BMW E36 M3 Alternator

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BMW E36 M3 Alternator

Postby ndporsche » Sat Jun 02, 2012 6:41 am

I built a track day car out of a BMW E36 M3 evo,the car is caged and 350kg lighter than standard,it does sub 8 min laps at the 'ring and sub 3min at Spa.
Under sustained high revs,full throttle use like the the 2km straight at the ring the engine will suddenly start running very badly-top speed 30mph-if you stop ,turn off the ignition and leave it for 30 sec on restart it will be running perfectly.EML light does not come on and no fault is recorded by the ecu.After several blind alleys and a baffled BMW Technical I have found the problem is the alternator which appears to have a leaky rectifier.I did an ac ripple test,see attachment,and it is feeding the ecu with a diet of 3-4 volt ac spikes.
The ecu has been tested and I had new capacitors fitted to the power supply section of the board,all wiring and connections have been tested and are good.I have tried three different alternators on the car-the original Valeo,a new Valeo and a new Bosch unit from an E36 320.
I proved that this is the problem by disconnecting the alternator and running the car on the battery alone-sorted.
Recently I was at Spa for a track day spannering on the M3 and a second M3 which is an '07 E91 also stripped and caged with a custom loom and a Pactel race ecu running the engine and this car had exactly the same issue which I again proved by running it on the battery only.
So has anyone got any ideas because I'm a bit stumped.
Thanks,Martin
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Re: BMW E36 M3 Alternator

Postby PicoKev » Sat Jun 02, 2012 9:06 am

Hi Martin,

First of all welcome to the Pico forum :D I am not seeing an attachment on my screen can you check at your end please.

So that the very knowledgeable people here can offer some opinions it would be good if you could post what will be the likely questions.

What year is the car?
How many wires does the alternator have?
Can you post a wiring diagram of the alternator circuit?
Anything electrical added / removed / modified?
What testing have you done, especially volt drop/load testing of powers and grounds?
Are the replacement alternators you have tried new / used / re-furbed?

Non standard cars are always a bit of a nightmare when you cannot see them, so the more info the better. 8)

Kev.
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Re: BMW E36 M3 Alternator

Postby Kev » Sat Jun 02, 2012 8:20 pm

First of all welcome Martin!
I'm quite sure that part of the regs for racing is that you have to have a master cut-out switch! I don't know whether this is the setup that you are using. If this is what you are using, then you have to use a resistor to prevent damage to the alternator, because what happens is that when you turn the battery cut-off switch to the off position to kill the engine it opens the circuit for the alternator, this action destroys the alternator as the alternator should never have the load removed from it whilst the engine is running. Let us know if this is the setup that you have Martin.

Kev
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Re: BMW E36 M3 Alternator

Postby ndporsche » Sun Jun 03, 2012 8:18 am

Kev,
No battery cut-out switch fitted,not required for track days
Attachments
M3 valeo rectifier 2.psdata
(207.08 KiB) Downloaded 230 times
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Re: BMW E36 M3 Alternator

Postby Kev » Sun Jun 03, 2012 3:59 pm

Hi Martin,
Thanks for the info back! The waveform that you posted could you tell us where the test point was made and under what conditions, for example was the engine idling or under hard acceleration, another interesting point is some of the earlier alternators fitted to some BMW cars were rubber mounted, is this one!

Kev
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Re: BMW E36 M3 Alternator

Postby ndporsche » Mon Jun 04, 2012 7:58 am

Kev,
Alternator is solid mounted.Best result was with the Bosch alt. attached is a screen shot of that one with 45amp load at 5000rpm in the workshop,testpoint was the output terminal onthe alternator.
Martin
Attachments
M3 bosch alt 2.psdata
(345.9 KiB) Downloaded 209 times
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Re: BMW E36 M3 Alternator

Postby Kev » Wed Jun 06, 2012 5:53 pm

Hi Martin,
I'm quite sure you've already done it, have you checked the condition and tightness of the earth strap from engine/gearbox to body/chassis. If you have and it's perfect go back to the alternator B+ output terminal and follow it to the starter motor check that it's tight and in perfect condition, if your happy that it's tight enough and that all your tests have been carried out with both cables in first class condition, the other thing that you could try is to check for any voltage on the body of the alternator itself whilst under load, If that checks out OK then could you remove the alternator B+ cable that comes down to the starter motor and transfer it to the battery positive terminal. The reason I want you to do that is just in case there is a grounding through the starter motor at high RPM due to resonance! Let me know how you get on!!

Cheers Kev
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Re: BMW E36 M3 Alternator

Postby ndporsche » Wed Jun 06, 2012 7:38 pm

Kev,
I don't have the car at the moment.I was talking to my tame electronics geek and he pointed out that the biggest and best capacitor on the car was the battery itself so he suggested taking the alternator ouput directly to the battery on it's own cable avoiding any junction points with power supplies to other circuits.The battery is in the boot on this car.There's no voltage on the alternator body,I would have seen it when I tested the earth path for voltage drop.
Regards,
Martin
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