Dim (weak) test light bulb on ground wire HEGO sensor

94 Ford F150 4.9l  I've had a no start condition for a while now, and today I tested the grounds and the HEGO (02 sensor?) Was very dim.  I front probed the disconnected harness for the test.  I then tried tracing the wiring and no luck in finding any damaged wires, however when I disconnected the 02 sensor, it still wouldn't start.  Should I start disconnecting the other 5v sensors?  One at a time?  I have a confirmed no spark situation, and no CEL.

No check engine light with key on with a Ford. Your computer is not alive.

That's correct.  It's really not alive when you front probe the harness thats un-hooked from the computer.

An oxygen sensor does not use the 5v reference circuit and is very unlikely to cause a no start.
If you suspect an issue with the reference circuit, a better test would be to check for 5v with multimeter at a sensor that is on that circuit, like the TPS sensor.
As for the dimmly lit test light, exactly what wire on the harness were you probing, and how was your test light connected? Regardless, no spark no start diagnosis generally won't involve any testing of the oxygen sensor circuits.

When you're connected to the upstream oxygen sensor heater ground, how dim is the test light? Compared to connecting the light directly across the battery. Is this an incandescent test light?

Disconnecting the O2 shouldn't have any impact on your no start. But you may be onto something with the test light being dim. The upstream oxygen sensor heater grounds at G101, near the battery on the right fender:

 

There's a bunch of other stuff that grounds there, too, including the PCM itself. Service info lists pin #40 and #60 at the PCM as power grounds, which end at G101.

I'd suggest a good visual of G101 and some gentle tugging on those ground wires in case any are barely hanging on with some green corrosion. While you're under the hood, I'd also suggest doing a quick 5V reference check. Either the MAP sensor, EGR position or TPS would be fine. Key on, pick a sensor and backprobe one wire at at time. DMM set to DC voltage, black lead on the battery negative post. At least one sensor wire should have 5V.

Pin 49, I think.  I made a comparison of the bright light just to make sure the picture of the dim light showed as dim.  I had some sun and shade interfering under the hood today.  Also, I posted a pic of what I think is pin #49

I posted some pictures on another reply. The pictures show how dim the light is. Thank you for your help.

Bronze, not sure how you are making out with this.Just to clarify my earlier comment a no check engine light with key on in a ford is a dead give away the computer is not alive, if everything is connected. The check engine light should go out while cranking if computer is receiving rpm signal during crank.

Yes Sir. I know exactly what you meant. And I appreciate you chiming in. I sent the computer off and it was given a good bill of health prior to me starting this thread. But now, I'm not so sure it's good. The guys who performed the repairs are highly spoken of, and I'm thinking they may have missed something? The repair guy told me he replaced all three capacitors (as they were very leaky), and he cleaned the board free of any leaking acid, then bench tested it, and then re-assured me yesterday that it had to be something else. So far, I've tested power and grounds to the harness, and it's all good. Also, I'm still not getting a 5v reference anywhere. The only part I've thrown at this thing is a new distributor. I did this prior to learning about scanner danner. Today, I did notice that pin 17 was very dim. I think I may have mis-posted that number earlier in this thread. I just learned the proper way to count the pins, and the dim light is definitely pin 17, but this was a power check, as pin 17 isn't really a ground? 17 shows STO or self test relay? But all my grounds appear fine. Sorry for the long wind, but if anyone could help, I'd sure appreciate it.... .

So with everything all plugged in, when you turn the key to on, engine not running, is your check engine light on?

No. CEL is not on with everything plugged in, and key on, engine off.

Also, I just finished doing a 5v reference check, and I'm only getting  .5 (one half of one volt) on all three 5 volt reference sensors.  

Do you have a wiring diagram for the truck? You will have to see how power feeds to the computer. You do not have a 5 volt reference because the pcm is not getting powered up. I see one power feed to computer on wiring diagram , pin 57 red wire. There are probably more , but need to look at a wiring diagram. Also pin 37 -red wire.

Bronze wrote:

No. CEL is not on with everything plugged in, and key on, engine off.

Also, I just finished doing a 5v reference check, and I'm only getting  .5 (one half of one volt) on all three 5 volt reference sensors.  

 

Awesome! You've got some excellent evidence of either PCM failure, or the PCM is missing something.

To recap, you should find power at the PCM connnector on pin #1, #37, and #57. Should find ground on pin #20, #40 and #60.

If those check correctly with your incandescent test light, cut the brown/white pin #26 in a repairable location near the PCM. That's the 5V reference from the PCM. If the brown/white on the PCM side now has 5V, there's a short in the 5V circuit. Recheck for MIL operation.

If the brown/white on the PCM side remains at .5V, the PCM is smoked. Which is funny, because you mentioned having the PCM sent off to someone that replaced some capacitors, right? I did the same to a customers PCM out of a '92 F-150. One cap was leaking, replaced all three just because. Afterwards, the PCM was still dead. Replacement PCM fixed it. 

Hey Travis, could you tell me where you bought your replacement computer for that F-150, in the event I need one? I did check all the powers and grounds by probing the harness only, while it was disconnected. Do I also need to back probe them with everything hooked up? And when you say check pin 26, you mean cut that wire while it's connected, and then back probe it to check the 5v reference, correct?