BMW M5 Forum and M6 Forums banner

1 - 12 of 12 Posts

·
Registered
Joined
·
11 Posts
Discussion Starter #1
Hi all,

Apologies in advance for the long post but I’ve had an ongoing problem with my car now that’s driving me crazy. So a while ago my battery kept dying overnight, I would have to jump start the car pretty much every day. There was a drain problem in the past but that was resolved, I checked the drain with everything off and it was around 0.05amps. When the engine is started, the alternator is only providing around 8-9volts, all electrics are very weak as you can imagine. Along with this, the ABS light on the dash stays lit. Once the engine revs hit 4k, everything comes back to life and the voltage jumps back to 14v as normal. This happens every time the car is started. Everything pointed towards a faulty regulator. I pulled the alternator out, and replaced the regulator with a new Bosch unit. While it was out I got the alternator bench tested and it came back all fine. I re fitted the alt with the new regulator but I still have the same problem. A few people on Facebook and some old forum posts told me to check the battery light on the dash as that has something to do with exciting the alternator. I pulled the cluster and the battery bulb is working fine, but there’s no power going to it. I also grounded the small cable on the back of the alternator (the exciter wire) to the same ground point (strut tower nut) that I use to jump the car, but still no battery light and still 8v on startup. I’m quite baffled at what this could be and was hoping someone here could help me out.

Many thanks in advance
 

·
Registered
Joined
·
29 Posts
What year is your car?

Check fuses F28 (15A) and F17 (7.5A), unloading relay X1201, and a decoupling diode V9501.

Good luck
 

·
Registered
Joined
·
11 Posts
Discussion Starter #3
What year is your car?

Check fuses F28 (15A) and F17 (7.5A), unloading relay X1201, and a decoupling diode V9501.

Good luck
Sorry I forgot to mention it's a 91 model.

All of the fuses are fine and the unloader relay itself is fine, and pin 4 is also getting voltage when the ignition is on. Regarding the diode, someone else has told me to have a look at it but I have no idea where I'd find it, and how to check it.

I've done some further tests in the meantime, regarding the ignition light on the dash. The bulb itself is fine, and there is voltage going to the pin (pin 7 on the yellow 26 pin connector) with the ignition on, but no light when its all placed together?? I even connected the bulb to pin 7, and turned the ignition on, bulb lit up, started the car, still low voltage.

Appreciate the help
 

·
Registered
Joined
·
29 Posts
Car's diagnostic connector has a alternator indication pin. It should be pin 12. In theory, with the key in run position, but engine off, grounding pin 12 will light up the battery dash light. If this is omitted, problem should be with what I mentioned in the post above, or potentially wiring.

Unloader relay is known to cause charge voltage issues. When you say it's OK, have you popped a replacement one in, for assurance? Have you made sure that the alternator is properly grounded?

Having said all this, working on electric circuitry, without proper tools, can be detrimental. If you're not sure what you're testing, perhaps you should involve a professional.
 

·
Registered
Joined
·
11 Posts
Discussion Starter #5
Car's diagnostic connector has a alternator indication pin. It should be pin 12. In theory, with the key in run position, but engine off, grounding pin 12 will light up the battery dash light. If this is omitted, problem should be with what I mentioned in the post above, or potentially wiring.

Unloader relay is known to cause charge voltage issues. When you say it's OK, have you popped a replacement one in, for assurance? Have you made sure that the alternator is properly grounded?

Having said all this, working on electric circuitry, without proper tools, can be detrimental. If you're not sure what you're testing, perhaps you should involve a professional.
Ok so I grounded pin 12 and the battery light does turn on with the ignition, of course even with the engine on it will stay on as its been hard wired, but still bad alternator function.

i haven't used a replacement relay, I haven't got a spare and it'll take a few days to order a new one in. Would you mind guiding me on how/where to find this uncoupling diode? I've managed to locate it on the electrical diagram but it doesn't really tell you where it is on the car. I take it it's somewhere between the OBD port and the fusebox, buried underneath there? Am I right in thinking that the unloader relay also has a diode in it too? Appreciate the help

938466
 

·
Registered
Joined
·
29 Posts
Yeah, you're reading it right, there are two diodes. Unfortunately, my car is not accessible for checking. It's been stored, awaiting better days. So I couldn't tell you where exactly the V9501 is located. It could be anywhere between the relay and the alternator really.

