voltage drops at idle
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
voltage drops at idle
On my new-to-me 1991 in my sig, I noticed an odd behavior. When it idles down to 750 rpm or so, the voltage drops noticably. The interior lights get dimmer. If I reve the engine a little, it brightens back up. The battery gauge moves correspondingly. It jumps from about the 'o' to the 'a' in 'normal'. I don't have actual voltages (yet - later today).
Is this a battery, alternator, or ground problem? Batt & alt are relatively new.
Is this a battery, alternator, or ground problem? Batt & alt are relatively new.
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Joined: Jan 2001
Posts: 57,025
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From: Virginia
It could be normal. Also you can check your idle system by unplugging the idle air control valve with the truck idling. When you do this, the engine should slowly lose rpm and die. If it keeps running, the idle stop is set too high. If it dies quickly, then the idle stop screw is set too low. If you find the idle adjustment is not right, I would take the throttle body apart and clean it and the idle air control valve, and then put it all back together and do the test again, and then make any adjustments.
The throttle body over time gets a sticky carbon build-up on it. There is supposed to be a certain amount of air going by the throttle blade at idle, and the rest of the idle air goes through the idle air control valve and around the the throttle blade. You have to have enough air going to the control valve for the computer to have good control over the idle speed, and it also has to be free and clean so it can move back and forth.
The throttle body over time gets a sticky carbon build-up on it. There is supposed to be a certain amount of air going by the throttle blade at idle, and the rest of the idle air goes through the idle air control valve and around the the throttle blade. You have to have enough air going to the control valve for the computer to have good control over the idle speed, and it also has to be free and clean so it can move back and forth.
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Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
OK, an update. This only happens with the headlights on. At idle, with the lights on, the voltage at the battery terminals is about 12.8. Goose it a little to 1400 or so, and it bumps up to 14.5.
I don't think this is normal, as my 89 doesn't have this behavior, and neither did my 1990 F-150. But I will clean out the throttle body as you suggest, Dave. That's probably a good idea at 145,000 miles.
I doubt it's ever been done.
I don't think this is normal, as my 89 doesn't have this behavior, and neither did my 1990 F-150. But I will clean out the throttle body as you suggest, Dave. That's probably a good idea at 145,000 miles.
I doubt it's ever been done.
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Joined: Jan 2001
Posts: 57,025
Likes: 2,758
From: Virginia
You can keep checking connections and the alternator belt condition, but don't lose too much sleep over it. With those readings, you should be ok, the battery is not being discharged. It will probably drop more if you turned the wipers and the defrost on, but on average, you will be charging more that you are discharging.
There is a slight chance that one of the diodes is open inside the alternator, reducing it's output a little bit. But it's a long shot, and you said it was pretty new.
Make sure you check the connections over at the starter relay too. Go ahead and take them off and clean them up. There is a smaller wire over there that feeds power to the whole truck.
There is a slight chance that one of the diodes is open inside the alternator, reducing it's output a little bit. But it's a long shot, and you said it was pretty new.
Make sure you check the connections over at the starter relay too. Go ahead and take them off and clean them up. There is a smaller wire over there that feeds power to the whole truck.
