BMW Z4 Alternator Issues – FIXED!!!

[MEV Z4]

I acquired a Z4 2.5 2004 from ebay really cheap.

After owning it for about a week I was driving along and I realized I could not tell the difference between main and dip beams. Then most of the Dashboard lights came on (Engine / Traction Control / EML / Tyre Pressure etc….). The head lights went even dimmer, the electric power steering failed, everything else failed and the car went into limp mode (Would not rev at all and was running really ‘lumpy’) . Having rolled it off the road and gathered my thoughts I suspected an alternator / charging / battery problem even though No Battery warning light illuminated (However it did illuminate when parked with just Ignition on). The AA man came along and confirmed with a Multimeter that the alternator was not putting out enough charge. He wacked it with a hammer and it came back up to 14v.

So I got myself a Multimeter and found out the following before using it.

i) A Multimeter has such high internal resistance that hardly any current will flow through it - Even if you connect it straight across the battery terminals it won’t melt!
ii) Also it does not matter which way round you connect the multimeter- You’ll just get neg figures instead.
iii) My Multimeter was about £8.50p and is perfectly adequate.

Once the car was back at my place -I did the following tests with it.

- Multimeter across battery terminals. 12.5V with car not running indicating the battery is well charged. Then 13.8V with the car running indicates the Alternator is charging it well. The running voltage should be about 1.0v-1.5v above the standing battery voltage).

- Multimeter across battery terminals with starter motor cranking - Take note of the lowest value. Should not go under 10.5V when cranking - That indicates a good battery.

- Multimeter from Alternator Body to Negative Battery terminal (Or Car body) with car running. I got 0v which is a good reading. This low reading indicates there is no resistance / bad connections on the Negative Side. The alternator earths through it’s body so a high multimeter reading would indicate the electricity is finding it easier to go through the mutimeter to the Negative battery terminal rather than through the earth / car body back to the Negative battery terminal. (Should be no more than 0.2V)

- Multimeter from B post on Alternator (Where the fat red cable joins the Alternator) to Positive Battery Terminal. I got 0.0V with car running- This indicates there’s no bad connections / resistance between the Alternator output and the positive battery terminal. (Should be no more than 0.2V)

- Multimeter from B post on Alternator (Where the fat red cable joins) to earth (12.5v with car off) (13.8v with car running- This indicates the alternator is charging).

- Multimeter on Small Alternator wire to earth gave 8.5V - 9.0V. Don’t know what that means.

All looks good but I put a replacement (used) Alternator on anyway as i thought it might be the internal brushes. Then cleaned up all the terminals /connections I could find on the car.

Went down the road and it ran out of power - No Battery warning light illuminated.

Got it back and all tested again and looked good.

Put on a new Regulator onto the back of the alternator.

Went down the road and it ran out of power - No Battery warning light illuminated.

Took the alternator to a local Specialist who put it on a bench test and said “Regulator problem.” Then he said “Aha” and put a nut on the B2 terminal (That’s the OTHER heavy terminal on the back that appears to do nothing). Yes a nut IS required here. This is where the regulator makes contact with the body of the Alternator so it MUST have a nut on there and it must be tight and it MUST make a good connection. It’s very easy to think that terminal is doing nothing and ignore it while lavishing attention on the B1 terminal. (When your alternator is still on the car you most likely won’t even see the B2 terminal as there’s a plastic cap on the end of it). Without that connection being good you won’t get a circuit for the Field Coils. Ie No magnetism in the Alternator. I removed the Regulator and cleaned that contact (I mean the contact that hangs off the bottom of the regulator and is Shaped like the end of a Spanner) then I refitted and tightened everything up.

There’s also a small screw on the end of the diode pack that appears to make a contact through a copper loop – Cleaned it up and tightened that as well.

Went down the road and it ran out of power - No Battery warning light illuminated.

Got it back - Did all the tests again and all looked good.

Then I tried to start it and just got a click. Flat Battery? No still 12.0v there. So I looked at the Starter Motor terminal – HANGING OFF! Yes the cable to the starter motor was not even finger tight. Because the route of this cable is totally hidden under the manifold it’s very easy to assume the cable goes direct from the back of the Alternator to the Positive Mounting point on the bulkhead – No it stops off at the starter motor and then a 2nd cable (On the same post on the Starter motor) goes up to the Positive Post on the bulkhead. I took the heavy cables off the Starter Motor and cleaned them up then put them back on really tight. (Note – Disconnect your battery while doing this as you’re using a metal spanner on a positive wire with lots of earth points nearby).

So despite all my multimeter tests being good there was a seriously bad connection from Alternator / Starter / Battery which only showed up as an intermittent fault. The moral of this story is – While doing multimeter tests get your assistant to give all the leads wires a yank to make sure there’s no itermittent fault.

Now at 13.5v – 14.0v after a lot of headache.


Note – The regulator I was sent was for a slightly different BMW model, this means the ECU on my Z4 does not ‘understand’ it protocol / language. That’s not a problem, it just means that smart charging is disabled. The ECU can no longer command the alternator to drop it’s output to save drag / fuel. (The ECU does this by varying the Alternator field coils). So my Alternator has defaulted to maximum output at all times. The only other thing to note is you’ll get fault codes “Generator Serial Interface 281C & 281D” . You’ll only see that if you run a scanner on it – No engine lights or other indications at all.

