Question about 2001 GMC Sierra Crew Cab

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Voltage spikes and dips

Volt meter coasts back and forth from 14-12 and sometimes under 12. I got a new battery and 2 new alternators from the dealer. Now on my third alternator and it has started happening again. It was fine for a few months after each new alternator. Is there an external regualtor to check out? I would have figured the dealership could find something other than the alternator if there is something else at fault.

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  • xsadrenaline Sep 09, 2009

    The ground strap stabilized the volt guage in the short test I made. Seems high rpm (over 3000) or a transmission downshift takes the voltage all the way down to 12 volts.

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  • Master
  • 1,187 Answers

Sounds like the ground strap is loose, damaged, corroded or missing.

The regulated alternator puts out 13.8 volts. 14 volts is not a spike; it is normal. If your idle is not steady, the voltage will flucuate around the numbers you are reading.

Install a ground strap from the engine to the chassis, even if the dealer doesn't want to...

hope this helps...

Posted on Sep 09, 2009

Testimonial: "Sounds like a fair suggestion. I'll have a peek."

  • Just Gort
    Just Gort Sep 10, 2009

    Although I wouldn't worry about it now, you may want to get a heavier-duty alternator.

    You may have a 65 amp alternator. There is an 80+ amp alternator available.



    Problem with the high-amp is if you let the battery die (leave lights ON, etc.) or if you "boost" someone's dead battery, it will burn-out the high-amp alternator.



    Don't give any "boosts" to anyone if you switch out alternators.

    In an emergency, you can give a boost as follows:

    1. With engine OFF, disconnect your Negative cable from the battery.

    2. Boost disabled vehicle.

    3. Re-connect your battery cable after disconnecting booster cables.

    4. Your clock and stuff should be ok if only disconnected for a few minutes.



    Hope this helps!











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How is the pigtail ground strap from engine block to body? Have you had engine diagnostic test done? Voltage rVOLTAGE DROP TEST
A voltage drop test is the only effective way to find excessive resistance in high amperage circuits. It's a quick and easy test that doesn't require any disassembly and will quickly show you whether or not you've got a good connection or a bad one.
To do a voltage drop test, you create a load in the circuit that's being tested. Then you use a digital volt meter (DVM) to measure the voltage drop across the live connection while it is under the load. Voltage always follows the path of least resistance, so if the circuit or connection being tested has too much resistance some of the voltage will flow through the DVM and create a voltage reading.
voltage_drop.jpg

If a connection is good, you should find little or no voltage drop and see less than 0.4 volts for most connections, and ideally less than 0.1 volts. But if you find more than a few tenths of a voltage drop across a connection, it indicates excessive resistance and a need for cleaning or repair.
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To check the starter circuit for excessive resistance, you need to measure the voltage drop at the battery, battery cable connections and starter while the engine is being cranked.
The first check is "available battery voltage." For the starter to crank at normal speed, the battery must be at least 75% charged (12.4 volts or higher). Low battery voltage can not only affect the starter but every other electrical system in the vehicle.
A. Set your DVM to the 20 volt scale, then connect meter positive (+) lead to battery positive (+) post (not the clamp or cable), and the meter negative (-) lead to battery negative (-) post.
B. Disable the engine so it will not start when it is cranked. (Ground the ignition coil wire, or disable the ignition circuit or fuel pump relay.) Limit cranking time to 15 seconds or less.
C. While cranking the engine, record the volt reading on the DVM. D. Next, connect your meter positive (+) lead to the battery terminal stud on the starter, and the meter negative (-) lead to the starter housing.
E. While cranking the engine, record the volt reading.
F. Compare the two voltage readings. If both are the same, there are no excessive voltage drops on the positive feed side.
G. If available voltage at the starter is not within one (1) volt of battery voltage, there is excessive voltage drop in the circuit.
The next test is for voltage drop on the positive side of the starter circuit.
A. Make sure the battery is fully charged.
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egulator checked with voltmeter?

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