[kcdoyle]

Morning Board-

Still too tired from celebrating Saint Patrick's week to do a meaningful search. Besides this is a pretty simple question:

If one's battery (read bat tree for you United Kingdom types) is old and/or ready to be replaced, will this affect the idle? I noticed the other day as I was backing into a slot at an underground garage (gar eg) and, at idle, noticed the reverse lights were getting brighter then dimmer with each "lope" of the engine. Any ideas?

Thanks for the time!! :15:

KCD

 

[greg]

Usually when the engine lopes at idle it is a sign that something else is wrong - like camshaft position sensor. But that should throw a Service Engine Soon light. It COULD be a weak battery. Here's why:

The idle speed is computer controlled, but too variable a load can outpace its ability to maintain a steady speed. The fact that the lights get dimmer and brighter with engine speed means that the idle speed is too low to supply all the current you're drawing, and a weak battery would contribute to that load. The voltage regulator sees the voltage sag, and increases the "field" current to the alternator so it will charge more. This places a load on the engine, slowing it down. The computer responds speeding it up. As soon as the voltage comes up, the regulator backs off, the alternator works less and the engine speed climbs due to lower load. Under the right conditions this could easily "cycle" or lope.

My experience is that the alternator makes enough current at idle, normally. But if engine idle speed is getting dragged below normal for some other reason, the alternator won't make as much charge and voltage will drop. So this all could just be a symptom of something else.

Battery condition is easy to check, though. Simple test is done with a "hygrometer" (I think that's what it is called) which measures the specific gravity of the battery acid. A better test places a stout load on the battery while measuring its ability to stand up to that load. Most auto shops have such a tester.

 

[kcdoyle]

Greg- You're awesome. Thanks a million.

Usually when the engine lopes at idle it is a sign that something else is wrong - like camshaft position sensor. But that should throw a Service Engine Soon light. It COULD be a weak battery. Here's why:

The idle speed is computer controlled, but too variable a load can outpace its ability to maintain a steady speed. The fact that the lights get dimmer and brighter with engine speed means that the idle speed is too low to supply all the current you're drawing, and a weak battery would contribute to that load. The voltage regulator sees the voltage sag, and increases the "field" current to the alternator so it will charge more. This places a load on the engine, slowing it down. The computer responds speeding it up. As soon as the voltage comes up, the regulator backs off, the alternator works less and the engine speed climbs due to lower load. Under the right conditions this could easily "cycle" or lope.

My experience is that the alternator makes enough current at idle, normally. But if engine idle speed is getting dragged below normal for some other reason, the alternator won't make as much charge and voltage will drop. So this all could just be a symptom of something else.

Battery condition is easy to check, though. Simple test is done with a "hygrometer" (I think that's what it is called) which measures the specific gravity of the battery acid. A better test places a stout load on the battery while measuring its ability to stand up to that load. Most auto shops have such a tester.