Every morning when I start my truck for the past month or so the batt light has stayed on for about 10-20 minutes of driving. I bought DashCommand and an OBD II reader and the batt gauge on dash command shows about 11.5 until glow plug relay shuts off then about 12-12.5 for the next 10 min. Then it shoots up to 14 at idle, and batt light on dash turns off. When I accelerate it drops to around 12.5 and off go pedal jumps back to 14. Does this for about 10 minutes then stays constant 14 the rest of the drive. Put a new alternator on about a year ago and have Optima Yellowtops that are about 3 years old. Thoughts?

 

Clean your battery posts and tight the connections to the battery.

If it continues, have your alternator checked.

One quick alternator test...in the morning before you have started the engine, touch the engine block to gauge its temperature and then put your hand on the alternator. If the alternator feels warm, its likely got bad diodes.

 

I have seen many on here go through a few alternators in a rather short time frame. Did you go with OEM or some off brand? I would check terminals to make sure they are clean and tight and if they look good, check the belt to make sure it is fine and after that I would focus back on the alternator. JMHO

 

I got a new, not reman, from O'Reillys. It was cheaper than OEM. Battery terminals are clean.

 

Not if it doesn't work or it craps out early.

Without being snooty - this is how I think of it: The more aftermarket parts I put in there, the less it's the vehicle that lasted this long. I don't want an O'Napa Schlockyduty from China.

 

Has the sequence of voltage changes you describe been consistent? With the voltage jumping around like that either the load on the system or the output of the alternator is changing. Bad wires/connections at the alt or batteries, etc. But it sounds like bad alt to me, and I'd try bigtexans suggestion to feel the alt temp before running in the AM. IIRC bad brushes and/or diodes will make it warm up and cause the voltage fluctuations you are seeing.

 

I often see people pose a question and then but an Alternator and most times it works. I thought I would just do a quick write up where anyone with a Digital Volt OHM Meter can quickly diagnose not only a Bad Alternator, but ensure field wiring is good as well ei: Battery Cables, Grounding, and so on.

Use the DVM to test the Voltage at the +/- Posts of the Battery and record your reading. This is the last time we will test anything to the Battery + Post.

Verifying a Good Alternator: Always use the B+ on the back of the Alternator when testing Voltage. Otherwise, there may be a problem with the wiring in the vehicle and you might throw money away when the problem is not the Alternator at all. This is why we recorded the Battery Voltage First.

The battery must be fully charged. Run the engine and verify that no-load voltage is 13.8 - 15.3V (everything electrical is turned OFF). Next, load the alternator to rated output current with a carbon pile across the battery. Now, most of you don’t have a Carbon Pile, so, turn on Head Lamps, Blower Motor to Max and Run the engine @ 2000 RPM. Check the current. The Alternator MUST maintain at least 12.6V. However, you will probably see 13.0 V as you're not using a Carbon Pile.

Checking Ripple Voltage: Ripple voltage or (AC voltage) can be measured by switching your DMM to AC and connecting the black lead to a good ground (this is the only other time you will use thy Battery (-) and the red lead to the "BAT+" terminal on the back of the alternator, (NOT AT THE BATTERY remember, we’re testing the Alternator!). A good alternator should measure less than .5 VAC with the engine running. A higher reading indicates damaged alternator diodes

Alternator Leakage Current: To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the Engine is NOT running. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps (don’t forget to change your leads and meter setting to milliamps). Use care when disconnecting the alternator output wire; make sure the battery is disconnected first.

Checking Field Current Worn brushes limit field current, causing low alternator output. To test: load unit- turn on Head Lamps, Blower Motor and Run the engine @ 2000 RPM and measure field current with current clamp (if you don’t have a current clamp) use 10A jack on DMM. Readings range from 3 to 7 amps. Field current should be between 2 - 5 amps, higher current with lower battery voltage (control battery voltage by loading turn on Head Lamps, Blower Motor and Run the engine @ 2000 RPM). ** Measure after the Blower Motor or Head Lamps have had a chance to start up and fun for a few seconds. This will eliminate fluctuating in your readings as motors have a "start-up" volt/amp vs Running volt/Amp draw.

Why do all the tests? Well, for instance. If you have two failures in a short period of time you will know what is failing and that will most often point you in a direction in the field circuit where it can be corrected to prevent another failure.

 

I agree with using OEM when possible. But, for the reason that OEM Standards are most often higher and have less tolerance for failure. Most Electrical parts we use in the U.S. are actually "Made in Mexico" and used internally to include the OEM Brandings across the board.

This is disheartening to any American. Especially those who "Buy American". I have disassembled "Made in the USA" Alternators for rebuilding only to find the Rectifier or other parts were stamped "Made in Mexico" And, this is one of those high-end, never fails, best you can buy, etc. advertisements.

IMO The OEM and some aftermarket brands to be better than anything "Made in China". They are quantity and not quality.

BTW: If you purchase a Motorcraft Alternator, before you purchase it, look at the Voltage Regulator/Rectifier on the back. If it's not Brilliant White, ask for another one. They re-use parts that were not the cause of failure.

So, even OEM has its shortcomings. We (America) have allowed ourselves to be an end market and not a manufacturing market even though we invented just about everything out in the world today - SAD!

 

The reality is that when a company goes to a manufacturer and has a product made, they provide a set of specs the product needs to meet. If the standards set by the requester are low, the product is low quality. There are plenty of products that come from China, Mexico and other countries that are manufactured to high standards. Conversely, there are plenty of products made right here in the US that are garbage.

 

Can the alternator still put out the correct voltage with damaged diodes?

 

Yes. however, it may put out 14.0 VDC at idle but may not at say 2,500 or 2,000 RPM. And visa-versa. That's the purpose of checking it at both. These are non PMC controlled units. The PCM controls the out put voltage based on signaling.

 

I took the alternator off and had it tested and all tests came up good, even after multiple times. When I went to put it back on and then put the neg cables back on the batteries, the one on the passenger side battery broke while I was tightening it. I replaced the end of the cable and now everything is working fine. 14V right from startup yesterday morning, no V drop. I checked that battery connection before and it appeared tight but it must have been just enough on its way out that it wasn't making full contact. Strange.

 

Apparently I spoke too soon. This morning was the same symptoms as before. Volts sat at 12 until I driven for about 10 min then shot up to 14 and stayed there. GPR was off within 60 seconds of starting according to my GPR LED and the DashCommand app that gets info from the OBD II port. I'm stumped.

 

I had the same symptoms as you are having.
Went thought all the things that could cause the problem and still same problem.
Last resort I replaced the alternator and problem gone.

 

ztodd, if you search for alternator on this site, you will find your story repeated many times over the years. Just because the alternator tests ok, does not mean it is always ok, just at that moment. Temperature, humidity, engine vibration, time running under these conditions are all factors which cannot be replicated during a test at a retailer or electrical shop. I had a similar electrical problem, had the batteries tested-they were "ok". I replaced them anyway. Solved the problem. My point is to look for the most likely point of failure in your situation. Did you check the major ground and positive cable connections for corrosion at the bolt and in the cable connector? If so, the odds are it is either the batteries or the alternator. Try an OEM alternator. Larry