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I posted yesterday about my low fuel and complete shutdown. Today I refilled the tank, bled the lines, and as Oliver mentioned, made sure the FSS was connected properly and she still wouldn't fire. As a hail mary, I kept in mind that the FSS is the likely problem (because Oliver knows his stuff), connected power directly to the FSS and bypassed the factory wiring and she started right up. There must be some electrical component upstream that is the problem but I am at a loss on where to look first: under the dash at the ignition, focus on a relay under the hood, etc?
Any help is much appreciated!
The easier way to test (sorry I couldn't tell you sooner) would have been to wire the fuel lever in the "up" position - preferably by disconnecting the FSS from the fuel lever, but you can leave it on if you don't mind fighting the spring. For the next time you go through this...
If you track the FSS wiring back from the connector, you'll see that the "pull"-circuit wire goes to a relay on the firewall. That relay is known to fail. Also, both the "pull" and "hold" coils in the FSS can fail. You need to check power to both coils - check power to the pull-coil in start mode and check power to the hold-coil when you release the switch from start mode to run mode. That will guide any further testing. If you have power to each coil at the appropriate time, the FSS itself is the problem.
Here's a diagram from LarryB's - you can see that the firewall-mounted relay for the FSS pull-coil is triggered by the relay for the starter. If you don't have power on the pull-coil while cranking, that is next place to look. Either the relay is bad, or one of the inputs to the relay is missing.
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I appreciate the information, I'm sure it will be VERY helpful. Another consideration I have while I'm breaking into the electrical is to do away with these fusible links that are known to cause trouble in the Cummins. Not sure if I want to replace them with a fuse box (video attached) that I have from an old parts Truck or with various circuit breakers like Deboss does.
Any input on that front is much appreciated and, again, thank you for the diagram and advice!
Personally I like the fusible links- cheap to replace, good safety record and if a cheap part goes TU after some decades - why not replace it with a known good one.
Nevertheless- let's start with some basic diagnostics (oposed to a wild goose chase)
Assuming charged batteries with good capacity (please test that before the next steps):
Ignition switch off -what works- lights, horn, etc
Ignition switch on ACC - same again
Ignition switch on run- same again
Ignition switch on start- what changes?
In the next step we might need a multi meter and a test light.
I will take that into consideration, thank you.
Battery is charged and working as it should.
Ignition switch off - Lights, Horn, Brake Lights
Ignition switch on ACC - Nothing, no buzz, radio, dash, gauges
Ignition switch on run - Same
Ignition switch on start - Same, no clicks, anything
Hope that helps and I really appreciate the insight!!
Ron, I sent you a red herring. The diagram I posted is for a 2nd gen with a p7100 pump. I mistakenly assumed that is what you're running. I went back and looked at your other thread and now I see you have a 1st gen. My apologies!
Hmm... does the light go out if you turn the key to start?
My wild guess would be somewhere in the supply to the ignition switch or maybe the switch itself.
I don't know for the first gens, but the 2nd gens do like to have wiring problems in the steering column if the steering wheel adjustment is used a lot...
If you have the truck running with the FSS powered by a jumper- does the gen light stay on?
Absolutely nothing happens unless I power the FSS. If the FSS is powered, everything works fine. I’m in the process of replacing all of the fusible links with a fuse box, the only problem I’m having is figuring out is what is powered by the “T” (picture)to find out if I need all of the lines or not. I’m having trouble finding a diagram of what they’re going to, if anyone has easy access please let me know!! BUT WHAT DOES EACH POWER???
Ah, OK, now it makes sense- you have a break on the way from the battery to the FSS- as soon as you put power on the FSS you also have power on the other circuits.
If you didn't already cut anything I would go and check the fusible links with a powerful test light- like a headlight bulb.
A multi meter will tell you that there are 12V even if only the thinnest strand of wire is still there- high resistance.
Without flow the Volts are still there, but if the Ohms pinch the hose you will not get the Amps through.
If you can get a multimeter probe or alligator clamp on both ends of the wire, you can do a voltage-drop test - that will show you what's happening under load. As Alpine says, doing a normal voltage or continuity test is misleading as long as there is at least one strand making contact all the way through.
Voltage drop test works only as long as stuff works. (And is agood tool to find a lot of problems.)
But if the backfeeding works you would need to be extremely lucky to find the place the break is.
I always have an H4 bulb with 2 wires in the toolbox- one goes to battery - and the other end is to look if the circuit is "really hot" Often quick and easy to find problems in the supply to a fuse box.
Hey guys, just an update, I replaced the fusible links with circuit breakers but still no power. Had to order a better test light to check the power to the FSS. I’ll keep you posted.
08-22-2018, 02:22 PM
I went back and looked at your other thread and now I see you have a 1st gen. My apologies!
