I noticed my engine seemed to be turning over just a little slower than usual the other day. It still started ok but it just didn't seem to have the snap when I turned the key. So I went to check the alternator output and it showed 13.8v. This was connecting to the big red terminal on the back and then to ground. I also got the same reading across the battery terminals. Then, mostly out of curiousity, I made my ground contact on the voltage regulator retaining screw that is marked Ground Here to test. I got a reading of only 3.6v I don't usually use that location because it is difficult to reach when things are hot. Have I been reading things incorrectly all this time? Is 3.6v the actual output? Have I been simply reading battery voltage by using the wrong ground location.
FYI A couple of months ago I had the alternator off while doing some other work and put new brushes in just beacuse it was so easy. The old ones were worn(112000mi) but not worn out.
I appreciate you taking the time to read this and thanks for any help you might give.

 

13.8v across battery with it running shows charging system is working. A battery load test wouldn't hurt, it could be getting weak. If you check batt voltage soon as you start it in the morning, alternator will probably be charging a little higher. Also check all connections/cables to starter. Starter may be getting old and drawing more amps to start engine. I'd load test battery first.

 

In general, it's not safe to probe behind the alternator with the alternator output (BATT or A+) stud exposed, because of how easy it is to short the alternator output stud and ground (cylinder head) with your meter probes. Do all your probing across the battery terminals; it's the same voltage as long as the battery cables and alternator wiring are in good shape.

You're saying you read 13.8 volts between the alternator output and ground (I assume somewhere on the engine block) and then 3.6 volts between the alternator output and the regulator mounting body? That can't be right. That's a voltage drop of 10 volts; the two grounds should be the same. What year/truck is this? I assume something older with an external regulator? Those trucks have the regulator grounded to the inner fenderwell, which bolts to the firewall, which itself is grounded to the block through a ground strap. All the "grounds" across the truck should be at zero volts with respect to the engine block.

 

The truck is a 97 F-250, but it is the early body style(90-96). 1997 was a splityear for 250's and 350's for the newer and older bodies. The regulator is mounted on the back of the alternator. If you look closely, there is printing on the small cover that protects the electronic components of the regulator. It says GROUND HERE to test, with a small arrow that points towards one of the retaining screws that hold the regulator to the bursh set. Not the screws that mount the regulator to the alternator case. If I touch the black(-) lead of my VOM to that screw and the red(+) lead to the post on the back of the alternator, that is where I get the 3.6v reading. If I touch the black lead anywhere else, I get 13.8v readings. I hope this clears things up. Thanks again for taking the time.

 

The screw you are testing is the “F” test point. It is on the circiut from the ignition switch thru the dash warning light to the Alt. at the VR. It is the circuit that turns the VR on. The VR then senses the battery voltage and regulate the voltage accordingly. The “F” test point ground is to bypass the VR and full field the Alt. It is a “quick” test to check the Alt. output.

<TABLE style="BORDER-TOP-WIDTH: 0px" class=tborder border=0 cellSpacing=1 cellPadding=6 width="100%" align=center><TBODY><TR title="Post 10916761" vAlign=top><TD class=alt2 width=125 align=center>gfw1985</TD><TD class=alt1>13.8v across battery with it running shows charging system is working.</TD></TR></TBODY></TABLE>

 

So if that point is supposed to full field the alternator, shouldn't it read more than 3+v's? Anyway, I just started the truck and it still sounds like it's turning slower than it should with a good/hot battery. When I came out fo the hardware store, I didn't think it was going to start. It turned over VERY slowly, then it caught. I went straight home. I'm waiting about 15min. and then I'm going to see if it does the same thing. I'll be back shortly. Thanks

 

You already verified the Alt. is charging ok.(13.8 at the battery). The problem you are describing is with the cranking system. Start with load testing the battery.

 

It had the same problem. Acts like a dead battery, but the battery has a fresh charge and shows 12.77v. I'll let it sit over night to see if it's a heat problem. Time for a beer. Thanks again. See you guys in the morning.

 

Fresh charge means nothing if the battery will not provide the cranking amps needed. Load test is the only way to check the battery. While you have it charged, take it to your local parts store and have them check it. You're wasting your time till you eliminate this as a problem.

 

Many thanks to all who have replied as well as you guys who just took the time to read.
Here's the latest update. Stone cold, it was cranking below normal speed but it fired right up. Drove around long enough to reach operating temp and went back to the garage. Turned over real slow, might have started if I kept at it but didn't want to burn anything up.
I swaped bateries with my car. The car battery acted the same way and I know it has no problems as it works fine in the car. Put the truck battery in the car and it spun it over like there was nothing holding it back.
Now I'm looking at the starter. The problem seems to be getting slightly worse each day, but is really bad after everything heats up. I haven't yanked the starter off yet because everything is hot and I'm getting a little tired of messing with it. Tomorrow morning will be soon enough. If anyone has any other ideas, I'd like to hear them before I get all greasy. Thanks again for sticking with me.

 

If you have a set of jumper cables, take one lead and connect to pos batt and touch the side of solenoid that runs to starter. If it turns over the same, you have eliminated the stater relay and pos batt cable leaving only starter cable and starter itself as possible problem. If it works correctly, then you have an issue with relay or pos batt cable. The other side of jumper cable connected to neg batt term and a clean ground on the block will prove the ground side is good or bad.

 

I think we've reached the end of this little adventure. Thanks gfw1985, that was good idea with the jumper cables. I never really gave any thought to the circuits actually being bad. As it turned out they were not the problem.
Put the new starter in this morning and all is as it should be. I really want to thank everyone who took the time get involved.