Note- I now keep a voltmeter plugged into the cigarette lighter socket. It has USB plugs on it so I think its main purpose is to charge up mobile phones and display the voltage for that purpose.
Note - My local Alternator specialist advised that the small wire plugged into the back of the alternator should run at about +9 volts (He put +12v onto it when doing a bench test). He said it’s function is the battery light wire (I assume it must also be how the ECU commands the alternator?). I’m passing on this knowledge as I could not find any info on the internet what the voltage there should be.

Note- All of the above text refers to a BMW Z4 2.5 Litre of 2004 which has a Bosch Alternator. Other / Later models may differ.

My intention in writing this is to repay the forums which have helped me so much.

[tug]

Excellent write up. And will be invaluable to electrical thicko's like me.

Also, welcome to the mad house mate.

[raymond.harper]

A very nice write-up, you have the patience of a saint! The AA seems to employ the "wack" technique in most applications. They did this to the ignition key on our Honda Jazz that was refusing to turn. This resulted in severe damage to the lock that could not be repaired!

[Reamesy]

A great shared experience which hopefully will help someone else in the future.

Just a thought; as you replaced your original regulator with a new one which turns out was not the fault, could you not put the original on to get everything working as it should?

Just a thought

[colb]

Had my 2023 2.5 Alternator fail, bought a rebuilt one from a supplier in Birmingham fitted that but still had a battery light, sent it back and it was replaced but replacement came with wrong pully so that went back and I had a refund. Took the failed one to a local rebuilder who put a new oem brand regulator unit into the back of it. Fitted that and all was well no lights and charging back to normal. I assume the replacements had iffy regulators installed and were not the variable charge rate spec.

[Reamesy]

Usually it’s 9/10 the regulator that fails. Had this with an Audi TT. Bought a genuine Bosch part £30. 6 hours labour in -5 all done.

[MEV Z4]

Thanks for your Positive comments! I'm going to leave the replacement Regulator on the 2004 Z4 2.5 Litre so I get full Alternator output 13.5V -14.0V all the time. (Took it 50 miles yesterday and my cigarette Voltmeter indicates those good figures all the way even with Main Beam / Seat Warmers etc all on).

The reasons for leaving it are -

i) It's working now and "If it Aint Broken I Wont be Fixing It".
ii) In my internet searches I've seen comments that the Smart Charging can limit the life of the battery as it never gets fully charged - (I don't know how true this is).
iii) I've also seen comments that the smart charging system may make if difficult to start another car with Jump Leads from the Z4 as the alternator is doing does the Minimum. (I'm not sure how true this is).
iii) I love the Electric Seat warmers, they're on all the time.

Note - The D shaped plastic Plug on the replacement Regulator had it's internal Tabs in a different place to the plug on the car- Which is to try to stop you putting it the Regulator on a different model / different Protocol for smart charging. I cut the internal tabs out of the regulators D Socket and the external tab off the outside of the Regulators D socket with a sharp knife. I Pushed on the plug from the car / ECU and so far it's done 100 miles at 13.5V- 14.0V and the Smart Charging is bypassed without issue. I have the original Bosch regulator as a spare as I know it works.

Note - Having cleaned up the Starter Terminals as an extra bonus the car cranks MUCH quicker. Yes that's obvious - but I didn't know it was supposed to crankover that fast!

Note - Our local garages don't want wont jobs like this. (Yes last year for my other car - a 2004 E46 330i Touring I was told "Take It Away We Don't Want To Be Working On THAT Car" - All it needed Front Damper tightening up and Handbrake Adjusting) - Whether that's because of labour shortages here in the UK or they just don't want anything that might have a seized bolt or that might involve a bit of thought / troubleshooting (And they know they could change some biddies Pollen Filter for the same money) - and when you can get them to take on a job - You see them standing around doing a lot of pointing at it when you go to collect. My reply to all that is - "If it worked it would not be in your garage. What do you want me do do? Wastefully crush it and go into debt wastefully buying something new to please you? If you don't want to fix cars then What Do You Want To Do? Why then do you exist?"). I also figured that a garage would simply stick on a new genuine Alternator for £400, act like they're doing me a favour, not bench test the old one and not figure out any other issues (As I have done) and not give a flip when it got down the road an ran out of power again. Which It would have done....

For those reasons I'm learning all this stuff for myself (and will be sharing it) so that I'm no longer beholden to / putting money in the pockets of people with such an entitled attitude.

I never quite made it into this century - So for me, 2004 is a modern / new car. That car's got a computing thing in it.

[Joez4]

Just to add if you have codes 281c 279d and any other codes related to bsd communication fault try cleaning the the connector to the voltage regulator with electrical contact cleaner.

I had those codes after changing my alternator, and voltage regulator and finally stopped them coming back after cleaning the connector.

And you can display the voltage on the dash using the hidden menu before starting the car, I have this on when I go on any long trips or I've done work to the car recently