Hello,

I have a 5.0 EFI F250 with the divorced module ignition system. I was unable to pass smog the other day because the engine will not run below 1500 RPMs with the SPOUT pulled (I had to adjust the throttle stop to maintain a high idle to set base timing at 10 BTDC). In looking at my system I've observed that I have 4.0V on the START circuit while in run. If I pull the START wire at the solenoid, the idle (with SPOUT connector plugged in) drops and becomes a bit laboured, though the timing seems unaffected. I'm thinking that this stray voltage on the START circuit must be confusing the ignition module somehow...maybe through the Neutral Drive Switch circuit. My computer harness is made from scratch, and I had to add an external diode on the wire going to pin 30 (later computers had this diode built in, but my '85 did not). Could it be that I'm somehow seeing board voltage from the ECU back-feeding through the diode and causing this? I really didn't think I had to use a very specific diode. so I just picked a big honkin' thing thinking it would be pretty durable when wrapped up in the harness under the hood of a truck. Any ideas? I really appreciate it!

Thank you,

Rick

 

Disconnect the wire at the computer and see if the voltage goes away.

 

Your Ignition system should look like this so just move the selector out of park or neutral and see if the voltage goes away.

1986 EEC Auto 5.0L:


/

 

Yes, I'll do that. I certainly have more testing to do. The 4V is present in the wiring on the START circuit, which although it could be coming from some strange not-quite-a-short in the ignition switch (which is new), my suspicion is that it's coming from pin 30 on the ECU. Would it be possible for someone reading this to check the START terminal of their TFI module with their ignition switch in the RUN position and see what you get? Looking at the schematic, I don't see how we all don't have this condition, leaving me to wonder if I'm barking up the wrong tree.

 

What does your idle do if you unplug the IAC?

 

I was able to do some more testing today. I'm at the conclusion that the behavior I see in the START circuit is normal as near as I can tell. It appears that the primary relay coil in the starter solenoid is the ground path for pin #30 in the ECU, and I confirmed that this pathway is only completed when the transmission is in PARK or NEUTRAL. The 4V (actually 5V at the board in the ECU) is not enough to trip the starter solenoid, and when connected, the measured voltage at the START lead at the ignition module is 0V while running. I also tested for both spark and injector firing when the SPOUT is pulled and found I DO have injector firing, but NO spark. At that point I really started to think the module was bad, though changing it out with a new one didn't change the symptom. At this point I'm going to look at the other leads going to the ignition module...perhaps I have some kind of ground issue?

 

wait, what do you mean no spark?

 

Correct. With the truck off, and the SPOUT pulled, the engine will turn over and fire for a second or so while doing so, then nothing. It will just crank and crank without ignition. If the SPOUT is replaced, the engine will start right up, but when you pull the SPOUT while running, it will die...UNLESS you either: start with SPOUT out and goose the throttle to 1500 rpm or more and keep it there, it will stay running OR while running with the SPOUT in, raise the rpms to 1500 or more, hold it there, then pull the SPOUT, and it will keep running. And when I say running, I mean running fine, without any change in tone or smoothness. Yet as soon as you drop it below, even barely below 1500 rpm, the ignition will shut off like a switch was thrown.

After spending some time reviewing the schematics for this system, I'm going to revisit what I actually did on the distributor harness. I made it from scratch, and I may have botched the grounding...Attached are two schematics I used for reference. On the one that includes multiple schematics, I made up a harness that looks like the second one from the top on the left hand side. If you compare it with the second diagram take from an EVTM, there are some inconsistencies..I also took some liberties with which end I grounded the shielding at after reading that, technically, it doesn't matter which end is grounded, as long as it's one end only.

 

What year truck are you working on?

 

It's a 1985...with a home brew harness. I converted to a closed bowl distributor/fender mount module.

One question that comes to mind; in one diagram the IGN. GROUND goes to ground at the distributor, while in the other it does not. What is the purpose of IGN.GROUND (to ECU pin #16)? I figure it must be a sensitive signal pathway as it's in a shielded wire, and as it's in a shielded wire, what's the point of dumping the signal to engine/chassis ground at the distributor?

Thank you very much for your help!

 

Honestly it sounds to me like you have a faulty TFI module or weak PIP sensor inside the distributor.

 

For all EEC-IV Ignition systems the Ignition Ground point (to ECU pin #16) source is inside the distributor.
Sorry I do not see a diagram above that shows it any different.

The shield ground is also the distributor and the other end of the shield can not be grounded or you can have ignition problems.

I do not understand what you mean by " what's the point of dumping the signal to engine/chassis ground at the distributor?".
No signal is dumped to ground at the distributor.

 

I wonder why the lead coming from ECU PIN#16 is shielded all the way from the ECU to the distributor of it's shorted to ground anyway? That would be the same thing as shorting #16 to the sheet metal in the cab at the ECU.

I am confident the CMP (pickup assembly) is working fine because it does produce a good PIP signal, which is all I think it's supposed to to. I'll recheck some pathways if I can steal some time between work shifts...

 

The same reason the ground wire inside a USB cable is also shielded. It helps to reject noise and interference.

 

I think the ground wire in a USB cable is shielded is due to a matter of packaging. The circuit in a USB application is made up of different devices that are consumer products in plastic enclosures arranged in a truly random configuration, whereas the devices in an automobile are fixed-mounted and enjoy a common ground plane. I could be wrong in my understanding, but that's how I see it at present...

One thought just occurred to me...What if I'm seeing a PIP signal at the ECU, but not the ICM? I did not get a chance to do any testing today, but I just thought it might cause exactly the symptom I'm having. The connector I got for the ICM fits pretty loosely, even though it's new. If I was getting a SPOUT signal the module might work fine as long as the SPOUT connector is in, but once the SPOUT connector is pulled the module would need to see a PIP to do anything. However that wouldn't explain why it works above 1500 rpm