Where to start looking for the cause of this one?

After about half an hour of driving at night I'm getting a strange lack of power from my 1990 325i. When climbing uphill, whenever the engine needs to work a bit harder, it suddenly loses power. It's not quitting altogether, it just feels sluggish as though it's thinking of quitting.

At first I suspected a fuel pickup problem, or water in the tank that only gets picked up whenever the car is going uphill. But it's not that. It's definitely an electrical issue.

It only does it when there is a larger electrical draw on the system, such as if I have the blower fan set to level-3 or 4, and have the headlights on high beam. If I dim the headlights to only low beam, or turn the blower fan down to level-2 or 1, then boom, normal full engine power is instantly restored. If I only ever run on low beams and the blower fan set to low speed, there's never any problem. But once this stumbling condition has started, if I switch to high beams or turn the fan to a higher speed, right away the engine loses power as though there isn't enough voltage to create a good spark, then it struggles to climb uphill.

There otherwise seems to be plenty enough electrical power as the lights aren't noticeably going dim, and the starter motor always cranks just fine starting the engine right away.

What's up with that? Any suggestions?

I think you have correctly deduced that there is a voltage drop affecting the engine. Another component that may be involved is the power to the fuel pump. One way to confirm this might be to put a volt meter on the cigarette lighter to see that the system voltage varies as you suspect while you vary the loads and driving.
The first thing to check is the alternator belt tension. It may be slipping and not driving it at full power.
There could be a problem with the alternator not being up to full power due to worn brushes or damaged diode rectifier pack. The easy verification is to watch the voltage when loads are applied. If this problem persists, it could damage the battery as well. I have had this happen myself. Then the bad battery causes the new alternator to be overloaded and it fails again. Then the new battery is damaged by the bad alternator. This can go on until both are changed at the same time.

So once you have verified that the belt is tight, the alternator is not keeping up with the loads, change the alternator and check the battery capacity to see that it is not already damaged. Some auto parts stores may be able to do this for you for free. The alternator test the store will do however may not correctly diagnose the bad rectifier pack as they cannot load the alternator sufficiently to find that problem. They simply measure the no load voltage while spinning it with a low power drive.

Bob in Lake Havasu

Thanks Bob! That sounds like a very reasonable explanation. I think you're on to something there.

It's definitely an electrical issue causing low voltage. When I turn the headlights on, or switch the blower fan to high speed, the needle on the engine temperature gauge jumps correspondingly higher. Fan off, and lights off, the temp gauge needle swings back down to normal.

I hope to get to it this weekend. Will let you know what I find.

I just had a similar experience. My car started running like it was about to die when I had heavy electrical loads, but ran fine when the loads were light (headlights on vs. headlights off). This was an intermittent problem. Sometimes I could make the problem go away by revving the engine up a little. The dash lights would also flicker a little bit when the engine was sputtering. It all looked like an alternator problem to me. I found that the alternator was only putting out 12 volts when the problem was present and a little over 13 volts when it was not. The alternator should be putting out nearly 14 volts. I had a local alternator shop rebuild mine, and since then all is well. They told me my brushes were all but gone.

John

The alternator belt is tight. Definitely not the problem.

Then I pulled the voltage regulator and carbon brushes pack from the back of the alternator. I'm disappointed to report the brushes, as well as the slip rings inside the alternator, all look fine and still have lots of life left in them. I was so sure it was going to be the brushes...

But then a closer look reveals a bit of corrosion built up on the blades of the electrical contact which the voltage regulator presses against when mounted.
I cleaned the contacts with a bit of fine sandpaper.

I haven't had a chance to test it out yet to see if that made any difference. Christmas stuff is getting in the way. It'll have to wait a few days now.

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Ferdinand
The alternator belt is tight. Definitely not the problem.

Then I pulled the voltage regulator and carbon brushes pack from the back of the alternator. I'm disappointed to report the brushes, as well as the slip rings inside the alternator, all look fine and still have lots of life left in them. I was so sure it was going to be the brushes...
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But then a closer look reveals a bit of corrosion built up on the blades of the electrical contact which the voltage regulator presses against when mounted.
I cleaned the contacts with a bit of fine sandpaper.

I haven't had a chance to test it out yet to see if that made any difference. Christmas stuff is getting in the way. It'll have to wait a few days now.

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Well I am curious to know if this is the problem. I do think that you have found the answer. Cuprous and/or cupric oxide are not very good conductors and this might be the cause of your problem.

Salut, Bob P.



Edited 1 time(s). Last edit at 12/29/2017 08:57AM by Bob P 325is 88.

