Alternator... with Alternating Blame

December 09, 2007, 05:19:28 PM

Question... in our Fox cars, specifically, 1988 Cougar XR7 5.0 SEFI and the 1988 T-Bird 3.8 EFI ("Salt"), when does the alternator engage?

I will be taking the alternator out of Salt in to a Canadian Tire store tomorrow for testing, and am curious if there is a difference in the answer to my question when you change from Cougar X to Salt.

I ask this question with some degree of caution... I know the pully turns when the engine is running.

My question is, do you need to be driving down the road at XX mph for the alternator to charge the battery or would revving the block at idle be sufficient to engage the alternator or is the alternator supposed to charge the battery even at idle? Is there a varying degree of effectiveness between all three of those choices?

My reason for asking this question is this observation. The engine ran for the first time in months (on Salt, after a headgasket job) perfectly for 30mins until I shut her off. Before I manged to get her running, I relied on a new battery with a charger and boosting. After she fired, I did not need the charger or a booster to get her re-started. However, now it is the case that she will start but only run for the "cold-choke" cycle, then stall. I can keep her going with my foot on the gas; I did this for about 10 mins, then I let her stall. Now, I can't get her started without charging up the battery; it's almost as if the alternator just 'decided' not to work. The other clue is a frayed serpentine belt (which will be replaced tomorrow). I have posed this question within the "Engine Technical Forum", seeing as how I did just do a headgasket job, it could still be an internal engine problem... I wanted anybody with any electrical abilites to brainstorm over the alternator with me.

Hopefully your suggestions will help me determine if the alternator is behaving as my 'less-than-subtle' culpret.

Alternator... with Alternating Blame

Reply #1 – December 09, 2007, 09:22:54 PM

The alternator is "excited" or turned on when the ignition is in run.
Put your volt meter on the battery terminals. You should see the battery base voltage of 12.5 volts with the engine not running.
With the engine idling you should see 14-14.5 volts if the alternator is doing it's job.

There is a type of alternator call a "one wire" or "self excited" that does require a higher RPM before it will turn on.
This is not what you have.

Alternator... with Alternating Blame

Reply #2 – December 09, 2007, 09:25:41 PM

Thank-you; I will perform that test tomorrow.

Alternator... with Alternating Blame

Reply #3 – December 10, 2007, 07:02:08 PM

As it turns out, the alternator was not doing it's job. My test with the voltimeter indicated very little change above the base battery voltage.

I took the alternator into Canadian Tire today, they concluded the same thing; the alternator is not charging the battery.

Could the alternator in this state be responsible for my frayed serpentine belt?