Alternator too small?

[DieselWarrior]

I noticed tonight, the voltage reading was inching closer and closer to 13v mark, down from the normal 14-15 mark when on the highway. This time it was night so I had my headlights on, defroster on and an additional fan blowing, and windshield wipers.

When I shut off the stock blower motor for the defroster, the voltage went up a bit.

I hate to ask, since Im supposed to be an 'lectronics guru, but would this be a good indicator that my alternator is just too damn small for the loads Im putting on it??

Its stock, so Id guess its either 40 or 60 amps...

Also, being that its a diesel, Im assuming the idle RPM is lower than a gasser, so when/if I have it rebuilt, should I ask it to put out 14v at my idle RPM?

WTF is the idle RPM for a 6.2l (NA) diesel anyway?!?!

THANKS for looking and any help...

Andrew

 

[Zeus33rd]

You might try putting a multi-meter on your battery with the truck running and all that stuff turned on. I was having the same issue as you not to long ago, which seemed weird to me because I have a 140 amp power master alt. The stock gauge was reading less than 13 volts with high beams, heater on high, wipers, radio and several different gadgets charging off an small inverter. Stuck the multi-meter on the battery and it read 14.4v. Don't put too much faith in what the stock gauge reads.

 

[ktmoutfront]

13 volts with all that stuff on is no big deal. If it gets down below 12, you should start looking for problems.

 

[dhcomp]

13 volts with all that stuff on is no big deal. If it gets down below 12, you should start looking for problems.

X2.

Also, try adding additional ground wires. Body to frame, frame to motor.

I like the cheap, "lawn and garden" cables from walmart for this.

I bet your gauge stays more constant.

 

[tRustyK5]

Check your belts too...

 

[Fordum]

Always double check with a known good meter at the battery. And again, a temporary drop to 13 is OK under a heavy load.
If it stays there, you might need to invest in a bigger alt. Otherwise, just keep it in mind if that one ever goes bad to get a bigger one.

Technically, the answer is yes, because an alternator should be able to maintain 14.5 at the max load it sees.
But, few factory ones will. Its just not cost effective to add an extra 50 or so bucks to every truck they build just to handle an occasional load.

As to your idle speed, its kinda irrelevant, given that you said you were at highway speeds........
If you are driving highway speeds at idle, you really need a gear change.

But if you are worried about it dropping too low at idle, remember that the critical speed is the alternator speed, not the engine speed.

When I changed the system on my old 8N Ford tractor from 6 to 12 volts and put on a one wire alternator, it would not kick in unless I wound the engine up past max by holding the governor for a second.
After that, it would put out all the way down to idle.
Rather than doing that every time I wanted to start it to charging, I just changed the pulley to a slightly smaller one so it wound up faster per engine RPM.

 

[DieselWarrior]

One reason I was asking, when I turned off the blower motor and the voltage came back up a tad, the headlights got a bit brighter too... I thought they were a wee bit dimmer than they should be. I dont think the gauge is "accurate", but I feel sure it represented the system status pretty well.

Belts are good... Ive got *2* ground wires directly to both batteries, and I also have a pretty hefty ground to chassis wire... dont think any of that is the culprit.

I was thinking the alt was a bit on the skinny side... BTW both batts are NEW and in great condition to boot.

Andrew