Hello,
I joined this forum to ask this specific question. I have a 97 XLT 4X4. It was parked for several years before I decided to give it a new life. Now the alternator isn't charging and I'm not getting power from the OBD2 data port. I have been driving this truck for a month now and the above problems have just presented. Below is a list of things that I have checked/done

Alternator
1.) old one was bench tested at part store. Passed test but bearing was not good and very loud - replaced
2.) tested continuity between all connections from alternator to battery including 175 amp fuse, 20 amp fuse, all cables/wires
3.) found center pin on 3 pin alternator connector was bad - replaced connector
4.) retested all connections from alternator to battery for continuity - all connections solid
5.) voltage reading on battery before start 12.5 voltage while running 12.2 and slowly dropping

OBD2 Data link
1.) tested all GEM/BCM related fuses. (the GEM was replaced recently and OBD2 wasn't working with old GEM)
2.) tested all fuses including the ones responsible for powering the data link - all passed
3.) tested for power at power port/cig lighter - both reading battery voltage
4.) check for continuity from data link ground pins to chassis - passed continuity (didn't check for resistance)
5.) checked for battery power on pin 16 - failed no power

I am at a loss on what could be causing theses issues. I have a distinct feeling that they are related but not sure. Can you wonderful individuals help me solve this issue?

 

Treat the issues separately.

Check the I terminal at the alternator to see what it is reading with key on, engine off.

Check for continuity between DLC pin 16 and its fuse (cigar lighter). Disconnect battery first.

 

ProjectSHO, I will check that in the morning. I have a feeling the issue with the charging lies in the circled section of the above schematic. I have read that the low battery lamp can blow and cause an open circuit on the system not allowing the alternator to get a voltage signal to excite. I already have the dash apart replacing some bulbs and will replace that one as well as take your advice and check for continuity between pin 16 and cigar lighter. What did you mean "check the I terminal"

 

The area you circled is the "I" circuit.

 

I'm not a total noob and have some relevant experience diagnosing/fixing automotive problems but I am not sure how to "test" the "I" circuit. I have checked continuity from the alternator to the C169F connector (which is behind the mega fuse) and I really don't know how to test the rest. I am assuming that this circuit is the feedback trigger loop that excites the alternator in a start/run condition. Would that wire be carrying 12V positive and if so would it be possible to "apply" a jumper from battery to the pin inside that disconnected connector to isolate the instrument cluster and everything else behind it to narrow down where the problem is.

Second, I have checked for continuity from pin 16 to cigar ligher/power point and pin 16 directly to fuse under dash. No continuity on the this pin. There is continuity on the grounds 4 and 5 pin. and power to the cigar ligher/power point. Could this be a bad GEM module again? Next step?

 

No, it's not the GEM module, it's a "simple" open circuit in the circuit. I gave you some bum advice earlier. I forgot that the DLC in the 97 F150 has it's own fuse and it's not connected to the lighter as it is in most Fords after that timeframe. Check fuse F12 under the dash instead, it feeds pin 16 of the DLC (and nothing else).

As for the alternator, backprobe the LG/R wire at the connector with the key on, engine off. You should read 1-3 volts DC and the BATTERY light should be on. If you read full battery voltage, either the connector or the voltage regulator is open. If you read zero volts, there's an open circuit "upstream" of the test point. If that passes, start the engine, the voltage should rise to full battery voltage.

Report results.

 

Are you referring to the fuse in the #12 position? The owner's manual shows that to be and unused position. Or, in Ford's infinite wisdom, did they put another fuse outside of the panel similar to the 4 lonesome fuses under the hood?



I assume that would be a positive lead backprobe to ground?

 

You're looking at the wrong panel listing.

Yes, that is correct.

It's actually a 5-position fuse block, only two of which were used.

 

Your are the man!!!
I truly mean that.
YOU ARE THE MAN!!!!
I had downloaded the fuse section of the owner's manual when I started restoring this truck so that I could have a printed version that I could write on and compare what was in the truck vs what was supposed to be in the truck. Apparently, the jackwagon that made this online version available screwed up the fuse layout. I had fuses missing that were supposed to be there. The data link fuse, and the fuse responsible for the battery gauge of the instrument cluster was missing.
The Data link and the alternator now work.
Thank you sir!!!