On Sun, 12 Jun 2016 11:33:39 -0500, Ignoramus13864
wrote:

I have a forklift with a Detroit Diesel 3-53 engine.

Unfortunately, the alternator on it does not charge at engine idle.

At idle it rotates at 1,250 RPM (measured with my tachometer).

That may be normal:
http://www.thesolar.biz/Balmar%2070-SeriesDIMENSIONS.jpg
Notice that it starts charging at about 1200 RPM.

If I push on gas, it charges.

This alternator is a rebuilt alternator.

Oh-oh. Was this a rebuild, or an exchange? If exchange, was the
pulley diameter the same as the original? Methinks you should check
if it's the correct diameter pulley (including the belt depth and
fit).

Someone told me that I should change the regulator on this alternator
to make sure that it does charge at idle.

Nope. If it's charging normally at higher RPM's, and it's not over
charging the battery, it's probably working normally. Minimum
charging RPM is set by the pulley diameter ratio, typically 3:1 for a
gasoline engine. No clue what it might be on a diesel. The idea is
to set the RPM so that it barely charges at idle, where the added load
of charging the battery and running the lights does not put an
excessive load on the engine causing it to stall at idle or have
difficulties starting.

Could someone comment on this, and also how can I find different
regulators?

The regulator is inside the alternator and possibly looks something
like this:
http://store.alternatorparts.com/ProductImages/35-103.jpg
Before attacking, you might want to pull the alternator and drag it
down to an auto electric shop and have it bench tested. Or do it in
the forklift:
https://www.google.com/search?q=alternator+test&tbm=vid

On a different forklift, I was able to solve it by making the pulley
diameter smaller, but here the pulley is already very small.

What problem did that solve and what problem are you trying to solve
here? Sounds to me like it's working normally. If it doesn't charge
at idle, does it need to charge at idle, such as it might need to
charge the battery just after starting? Does it deliver any current
when you turn on the lights?

Note: The dashboard ammeter might be giving you a false impression
that the alternator is not charging. It's connected to the positive
wire going between the battery and the alternator. The various loads
in the forklift go to the battery. If the engine is at idle, the
battery is fully charged, and you have perhaps a 5 Amp load
(headlights), if the alternator is delivering 5 Amps to the lights,
the meter will read zero because the battery is not delivering any
current or being charged.

Basic test. Put an inductive ammeter:
http://www.stuttgartperformanceengineering.com/inductiveammater.html
on the battery (not the alternator) positive wire and see what it's
doing. At idle, it should barely charge (less than 5A). Turning on
the lights, will make it charge more. Revving the engine should
charge even more, but only if you have a battery that needs charging.
If the battery is fully charged because the forklift has been running
for a while, you won't see much charging current. Moving the
inductive ammeter to the alternator positive wire should should what
the alternator is doing.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558