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Old 09-12-2010, 11:21 PM   #1
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Exclamation Quick electrical question: battery/alternator/tach stuff

Summary of events:

Drove the car to work the other day. Started fine that morning, maybe a tad sluggish but I hasn't been driving it regularly so I didn't think much of it. When I left work, it was noticeably slow to start, but does. As I'm driving home, I noticed the A/C isn't working as well as I recalled, though it was a stupidly hot day.

As I near home, the tach started acting funny...doesn't follow revs from idle, and then jumps up to 4 or 5k. Turn the A/C off and tach acted more normal, but still a little funny.

I get home, shut it off, try to restart...not much juice left, won't start.

So:

Popped the hood, alternator belt looks okay, isn't loose. No squealing from the belt, though I can't say for sure that it's not slipping. Also, last time I replaced the battery I got a cheap reconditioned one from O'Reilly's.

So...take the battery to get checked, or just go straight to replacing the alternator belt, and then the alternator if the belt doesn't fix it?

Last edited by mgeoffriau; 09-13-2010 at 10:57 AM. Reason: Corrected all my inconsistent verb tenses.
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Old 09-13-2010, 12:49 PM   #2
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Advice? I'm heading to the parts store after work with the battery...hoping it's just a dud, but if it checks out then the next step is alternator belt, right?
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Old 09-13-2010, 01:10 PM   #3
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I'd think if it were an alternator belt issue, you'd have some evidence- squealing, glazing of belt, etc.

My first step in the process here would be to stick a multimeter directly on the battery between + and -, and see what the voltage here is when the engine is running. Should be at least 13.5 volts, preferably 14 or so. If it isn't, and if the alternator belt isn't whining like hell, then I'd be looking at the alternator itself as a suspect.
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Old 09-13-2010, 01:30 PM   #4
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Man I hate electrical stuff.

So, since the car won't start, jump it off my truck and then remove jumper cables and check voltage across the battery terminals. I have this multimeter:

http://www.harborfreight.com/7-funct...ter-90899.html

Which I don't really know how to use. I'm assuming I want it set to one of the DCV settings?
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Old 09-13-2010, 01:53 PM   #5
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Wow. I have literally never seen a $2 multimeter before.

Ok, yeah. Jump-start the car off your truck's battery. Remove the jumper cables. Set your cheap-*** meter to DCV-20. At the meter itself, plug the black lead into the bottom hole (COM) and the red lead into the middle hole (V Ω ma).

Poke the red lead to the + terminal of the battery, and the black lead to the - terminal. You should see > 13.5v or so.

If you see less than that, try moving up to the front of the car, plugging the red lead into the car's blue service power connector, and the black lead to the head. If that fixes it, you have shitty ground connections either between the engine and body or between body and battery. But my guess is that it won't make a difference, and you'll still see a lower voltage which most likely points to a bad alternator.
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Old 09-13-2010, 01:57 PM   #6
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Thanks, Joe. Appreciate the dummy-level instructions -- I need them. Will test it out tonight.
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Old 09-13-2010, 04:20 PM   #7
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I have one of those $2 meters, it was in the bucket of free stuff at an SCCA Banquet, lol.
Quality control isn't so good. It's fine if you only need it to be accurate +/- 1v. There's some calibration adjustments inside that might make it more useful. I've not tested the Ohms scale, just used it for Open/Closed circuit checks.
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Old 09-13-2010, 04:29 PM   #8
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sounds like a crapped out battery.

Mine did that too right before it died.
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Old 09-13-2010, 04:36 PM   #9
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sounds like a crapped out alternator.

Mine did that too right before it died.
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Old 09-13-2010, 04:44 PM   #10
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Would a dying battery make the tach jump around like that? I was hoping it was just the battery, but I've had plenty of dead/dying batteries before and I've never seen the tach act like that.
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Old 09-13-2010, 11:08 PM   #11
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I'll bet you hustler's cat that this isn't a battery problem. The jumpy tach is a bit of an odd symptom (especially if the engine is running fine when it happens), but whatever is going on here, I'm pretty sure the battery isn't at fault.
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Old 09-14-2010, 12:03 AM   #12
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Quote:
Originally Posted by Joe Perez View Post
I'll bet you hustler's cat that this isn't a battery problem. The jumpy tach is a bit of an odd symptom (especially if the engine is running fine when it happens), but whatever is going on here, I'm pretty sure the battery isn't at fault.
This.

Your alternator is dying. When mine went over the course of two days I'd gradually lose accessories, then lights, then gauges...then power. I could charge the battery at home, and barely make it to work.

$20 and a visit to a local LeMons crew later and I had a new-used alternator and kept happily motoring along.
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Old 09-27-2010, 07:51 PM   #13
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Haven't swapped the alternator yet, decided to charge the battery first. Hooked it up, started the car, then followed Joe's instructions.

Did exactly as instructed...meter read about 14.2 to 14.3.

Sooo....not the alternator after all? Just the shitty reconditioned battery I bought?

Quote:
Originally Posted by Joe Perez View Post
Wow. I have literally never seen a $2 multimeter before.

Ok, yeah. Jump-start the car off your truck's battery. Remove the jumper cables. Set your cheap-*** meter to DCV-20. At the meter itself, plug the black lead into the bottom hole (COM) and the red lead into the middle hole (V Ω ma).

Poke the red lead to the + terminal of the battery, and the black lead to the - terminal. You should see > 13.5v or so.

If you see less than that, try moving up to the front of the car, plugging the red lead into the car's blue service power connector, and the black lead to the head. If that fixes it, you have shitty ground connections either between the engine and body or between body and battery. But my guess is that it won't make a difference, and you'll still see a lower voltage which most likely points to a bad alternator.
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Old 09-27-2010, 07:59 PM   #14
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Ok, its been idling for several minutes now, just checked it again and now its showing 14.50.
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Old 09-27-2010, 08:01 PM   #15
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Hard to say, now that you've changed a variable. If the battery was already at full charge, the alternator may well be able to float it into the 14s. Turn on the headlights, fans, and HVAC blower, and then see what the system voltage is. Also, measure battery voltage with the engine off.
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Old 09-27-2010, 08:06 PM   #16
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11.76 with lights, fans, blower running.

12.44 with engine off.
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Old 09-27-2010, 08:30 PM   #17
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Ok, even holding a steady 2500 rpm, it's reading 11.5 to 11.7 when lights, fan, blower are all on. So, time to swap the alternator?
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Old 09-27-2010, 08:55 PM   #18
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Alternators usually take a toll on batteries on their way out. Thus the two symptoms/two variables jazz.

Sounds to me like an alternator issue. It's easy for them to create potential (ie voltage), but not be able to fulfill that potential (amperage), demonstrated by not being able to keep up with load.
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Old 09-27-2010, 09:41 PM   #19
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No matter what is on, your voltage should stay between 14.5 - 14.9

my initial drop from turning the lights on goes from 14.7 to 14.4 then probably 2 seconds its back up.
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Old 09-27-2010, 10:18 PM   #20
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Got it...alternator is coming out tomorrow if I can get home in time. Thanks guys.
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