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Discussion Starter #1
Hi all, I need some advice on this weird issue. I put in a new alternator a few years ago, and the problem never really went away. I get just over 14V when I start the car, but it drops to 13.2- 13.6 once the engine has been running a while, and the battery will loose charge if I run the lights or heater motor. It's a higher output Bosch that I bought from Ebay.

There is a voltage drop from the junction box under the hood to the battery terminals of .2-.4 volts, new positive battery cable and new thicker ground strap. I don't know if that's normal, or not.

I'm going to take a closer look today at how much the voltage changes over time, but I'm convinced it's due to temperatures increasing. Has anyone seen this? The original alternator was doing this also. New voltage regulator didn't help, IIRC. I have a few of them that I had used for testing purposes.
 

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I would suspect the voltage regulator, particularly if the alternator will deliver 14 volts with the lights and heater fan on when it is cold.
 

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But Mad North-Northwest
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I've noticed the same effect on my S4, but not to the point of the battery draining.

My car didn't have the rear alternator cover (the one with the air tube facing forwards) so I added one to try to help improve this a while back, but I don't really recall if it made a difference or not.
 

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I would suspect the voltage regulator, particularly if the alternator will deliver 14 volts with the lights and heater fan on when it is cold.
Yup.

I assume that John533i's "higher output Bosch" alternator has an internal regulator. If so, the regulator/brush assembly can be changed easily and inexpensively.

John533i: Although a "higher output" alternator has the capacity to delivery more amps of current if the car requires it, it still only delivers 13.5 - 14 volts when working properly and can fail in the way you describe (e.g., low voltage).
 

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But Mad North-Northwest
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I can't find a pic of exactly what I installed, but it sort of looks like this. It covers the rear of the alternator and VR and has a forward facing tube. I assume it acts as a heat shield and also ducts cool air to the alternator.

I believe the S3 cars came with these stock but the S4 did not. I added one as an "it can't hurt" thing. No idea how much it helps, but I wasn't having problems as bad as you are.
 

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Discussion Starter #6 (Edited)
Yup.

I assume that John533i's "higher output Bosch" alternator has an internal regulator. If so, the regulator/brush assembly can be changed easily and inexpensively.

John533i: Although a "higher output" alternator has the capacity to delivery more amps of current if the car requires it, it still only delivers 13.5 - 14 volts when working properly and can fail in the way you describe (e.g., low voltage).
Thank you! I'm in great company here. Knowing that I was installing the halogen headlight kit that IAP used to sell and an electric fan, I opted for the cheaper, higher output rebuilt Bosch unit from Ebay. I was chasing this same issue back then, which was around 2015 IIRC. I would turn on the AC fan just to move air, and the volts gauge would dive to the left. I almost had the issue licked for a while- I installed a new ground strap and found the old one to be loose anyway. Then an engine rebuild occured. Also, I finally installed an electric fan...

If I don't charge my battery in the winter months regularly, it will drain to the point of no-start and I need a jump. The ECU needs that 10.5V or whatever...stranded a few times at work...

So I did some testing yesterday, and found something that I hadn't taken into account. When hot, at some point the electric fan kicks on, which together with the heat-related voltage drop, I see really low charging voltage. Also, my headlights are now dropping the Volts pretty hard. It seems to me that the lights didn't used to draw things down that much. I know that it didn't. Below are some results.

Alternator Behavior

Voltage Cold 13.9 BATT, 14.0V Junction Box (under hood, left fender) No lights or accessories, just started and warming up.
-with lights on at BATT: 13.3V, 13.4 box
-no lights, HTR fan BATT: 13.8V , 13.9 at box

Voltage HOT
-Nothing on: 13.8 BATT, 14.0 Junction box
-with lights: 13.5V, no reading at box, cooling fan kicks on
-with cooling fan: 13.42w/eng fan at batt, 13.53V box
SO, with eng. fan, htr fan and lights on...
12.95V Batt, 13.3V at junction box

Do you guys think a grounding issue has come back? Should I add a ground? I'll check the battery ground first, I haven't looked at it since I replaced the original. Grounding lug was good and rust free. How about a second ground!

As far as the regulator goes, I've been through bad regulators before on my BMW's with Bosch alternators, and they had other failure modes, like too much voltage and blowing out my headlight, or just constantly putting out 13.1, .4 volts with nothing on, or lights dimming at idle...


Enough of my jibber jabber. Thanks for all of your input. I really do like driving this thing all year around when I can...

John
 

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The short wire from the back of the alternator to the junction block on the fender was the culprit on my similar condition. As the engine compartment heated up I would drop up to 1 volt across that cable or it’s connectors. I replaced it with a larger diameter wire as I had it already. That fixed that issue. The alternator output would still drop a little as it heated up but not enough to be a serious problem.
 

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Have you done a proper battery test? Sounds more to me like the battery is not holding charge.
 

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Discussion Starter #10
Have you done a proper battery test? Sounds more to me like the battery is not holding charge.
Battery is fine. <2yr old Interstate. The car itself doesn't have a drain, if that's what you are thinking here. I can leave it with out a tender for weeks, and it holds it's charge.
 

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Battery is fine. <2yr old Interstate. The car itself doesn't have a drain, if that's what you are thinking here. I can leave it with out a tender for weeks, and it holds it's charge.
But you said:

"If I don't charge my battery in the winter months regularly, it will drain to the point of no-start and I need a jump. The ECU needs that 10.5V or whatever...stranded a few times at work..."
 
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