Electrical problem?

MARTINI-22

MEMBER
1991 NISSAN STANZA
145,000 MILES

I currently drive an automatic 1991 Nissan Stanza XE with a KA24E inline 4 cylinder engine. I've always noticed that when I am at a stop the car sometimes surges when it idles, but sometimes it is perfectly fine. When I use any electrical component, like the AC for example, the car idle lowers to about 500 RPMs at a stop. Another thing I have noticed is when I park and go to roll the power windows up that if I hold onto the switch after the window is all the way up the idle surges too. I find this car to be really weird because when I'm driving it runs perfect. Since we got the car we have replaced the Idle Air Control, Throttle Position Sensor, cleaned the EGR valve and the Mass Air Flow Sensor, but still there seems to be an issue when I use anything that needs electricity. I also tested the alternator. I turned the car on and unplugged the battery, the car kept running so I don't see it as the issue. I think a wiring harness might be starting to go, but I don't know if that's the problem or if it could be something else.

5 Replies

CARADIODOC

MECHANIC
33,916 POSTS

NEVER disconnect the battery with the engine running. That's a real easy way to do a lot of expensive damage. That was a test done many years ago by mechanics who didn't understand how these simple systems work. All AC generators develop "ripple" voltage. The battery smooths that out which eliminates the problem it can cause. If you want to tell if the generator is working, use a digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts.

If the generator has one defective diode out of the six, you will lose exactly two thirds of its output capacity. Ripple will also increase greatly. That can cause the symptoms you described, especially at idle where maximum output is going to be even lower. I'd start with a load test on the generator. Most professional testers display ripple as a bar graph, not a number. If ripple is high and maximum output is around 20-30 amps, suspect the generator.

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Sunday, April 22nd, 2012 AT 1:46 AM

KHLOW2008

MECHANIC
41,814 POSTS

If the engine is not idling at optimum efficiency, the problem can occur. It is not necessarily electrically related though increased draining of electrical power can cause it.

Improperly adjusted ignition timing and valve clearances are possible causes,

Did you clean the throttle body while rlacing the TPS and IAC?

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Sunday, April 22nd, 2012 AT 5:12 AM

MARTINI-22

MEMBER
3 POSTS

I will definitively check the voltage of the generator. We did clean the throttle body and the IAC as well. The only other thing I could think of is either a faulty MAF sensor or EGR vavle. The problem only occurs when I'm using stuff that requires a lot of power though

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Sunday, April 22nd, 2012 AT 5:51 AM

MARTINI-22

MEMBER
3 POSTS

Ok did the battery and load test. Battery was at 12.7 when the car was on it was at 13.7 I turned on the lights and the car rippled but came back up. I turned on the brights and the radio and it came back up, but when I turned on the AC it went down to 12.5 and didnt come back up. The Idle was low too. We tested the alternator itself and it was perfectly fine

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Monday, April 23rd, 2012 AT 2:59 AM

CARADIODOC

MECHANIC
33,916 POSTS

I think there might be some confusion. The generator is not perfectly fine if the voltage dropped to 12.5 volts. Unless the belt is loose, that sure sounds like there's a bad diode in it.

The load test you need to do is with a professional load testing machine. It will draw the maximum current your generator can deliver while maintaining at least 13.75 volts. By turning on the lights and AC, you were doing a similar test but the generator should have easily kept up with that minimal demand.

Your generator is rated at 80 amps. If one diode is defective, you will lose exactly two thirds of its capacity so about 25 amps is all you'll get. Head lights, tail lights, and the heater fan will draw around 20 amps leaving not much left over. Once that capacity is exceeded, voltage will start to drop.

One other thing to consider is generators are less efficient as they slow down so load tests are always performed with the engine at 2000 rpm. If you made your observations at idle, you may not have a generator problem although the voltage still shouldn't have dropped that low.

It also sounds like you might not be understanding exactly what "ripple" is. It has nothing to do with engine idle speed as your comment suggests. It is the variation in voltage developed by the generator. For a better explanation if ripple, look at the bottom of this page under "Excessive Ripple":

http://randysrepairshop.net/charging-systems.html