I did a quick little check today. Haven't taken it out for a run yet though, still entertaining Christmas guests.

The car has been sitting for over a week. Before starting it up, the battery voltage was 11.4v. It cranked fine and started right up.

At idle the voltage was 13.4v. I backed it out of the garage and revved it up a bit and the voltage climbed steadily 13.5, 13.6...

I put it back in the garage and switched it off, and the voltage at rest is now 12.5v.

I didn't have a chance yet to take it for a drive, or test it under full electrical load with lights and blower fan and rear window defroster on. But it does seem like the alternator is doing the right stuff at the moment.

Fingers crossed. It might be okay now.

Great to have a problem solved...and stay solved.
I had to change an alternator last week as the bearings are making noise. I had a spare from a part out. It is working fine. The challenge now is to figure out how to change that bearing.

Bob in Lake Havasu

Well, dang. It's not fixed.

I had to do a quick run to the local grocery store and thought I'd take the BMW for a run to see if it's cured.

Nope. I didn't even make it out of the driveway. The moment I hit the high beams, the engine almost died.

It's super cold here right now, -21°C, with windchill it feels like -34°C. Definitely not a good time to be going for a test drive and potentially getting stranded.

I had to take my wife's Santa Fe instead. I could cry.

There is a rectifier diode set inside the alternator and one of them could be the culprit. I think those are soldered in place though. Not sure it is a replaceable item at home. The result of it is that it will indicate full voltage under low load because only half of the rectifier is working. The volt meter averages the pulsating DC voltage to give a reading. Under load, the voltage will drop and not give full current output.
Looks like you may be in for a rebuild. One other possibility is that transistor on the brush set that regulates the voltage might be faulty and the brush pack could be changed but it would be risk of guessing wrong on a part that would not be returnable. Too bad you do not have a working one to swap parts out of to verify.

Bob in Lake Havasu

I put in a new (refurbished) alternator today. Went for a test spin and it seems like that fixed it.

It's back to its usual snappy and peppy performance with no noticeable difference when high beams and blower fan are on full. Yippee!

Glad you are back on the road. It is a good feeling to have solved a problem and implemented the solution successfully.

Bob in Lake Havasu

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Ferdinand
I put in a new (refurbished) alternator today. Went for a test spin and it seems like that fixed it.

It's back to its usual snappy and peppy performance with no noticeable difference when high beams and blower fan are on full. Yippee!

That's great - I guess the dirty corrosion diagnostic was not the problem. Sometimes one just has to bite the bullet and change the whole part. Now we can hope that the battery wasn't too affected. Salut, Bob P.



Edited 2 time(s). Last edit at 01/20/2018 06:08AM by Bob P 325is 88.

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Bob P 325is 88
I guess the dirty corrosion diagnostic was not the problem.

I have a slightly embarrassing confession to make. Cleaning the corrosion may have actually fixed the problem.

Previously the issue only ever cropped up when there was a large electrical load on the system, high beams, blower fan on high, when I put my foot down on the throttle the engine would stumble. Blower turned down, low beams, there was never a problem. So it seemed like there wasn't enough voltage to give a good spark to the engine under those conditions.

Then I cleaned those corroded contacts in the alternator behind the voltage regulator module and that seemed to fix the problem when the car was idling. The alternator appeared to be charging fine now. But I didn't have a chance to take it out for a good test run that day.

Then the other day I thought I'd take the BMW for a quick run to the grocery store, but...

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Ferdinand
I didn't even make it out of the driveway. The moment I hit the high beams, the engine almost died.

It was much worse than before. Previously the engine just felt flat, blah, wouldn't pull like normal. But this time it just about quit altogether. It would idle, just barely, but it wouldn't accept any throttle at all. Damn.

So I went ahead and ordered a new alternator. I could have tried just a new voltage regulator and carbon brush assembly, but if that didn't fix it I'd have to buy a new alternator anyway which comes with yet another new voltage regulator. I did that with my previous E30. The brushes wore down to the point where they weren't making contact anymore. I replaced the brush assembly. Less than a year later the new brushes wore through what was left of the commutator rings inside the alternator, so I had to buy a new alternator anyway. I wasn't going to do that again.

My new alternator arrived and I popped the hood on the car to begin the installation. When I had previously removed the voltage regulator, cleaned the contacts, and put it back, I first had to take out the air flow sensor box to make room. Well, it turns out...

I forgot to plug back in the electrical connector to the air flow sensor! D'oh!

