose. It also makes huge sense to me that the designers would have considered the possibility of the Starter battery being incapable of starting the car, yet with full power available from the System battery. How daft would it be to design a system where with plenty of power actually available from one battery, the car was unable to start due to the other battery having a problem - totally incapacitating the car, despite it's good main/System battery (which will now go flat as the car cannot be started).

Of course the Starter battery has priority so if left, the System battery is what drains, while the Starter battery has no drain and should still be good to start the car (and then of course the System battery gets recharged and all systems are again fully operational). But if the Starter battery fails, it makes HUGE sense that the Control system recognise this and give the engine extra shove from all the available power still in the System battery. Otherwise, why have any interconnection at all between the batteries. They could be entirely independent circuits. There's no need for the Starter battery to ever assist the System battery, but it does make sense the other way.

Which opens up the possibility that this could explain the symptoms I have seen. It WILL take time with the engine off for any Control system to conclude that the Starter battery is low and needs help and if there has been insufficient time to do this after stopping the car, the too quick re-start will be relying on the failed Starter battery with the result I have experienced.

The above is all my best guesstimation of what could be occurring, but relies on the System battery being able to assist the Starter battery. However, nothing else fits the symptoms. So I need to disconnect the Starter battery and test it independently from the car. If it's good, then I'm stumped. We'll see.
 
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ivan1983

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I had the same symptoms with my w140, I thought it might be the starter breaking down under high temperature. I had a spare battery so swapped it out and that solved the issue
 

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