when I first start my truck up it doesn't charge for like a minute or 2 then charges fine after that.
so I replaced the alternator and it fixed it for about half a day and started doing the same thing. batteries are interstate mega tron plus FEB 2011, so just over 2 years old. they both checked out fine on the analyzer at work.
any thoughts?
any schematic for the charging circuit?

 

How are you determining that the alt isn't charging? Is the battery light on or are you just watching the volt gauge? The volt gauge will stay over to the left side of the range for up to two minutes after you start the truck because the Glow Plugs are still running. If that's what you're seeing, it is completely normal and means that the GP's are working as they should be.

 

That's perfectly normal. It is charging. It's just not able to keep up with the load of the glow plugs, which run for 1-2 minutes. Your old alternator was probably fine.

 

As stated, that is perfectly normal. The GP system pulls a big draw and until the GP system turns off(up to 2 minutes), you will not see the 14v+ that the alternator normally puts out.

 

 

I had my associated battery tester hooked to the main before starting the truck then started it and it said it was not charging and then all of the sudden it went into the green as does the dash cluster reads. last week I had to charge the batteries because it was turning over very slow. it seemed to turn over slower and slower day by day.

a guy I work with said there was probably a diode issue causing the batteries to drain

 

Your tester is just reflecting the same thing -- the added load of the GPs.

But you may, separately, have a parasitic drain. You need an multimeter with a fairly high max rating for current (amperes) to test for this. Couple common parasitic drains are the alternator (perhaps that's the "diode" thing your friend was referring to) and the glove box light (can't see in there if it really turns off....). Try disconnecting the small connector on the alternator next time you park it (remember to reconnect it before driving it again), see if that allows the batteries to hold a charge.

'Course, it could just be your batteries are old and don't hold a charge any longer. Can your tester do a _load_ test? If so, you need to isolate the batteries (disconnect one of the negative terminals) and test them separately.

 

I agree with everyone else on this. a simple test would be to disconnect one of the smallwires from the glow plug relay. the glowplugs on these things pull down alot of juice and it is very noticeable on the volt meter in the cluster when theyre on.

 

^^^^^ No need to do that test. If it's charging properly after a minute or so, it's working right. There's no point in testing the charging system before the GPs cycle off.

 

ok, why was my battery slowly draining to the point it starts so slow?

 

unless you wanted to prove that it is in fact the GPs cycling that caused the original post^^^ if the relay is disconnected the volt meter wont show the tremendous drop.

 

Did you actually measure battery voltage? could be the starter.

 

is there a video on youtube that shows someone elses truck doing the same thing with the gauge on the dash? I just don't see why it has to look like its not charging. To me its like the brushes are not seated or worn away

 

It **IS** charging. The load of the glow plugs is greater than the capacity of the alternator. When the glow plugs cycle off, the voltage returns to normal.

You don't need a video. Every OBS PSD does this.

 

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