Hello FTE,

This is one of my first posts, so let me introduce myself and, more importantly, the truck.

She's a 1997 7.3l diesel. I've had her for only a year, but she has 300k miles and runs great. I've known since I got her that I would need a new alternator but the old one survived. Most of the time I'm just around town so I figured I could wait. Worst of it was just that the voltage wouldn't hold at 14.4V, from what I read on the dashboard. It would fall down to maybe 12.5V (Edit: 13.2V) at any RPM and all the time, more when idle and hot though.

I got enough saved to have the mechanic put in a new alternator. I guess I thought that was all I needed to do, but the drop outs are still there.

Definitely better than before, it used to drop all the time when idle and warmed up. But now it will drop out every 10 minutes or so.

I replaced both batteries in august so new batteries new alternator.

How do I go about troubleshooting a short? Or I guess convince me that it's the alternator and I need to have the mechanic replace it.

Thanks in advance!

 

Welcome to FTE
Welcome to the obs brethren

It be better if you did some of your own work that way you get to learn how to work on the beast, remember they are over 20 years old .
As to your original question
How are you testing your voltage?

Do Not run your truck if your voltage is dropping below 12v your IDM (injector driver module) may suffer the consequences and mulfunction or die all together.
Maybe someone with more wlectrical expertise jumps in.

 

When the voltage drops out, how log does it last, and how low? Any lights on in the dash while low voltage?

 

Thanks for the tips!

I *think* I'm seeing a short happening. When normal, the dashboard needle (pointing to the word NORMAL) is on the R. When it drops out, it drops to N. Then it slowly climbs back, though there was one time it quickly recovered.

When it drops out, the battery light lights up a little bit. Enough that in the dark it's clearly lit, but in the daytime you probably wouldn't see it.

I'm going to try to troubleshoot this one myself. Unless I decide the alternator is the problem... then I'll take it back to the mechanic and ask for warranty coverage.

 

When does this happen? Upon start up only or randomly while driving?

Rusty

 

Randomly while driving.

I'll try to get more accurate measurements. I have a bluetooth-OBD adapter and Torque Pro, so I should be able to get actual voltages (measured at the OBDII port).

 

Have you tried wiggling wires and harness?

 

If you see the battery light , it is the alternator not working

 

Saturday is finally here! So I wiggled the wires and harness and watched the voltage and...nothing happened. If there is a short, it's not with the wires in the engine compartment.

Thanks for the tip!

I used Torque to watch the voltage at the OBDLink adapter, AND the voltage reported by the ECU. The two didn't read exactly the same. But at cold idle, the voltage would drop to 13.5 V steady and even further to 13.2 V (as reported by the ECU).

When I held in the pedal at just over 1,000 RPM, the voltage picked up to 13.8 V and held steady for almost 5 minutes.

Then I let it idle again, now it's a warm idle, and it holds at 13.8 V for a minute before falling to 13.5 V with drop outs to 13.2 V.

Now I'm looking around for recommendations on a good high-output alternator. Any suggestions?

I've edited my first post to point out that the voltages I said up there were off, now that I got Torque hooked up.

 

Ok, got the alternator replaced with a second stock 130 A alternator. The problem is still there.

I'm scratching my head on this, it might be a loose wire somewhere, but I can't find it.

 

Check all your grounds specially the one going to the chassi.Good luck

 

Are you positive the wiring harness is good ?

 

My thought also

Check the one wire that directly connects the alternator to the distribution box (under hood fuse box) or even the one under the dash.

 

Thank you for your tips!

I traced as many of the wires as I could, starting from the alternator and working my way toward the battery. It took a while but I finally noticed that the ground wire on the battery is loose. I took it apart, cleaned it, and replaced the battery terminals.

Now the voltage sits right at 14.4, steady as can be.

 

_Hooray!!_