High elecrical load trouble

[Tim McKay]

Im new here, and been looking at this forum before and after a recent putchase of a 1993 F250 IDI. This is my first diesel, but not new to wrenching. It just developed an issue that manifests itself as a cyclycal high load dragging the voltage down. I see it turning on and off on the volt meter gage, and all the lights, cabin and head lights, dim with the switching on and off that correspondes with the volt meter fluctuations. A quick check by a parts store yielded the voltage regulator is bad, hence I changed the altenator. No change to symptoms. I read a post suggesting a faulty relay, so I changed the relay labeled engine management. No luck there. I installed new batteries and cable when I got it. It starts and runs great. The wait to start engine light takes about 15-20 seconds to turn off before starting, so that suggests at least some glow plugs are operaring correctly. If the culprit creates a high enough load to dim all the lights, maybe some glow plugs are switching on and off? This even happens ar highway speeds, 70mph, not just idle.
Any Ideas where and how I should begin trouble shooting? Thanks in advance for all suggestions.

 

[Thewespaul]

Sounds like your glowplugs are triggering. I’d disconnect the controller at the back of the intake of you are na or on the valve cover if you have a turbo, then take it for a drive and see if it’s still doing it

 

[Tim McKay]

Great, thanks for the suggestion. I will give that a shot and see if thats it. My bet is your correct, as it must be a high amperage source, and it cycling on and off regularly.
By the way, this one is a normally asperated 7.3.

 

[franklin2]

I have had that happen before, and it was a poor connection on one of the small wires in the plug that plugs into the alternator.

 

[Tim McKay]

To help others that may search this in the future, here are my findings. Both Franklin2 and Thewespaul were correct. I started by checking all glow plugs, 7 out of 8 tested open, only one had measureable resistance. I replaced all GPs with new Motorcraft units. Tested it again, and the relay was switching less, but still doing it what I thought was beyond normal. I then checked connections on the relay, and the connection of the wires that feeds the GPs voltage was loose. I tightened that and no change to the behavior of the relay. I pulled the relay and noticed signs of high heat, including some melting of the epoxy under the black controller. I decided I would replace it. The new unit did the trick. The dash volt meter now behaves properly, in the middle of the scale, and the relay is no longer clicking on and off constantly.
I also want to share 2 videos I found that were of help after I made the original post:

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Thanks guys for sharing your expertise. The truck runs excellent now. Total cost of a complete new GP system was about $225