Alternator Excite wire pulls voltage down

Thread Starter

250ptm

Joined Jul 29, 2010
34
Working on my car today turned ign off and the car ran on for about 10 seconds before shutting off. Noticed that when ignition is on, the battery voltage dropped by 0.6 volts. Removed Battery and had it checked by a mechanic who pronounced it good. Cranking amps are at 95% of the original spec of 540 and batt voltage was 12.6 at rest.
By removing fuses I have traced it to the alternator excite feed wire from the ignition switch. I removed the ALt excite wire from the alternator and checked the wire itself, it has 0 ohms resistance and all connectors are free from rust. With the wire removed from the Alt, the batt drop at the gauge is less then 0.2v.

So it appears that when the alt excite wire is connected to the alternator it pulls the battery voltage down by 0.4 v with ignition on. Is this normal or is my alt on its way out ?
It stills charges at 14.7v, and I don't know why the engine ran on after selecting the ign off.
 

BR-549

Joined Sep 22, 2013
4,936
The voltage is normal. The 10 sec. shutdown is not. Is it in knocker mode?
 

bwilliams60

Joined Nov 18, 2012
1,177
I am not sure which wire you are calling the exciter wire but I am going to assume it is the lamp wire. When the key is turned on, the lamp in the instrument cluster is going to cause a draw as it is grounded inside the voltage regulator. When you disconnect that wire from the alternator, you break that circuit so there is no longer any draw. Current draw should be 30-50mA (0.03-0.05A) in a normal vehicle. I would use an inductive amp clamp at the battery and see what the draw is after it goes to sleep. If it falls within spec, alternator is good. I suspect it is okay.
As for the 10 second run on, I am not sure what knocker mode is but I would say that it may be overfueling somehow and I would suspect there should be a code being set somewhere. A good tech should be able to pinpoint the problem fairly quickly.
 

shortbus

Joined Sep 30, 2009
6,976
Did you change your instrument panel charge light from an incandescent to led light?
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
The older model auto's that ran on was a sign of 'Dieseling', something heats up in the cyclinders and the motor will run on for 10secs or so after the key is off, shouldn't happen with modern fuel injection.
Knocker Mode?
Max.
 

shortbus

Joined Sep 30, 2009
6,976
The older model auto's that ran on was a sign of 'Dieseling', something heats up in the cyclinders and the motor will run on for 10secs or so after the key is off, shouldn't happen with modern fuel injection.
Knocker Mode?
Max.
Oh the good old days! Around the middle 1970s.

I thought Knocker mode was a convertible full of young girls? :p:eek::oops::rolleyes:
 

bwilliams60

Joined Nov 18, 2012
1,177
Knocker mode and 10 seconds.....hmmmmmm?
Anyways, dieseling is usually caused by carbon deposits burning unburnt fuel after the ignition has been turned off. As stated, it generally doesnt happen on todays cars. Did it happen only once or several times? If the latter then it should be easy to diagnose but I dont think your alternator is a problem.
 
Last edited:

narkeleptk

Joined Mar 11, 2019
370
Year and model is one of the most helpful pieces of information you can attach to any question relating to automobiles. Many times the problems may be known problems for some specific year/model car and easier to diagnose.
 

Thread Starter

250ptm

Joined Jul 29, 2010
34
Thanks for all the responses, particularly to ShortBus.
BWilliam60 : Current draw is ;ess than 30 mA with Ign Off. (car asleep)

Narkeleptk: what you say is true, problem is mine is a hot rod with custom wiring harness etc etc.

So the comment from "shortbus" got my interest. I removed the ignition light and found that although it is not a true LED it has a very very low current draw of 0.062 amps. According to sources on the internet, the ign light needs to be approx 2 watts, so it should normally draw almost 3 times more than mine. 2/12=0.166.

From what I read this is to prevent running on when the engine is shut off as the ALT excite wire can feed back through the ign switch to the coil and distributer. So with that in mind I changed the ign light to a true incandescent which pulls the correct amps. Will try to test it tomorrow.
 
Last edited:

bwilliams60

Joined Nov 18, 2012
1,177
Sounds like you are on the right track. Run on in older cars was a common problem. Make sure the idle and mixture screws are set properly and that should take care of your problem. High idle will make it run on. Set your mixture using a vacuum gauge and make sure the float level is correct.
 

shortbus

Joined Sep 30, 2009
6,976
I removed the ignition light and found that although it is not a true LED it has a very very low current draw of 0.062 amps.
The reason I asked was from something that I did when younger and building my first hot rod. I didn't use a charge light and had a similar problem. But then again this was back in the day when we still had separate regulators, but thought it may be a problem with an integral regulator too.
 

Berzerker

Joined Jul 29, 2018
542
Don't think that an alternator will cause your car to knock/continue to run or "Dieseling" is the term. This is usually cause by problems such as Fuel injection problems, Ignition timing, Carburetor/throttle body problems and can be as simple as the wrong Spark plug being used or carbon build up on them. Fixing a car over the internet is like programming over the phone "You never know if the person on the other end has done what you said or just made it worse". Have it put on a diagnostic machine it's better than guessing.
Brzrkr
 

Thread Starter

250ptm

Joined Jul 29, 2010
34
Ok, ran the car today, starting and stopping over 20 km of hard driving. Engine stops immediately the key is set to Off, it did not run on. Only time will tell if this fix worked permanently. Thanks for all the advice!
 
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