Well that sure explains why it would idle okay, but wouldn't run at any speed above idle. I'm so stupid. So the alternator corrosion fix actually may already have been successful.

In any case, it does run perfectly again now with a new alternator.



Edited 1 time(s). Last edit at 01/21/2018 07:40AM by Ferdinand.

So I guess you aren't going to try the old alternator to prove the dirty/corroded theory? Its the engineer in me - I gotta know!

Salut, Bob p.

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Ferdinand

My new alternator arrived and I popped the hood on the car to begin the installation. When I had previously removed the voltage regulator, cleaned the contacts, and put it back, I first had to take out the air flow sensor box to make room. Well, it turns out...

I forgot to plug back in the electrical connector to the air flow sensor! D'oh!

What?! You mean I'm not the only who does this kind of thing?

John

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Bob P 325is 88
So I guess you aren't going to try the old alternator to prove the dirty/corroded theory? Its the engineer in me - I gotta know!

It's bugging me too. But I'm almost certain the corroded contacts were the cause of the original problem. The old alternator definitely was charging subsequent to cleaning the contacts.

But also stupid of me, I never actually checked the voltages beforehand to compare before/after cleaning the corroded contacts. I was so convinced I'd find the carbon brushes worn and needing to be replaced that I didn't bother to measure the system voltage beforehand. So I can't say with total certainty that cleaning the contacts made any difference other than I know the alternator definitely was charging at idle afterwards.

I wasn't going to reinstall the old one in any case, as it was a bit of pain swapping the pulley from the old to the new alternator. And it's too late now as I already sent the old alternator back for a core refund.

And why the heck does the power steering belt need to sit in front of the alternator/water pump belt?!? That's an inconvenient design. To add a new alternator belt, first the power steering belt has to come off too.

I too have had similar faux pas but not so much that I have not saved a lot of money by doing my own work. Have had "professionals" do the same once in a while too.

Bob in Lake Havasu

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Bob in Everett
Have had "professionals" do the same once in a while too.

Here's a classic example.

As much as possible, I try to do all my BMW repairs on my own. But my wife's Hyundai Santa Fe gets regular servicing at the dealer. It's expensive, but I prefer to let the "professionals" look after that one.

Last summer it needed some brake work done. A couple of weeks after that my wife reported hearing a new rattling noise from the right rear corner of the car. Sure enough there's a new rattle, and it goes away whenever the brakes are applied. Hmmmm, I better take a look at that.

This is what I found.

[www.youtube.com]

Ah yup, that's not good.

Yikes! That kind of thing is exactly why I do almost all my own work, even on our new Mazda. At least if I leave bolts out I know who to blame. :-)

__________
Dave
'91 325iX

Update.

Well, it wasn't the alternator. It's still doing the same thing.

The instrument panel temp gauge jumps up a step whenever any additional electrical load is switched on. If everything is turned on, high beams, blower fan on full, rear defrost on, sometimes the temp gauge needle even goes all the way over full hot into the red!

When that happens the engine starts to lose power. All I need to do is switch anything off, like turn the headlights back down to low beam and, presto, the engine clears right up.

It's a bad ground issue.

I removed the main ground cable below the alternator that joins the engine block to the left frame rail and cleaned the mounting surfaces. They didn't look all that bad.

However! Giving the ground cable a little twist it practically crumbled in my hands!! What was left of the insulation around the cable all fell off. The ground cable is totally corroded. Bits of copper wire fell off and the remaining strands are all green.

A brand new ground cable has now been installed and, fingers crossed, that seems at last to have cured everything! Yippee!

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Ferdinand
Update.

Well, it wasn't the alternator. It's still doing the same thing.

The instrument panel temp gauge jumps up a step whenever any additional electrical load is switched on. If everything is turned on, high beams, blower fan on full, rear defrost on, sometimes the temp gauge needle even goes all the way over full hot into the red!

When that happens the engine starts to lose power. All I need to do is switch anything off, like turn the headlights back down to low beam and, presto, the engine clears right up.

It's a bad ground issue.

I removed the main ground cable below the alternator that joins the engine block to the left frame rail and cleaned the mounting surfaces. They didn't look all that bad.

However! Giving the ground cable a little twist it practically crumbled in my hands!! What was left of the insulation around the cable all fell off. The ground cable is totally corroded. Bits of copper wire fell off and the remaining strands are all green.

A brand new ground cable has now been installed and, fingers crossed, that seems at last to have cured everything! Yippee!

So it wasn't the alternator, but was corrosive contacts (grounds) of some sort!

I love it when a "mystery" is solved.

Salut, Bob P.