2010 Expedition Intermittent No Start/No Crank

[touchdowntexas]

Thanks again, and Happy Thanksgiving! I did a little bit more diagnostic work with the multimeter and I feel like my truck should be starting. I'm in the process of charging the battery as that was reading low and maybe that will miraculously fix it. But here's what I found. I also attached my wiring diagrams which is a little bit different from yours but more or less the same.

Starter Relay:
Pin 1 key out: 4.3 mV
Pin 1 key in: 28.3 mV
Pin 1 key start: 11.44V then drops to 28 mV after a second or two

Pin 2 key out: 60 kOhm
Pin 2 key in: 0.L (open/infinite resistance)
Pin 2 key start: 0.L => 35 MOhm => 9 MOhm => 7 kOhm => 0.7 kOhm => 0.L (all of this happens within a span of ~2 seconds which may be too fast for the multimeter to display the lowest readings, I did see minimum values of 0.08 kOhm and 0.1 kOhm)
It seems like it's functioning properly? Like the PCM has an open circuit but when I turn the key to start the resistance quickly drops, although maybe not enough?
Pin 2 to PCM connector: 0.3 Ohm (so the wire going from the junction box to the PCM is good)

I also measured some ground points around the truck just to get a feel for what a ground resistance should be reading.

Battery to passenger firewall: 72 Ohm
Battery to passenger fender: 74 Ohm and 68 Ohm
Battery to passenger frame rail: 52 Ohm
Battery to random body bolt: 84 Ohm
Battery to battery: 0.3 Ohm

So what I know is, I can jump the starter relay and start the truck no problem (at least I could a few days ago, maybe the battery is low). I get battery voltage coming in to Pin 1 when I try to start, and I get the proper ground (I think) to Pin 2 when I try to start. These things should function the relay and start the truck, but it's not. Swapping relays was one of the first things I tried, so I don't think it's a bad relay.

Got any other thoughts on what to try? The battery could be a legitimate issue; I just made a 12 hr drive last weekend so it should have been fully charged. Either battery is on its way out or the alternator is not doing its job. I'm not all that hopeful on the battery though; last time I was having these intermittent starting issues, charging the battery did not fix it.

 

[dkarrels]

Thanks for sharing your wiring diagram. It looks to me like the PCM controls both the input voltage to the starter relay Pin 1 and the connection from the starter relay Pin 2 to ground. With this design, the fact that you see +12V at Pin 1 when the ignition is in start position indicates the PCM thinks all conditions are met to start the engine. This reduces the risk of not knowing about some PCM internal logic or sensor issue.

The fact that you see Pin 2 start as open and then reduce to a relatively small number further indicates that the PCM is closing Pin 2 at the starter relay (Pin 7 at the PCM) to ground, suggesting it is working properly. The timing of both Pin 1 and Pin 2 moving around is new to me so I'm not sure if it's a problem. One thing you could try is to jump Pin 7 to ground directly, and then try starting the car. If the starter activates, then the PCM Pin 7 is not closing fully enough to ground. I don't know that we can draw any conclusions if it doesn't work -- it may or may not be a problem.

Do you hear a click on the starter relay when turning the ignition to start? A lot of what you write above sounds like a starter relay issue. You could also bench test the starter relay by removing it, then connect Pin 2 to ground, and touch Pin 1 to +12V. You should hear a click. If not, then the starter relay is bad. Since this is a low-power circuit, be sure not to hold that circuit closed for long -- touching Pin 1 via jumper to the positive side of the battery for momentary testing is sufficient.

Another thought to check the PCM is to remove the starter relay, connect an ammeter between Pin 1 and Pin 2, and try starting via the ignition switch. If the PCM is providing +12V to the starter input (starter relay Pin 1) and closing its internal relay to ground (PCM Pin 7), then you should see some current flow across those two pins when turning the ignition to start.

A third option to check is if power is reaching the starter solenoid when turning the key to ignition (and everything plugged in). That check will test whether anything upstream is broken.

 

[touchdowntexas]

Well...the freshly charged battery did nothing, which I'm not surprised about. I did bench test the relay and it clicks so seems to be functioning properly. I'm thinking whatever I'm seeing on Pin 2 is likely not a good enough ground, and grounding it separate of the PCM would likely fix my issue. Before going that route I'm going to check the ground right next to the PCM connections and make sure it's as good as the others. I checked the ground ~12" to the right of the connections and it seemed fine. But I didn't notice the ground just to the left of the PCM connection until today. If for some reason that ground has a bad connection and the PCM uses it, maybe I can clean it up and get it working. Otherwise I'm either wiring up a permanent ground to Pin 2 or forking out $ for a new PCM.

 

[touchdowntexas]

Fixed it! And it was a very very dumb fix. Unplugged all my PCM connectors and cleaned everything with electronics cleaner and the truck started right up. I'm gonna guess a bad connection was preventing that ground from occurring. So simple, yet so frustrating!

 

[dkarrels]

Awesome