Wiring Troubles 1993 7.3 IDI

[scooterrr]

1993 F250 7.3 IDI 4x4
I've done a fair bit of searching for my issue and I haven't come up with too terribly much. My problem is, my truck intermittently loses key on power and decided not to crank. This has been going on for about a month or so and was easily resolved by doing a little fiddling with the battery cables. The symptoms were as follows: (1) low voltage in cab (i.e., lights on dash are dim and door chime freaks out); (2) would have good voltage in cab with key turned on RUN position, but would click and lose power (symptom 1) when turning the key to START. I dealt with this for a while before my truck finally decided to die out of nowhere, kinda like a motorcycle where it revs out running out of gas, when I was going through a military training base of all places. Thought I ran out of fuel, tinkered with it for a few hours (which included lots of cranking because I thought there was air in the lines), before I realized the IP was not getting any power. So, to get home I just decided to run a jumper to the FSS from the battery and limp it home. At this point, I think it's important to note, I haven't experienced either of my prior low voltage symptoms.

Fast forward to the past few days, I've been trying to track down what's going on with this thing. I figured out that Fuse K in the EC fuse box was blown which quickly resolved my no FSS power problem, but I was still battling the intermittent low voltage/no crank symptoms (fiddling with the wires was much less consistent in resolving this at this point). I figured out one of my batteries was pretty low on charge (loose battery connector I assume; both brand new) as well. For each of these two previous fixes, the truck would crank AND start just fine afterwards leading to a very premature moment of happiness, quickly followed by frustration when the low voltage symptoms continued a few hours later. I found some wires going to something that looks like a MAP sensor in the EC (not sure what it really is) that were shorting bad, which I found out after getting shocked. Fixing those shorts also did not resolve the low voltage issue.

Here is what testing my voltages at different locations looks like right now:
At the fuse block for Fuse K, when low voltage issue is occurring: 9-9.5V
At the fuse block for Fuse U, when low voltage issue is occurring: 9-9.5V
R/LG wires leading out of EC fuse box: 4.0-4.5V
FSS, Fuel heater, and whatever the other ones are that are R/LG and attached to GPR: 9-9.5V
Heavy gauge, always hot wire on GPR: 12V
Smaller, key on hot wires on GPR: 4-4.5V

Glow plugs look fine, and so do all the wires/fusible links associated with the circuit. I'm quite confused, and having a hard time making any sense of this. I've also replaced my ignition switch under the column which didn't make a difference. Any help would be much appreciated, I'm running in circles here.

 

[rreegg]

how is the health of the pos/neg battery cables?

 

[IDIBRONCO]

Have you checked the fusible links under the hood?

 

[Noiseydiesel]

Izza Ford. remove all the grounds, clean with wire brush, clean mounting surfaces with wire brush. Dab of grease and reassemble.
Voltages are whu again?

 

[onetonjohn]

Check resistance from gnd of fuse block back to battery - terminal. Also resistance from positive battery post to fuse block source. V = IR so your voltage drop depends on how many things are pulling current.

Looking at it from a different angle, If you have 9.5V into fuse box, and 4.5 volts out the actual box connections are suspect. Are the slots for the fuses corroded or burnt?

To me it looks you have bad ground connections/load between battery and fuse box. Like Noisey diesel said, clean any junctions/connections between the two points so you get on the order of 1 ohm resistance to the fuse box. In addition, you have (another problem) more drop across the fuse box, OR you have a terrible ground for the box. Checking resistance of each hop. Sounds like you have bad connections all over. Clean them up and retest.



12V X Battery + termnial voltage
(loss here)
9.5V X First hop to input of fuse box
(loss here)
4.5V X output of fuse box.
(loss here)

0V X GND Battery - terminal voltage

 

[Schnepel]

As others have said it is ground. I had this issue. Cleaned the connectors on the battery cable and ground wires and fixed the issue.

 

[onetonjohn]

Trying to sort out wiring on my truck. Had several wires all spliced together and there was short to ground along with fusable links. Just fiddling with it removed the short - hate that. Cut the splices and added ring terminals and a terminal post. Now I can debug one at a time. One of my buddies said stacking connnectors increases corrosion, so may need to clean/maintain them more often, but at least its easier to understand what's what.

 

[DaveBen]

Use dialectic grease on the rings when you put it together. This will slow the corrosion if not stop it.

 

[BeastMaster]

I have read that a failing ignition switch will do this.