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Blue Streak
 
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Charging System Question

Ever since acquiring my 914 two years ago, I've always wondered about a small quirk with the idle speed.

Battery charges good while running and I get plenty of juice but when I turn on the running lights or go to full headlights lights, my engine idle speed changes by up to 75 to 100 rpm's slower.

Is this normal? I know AC systems can pull a significant load on the alternator but I never really thought that a lighting system would?

Thanks for any replies.

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1971 Porsche 914 1.7L with 73 Engine
Converted to MegaSquirt 2 PCB3.57 with 2.0L intake
Old 05-14-2012, 12:39 PM
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I don't think it's normal. Check your voltage regulator or ground points
Old 05-14-2012, 12:48 PM
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It's about as normal as your voltage gauge dropping back and forth as your turn signal blinks.
Old 05-14-2012, 01:42 PM
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I think maybe it's time to get new parts. Alternator and regulator should fix the problem as well as checking the ground points.
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Old 05-14-2012, 02:44 PM
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I don't think alternator and regulator will have any effect. Cleaning up the ground points and such might.

I won't say that they're "supposed" to do that, but it is relatively common. In some cases tweaks to the idle mixture have helped; see Brad Anders' D-jet website for one example.

--DD
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Old 05-14-2012, 02:52 PM
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Quote:
Originally Posted by Dave at Pelican Parts View Post
I don't think alternator and regulator will have any effect. Cleaning up the ground points and such might.

I won't say that they're "supposed" to do that, but it is relatively common. In some cases tweaks to the idle mixture have helped; see Brad Anders' D-jet website for one example.

--DD
I think Alternator might. My car is completely stock with the exception of the MS2 ECU, injectors and the Crane ignition. Everything is 40+ years old. I don't know the estimated life span on alternators but you would think that a alternator that was new in 1971 was only manufactured with maybe 10 years intended lifespan.
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Old 05-15-2012, 08:40 AM
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I would say this is quite normal and in fact it proves that the charging system works; it increases the load on the engine as the electrical load increases.
I dont se why an old alternator would add more load than a new one except for the new one possibly having a few % better efficiency.
As long as you do not have any charging problems this should be ok.


/Lars S
Old 05-15-2012, 11:43 AM
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Quote:
Originally Posted by lars s View Post
I would say this is quite normal and in fact it proves that the charging system works; it increases the load on the engine as the electrical load increases.
I dont se why an old alternator would add more load than a new one except for the new one possibly having a few % better efficiency.
As long as you do not have any charging problems this should be ok.


/Lars S
I agree that this displays that the Regulator is doing its job but what if say the Alternator is no longer putting out 700W@50 Amps, maybe about 10% less or that to put out 700W@50 Amps would require to run the engine at 1K rpm idle speed rather than 850?

Because I actually can idle at 1K and turn on the lights and the engine only backs down maybe 25-50 rpms. but if I set my idle speed to the Factory 850, once I turn on the lights it backs down about 100 rpms. Hmm, maybe I'll check my belt first before buying an alternator.
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Old 05-15-2012, 12:34 PM
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Alternators either work or don't work.
Resistance in the circuit ( old wires, bad connections, weak battery) will make the difference.

You need to hunt down EVERY SINGLE brown wire connection on the car and clean it.
Old 05-15-2012, 12:40 PM
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[QUOTE=falconfp2001;6749329]
Because I actually can idle at 1K and turn on the lights and the engine only backs down maybe 25-50 rpms. but if I set my idle speed to the Factory 850, once I turn on the lights it backs down about 100 rpms.QUOTE]

This is because at higer engine rpm the engine is less sensitive to the change in load, at 1K it can better match the alternator load.

/Lars S
Old 05-15-2012, 10:52 PM
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My guess is it is related to the Megasquirt itself. What are you AFRs doing when the idle speed drops?

Turning the lights on is probably dropping the overall system voltage and affecting your pulsewidth. If you see the AFRs change under electrical load use the battery voltage correction setting to adjust for it. Its another one of those trial and error settings so you just have to play with it for a bit.

Old 06-10-2012, 12:08 PM
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