I just reread your original post. You've mentioned that you've grounded the exciter wire. This method works with alternators that are ECU controlled ("two wire" field control). Grounding your alternator's field post will not produce a desired effect. You should feed it with 12v, perhaps through a test lamp (make sure other part of the field control circuitry is disconnected). Be wary though, you could push your alternator to produce a lot of voltage, potentially in the region of 15-18v. This may cause damage to other electronic bits. You should never do this test for long periods of time, and perhaps it's best done when your battery is already running low. Just to have the battery act as a buffer and "wick off" some of that extra umph. Of course, engine should be running, and you'd be measuring the voltage between the +B and ground (both on the alternator). If you get desired result, then repeat with measurements across the battery terminals.

Before you make any above attempts, did you check the alternator ground, I didn't see it written? You need a volt meter to properly check it. Specifically, you need to check the DC voltage difference between the alternator and battery grounds (voltage reading scale of 20V or thereabouts). It shouldn't be anything more than 0.1-0.3v. If it is more then it means you have a poor ground. You can apply the same test, between your alternator's +B terminal and the battery positive terminal. Again, not much voltage difference should be noted. If it is, then you have poor positive wiring. Considering the battery's location, this will require long test leads.

Keep us posted
 

·
Registered
Joined
·
11 Posts
Discussion Starter #8
Right, unfortunately over the weekend things took a turn for the worst, I’m probably going to have to book it in at my BMW specialist unless there’s something very obvious I’m missing.

On Saturday, I started it up and as usual was on 9v, after letting the engine warm up I gave it a rev to 4k to excite the alternator but it didn’t do it like usual, it stayed on low voltage for a couple of minutes, then while driving it suddenly spiked to around 16.7!!! Which is obviously way too much, the lights were the brightest I’ve ever seen them. In a 5-6 min journey it was fluctuating between 9-16volts but it still drove as normal.

Started it up again this morning to see if it had reset itself (I disconnected and charged the battery overnight) and in addition to the ABS light, the red brake warning light was also on, Speedo and rev counter was stuck on 0, I didn’t get a chance to measure the voltage but I assume it was well below 9 as the engine cut out after about a minute, was running very rough and would barely stay on. I’ve looked at everything I can, and will probably have to take it to Munich Legends which is about 45 min drive away so I’ll have to look at getting it towed as it won’t make it there in this state.
 

·
Registered
Joined
·
29 Posts
That's a pity. Sounds like the voltage spike has blown your instrument cluster, potentially more.

Good luck with the repairs.
 

·
Registered
Joined
·
11 Posts
Discussion Starter #10
That's a pity. Sounds like the voltage spike has blown your instrument cluster, potentially more.

Good luck with the repairs.
Well the cluster lights all work fine, and the speedo/rev counter also works fine but strangely they both drop to 0 when I release the handbrake??? Very strange, thank you, I’ll keep the thread updated with any news
 

·
Registered
Joined
·
29 Posts
Parking brake gives your cluster a ground it may be missing. Bare in mind that temperature sensors also go to the instrument cluster. Cluster could mess up your temperature readings, causing engine running issues.
 

·
Registered
Joined
·
11 Posts
Discussion Starter #12
Guys just an update, thankfully I’ve finally solved the problem!



So I did a few more tests with the help of an auto electrician, and he concluded that there was 2 problems, 1. there is a break in the exciter circuit somewhere between the alternator and firewall, and 2. possibly a fault with either the alternator or regulator (which was only installed a few weeks ago).



So we pulled the alternator out again, and for some reason one of the screws holding the regulator had loosened to the point where the back side of it wasn’t making contact with the alternator casing, re-installed it and tightened back up.



As for the exciter wire, I cut open the main loom right by the firewall, found the blue cable and stripped a small length of it, and wired in a fresh piece of cable just temporarily to the back of the alternator, creating a new exciter wire. I left the old one there for the time being as I’ll drive it like this for a few weeks just to make sure everything is ok, then go about making it look pretty. Unbelievable how it was such a simple fix but of course with electrics its always the fault finding and investigation which is time consuming. Put everything back together, battery light comes on with ignition, started the car and got a perfect 14.2v on startup, all electrics at full power, very happy.



Thank you so much for all the help and advice everyone has given, I really appreciate it
 
1 - 12 of 12 Posts
Top