Fox Please help: Electrical load creates missfire ?!
- Thread starter martroy
- Start date
CassieL
Active Member
Ok so other than MS it's stock.
So what I'm getting is at or around 3k rpm,
If you turn on a load ( blower) you get a misfire and the rpm reading goes nuts.
If no electric loads does it misfire?
When it does misfire, which cylinders?
I suspect o short to power. But some things need to be eliminated. Have you connected the condenser to the coil? If not, do that first. Also is the shielding in the tfi harness intact?
So what I'm getting is at or around 3k rpm,
If you turn on a load ( blower) you get a misfire and the rpm reading goes nuts.
If no electric loads does it misfire?
When it does misfire, which cylinders?
I suspect o short to power. But some things need to be eliminated. Have you connected the condenser to the coil? If not, do that first. Also is the shielding in the tfi harness intact?
CassieL
Active Member
CassieL
Active Member
So here's a test. Once the condenser is connected take either your blower out or use a spare, with the car running, connect the blower directly to the battery with jumpers or a power probe if you have one.
Does the symptom return? If no, then you have a short in the dash harness and i would definitely check the ignition switch.
Do this and report back.
Does the symptom return? If no, then you have a short in the dash harness and i would definitely check the ignition switch.
Do this and report back.
Just to make it clear, misfire is cause by revlimiter cause rpm reading goes nuts.
No major electric load and higher rpm, no wrong rpm reading.
Condenser, I havent cause I dont have one since I owned the car (past 20 years)
Shielding is intact and ground only 1 end near PCM.
I've done a similar test (without condenser) spare electric fan straight on the battery and I repeat the behavior.
At the beginning, I was focusing on blower wiring and with that test, it confirm load related.
CassieL
Active Member
So really it's the crazy rpm pip signal input to MS that triggers the rev limiter. Have you tried turning the rev limiter off.
I'm sorry if I'm not getting everything, I'm not there so diagnosis over a forum is difficult at best.
I'm sorry if I'm not getting everything, I'm not there so diagnosis over a forum is difficult at best.
I could turn off rev limiter and that will stop spark cut but will still have pip signal messed up.
CassieL
Active Member
Try this.
Leave the key off. Make sure your in park or neutral.
Find the eec relay and jump it on.
This will power on everything needed to run engine. Jump the starter solenoid to start engine. Does the symptom return?
What i want to see is the system running and not connected to key on power.
Also the condenser does more than just cut emi from the radio. This still sounds like a short to voltage.
Now this is important to note. I have had a fair share of running my tail off chasing running problems only to come down to multiple faulty tfi modules.
Leave the key off. Make sure your in park or neutral.
Find the eec relay and jump it on.
This will power on everything needed to run engine. Jump the starter solenoid to start engine. Does the symptom return?
What i want to see is the system running and not connected to key on power.
Also the condenser does more than just cut emi from the radio. This still sounds like a short to voltage.
Now this is important to note. I have had a fair share of running my tail off chasing running problems only to come down to multiple faulty tfi modules.
CassieL
Active Member
I'm not sure jumping eec relay will give me power to TFI and coil, will have to look schematic. Several things to try in the next days. I want too to remove all wires from EEC harness that are not needed to run engine. (Speed aensor, pressure sensor, etc)
As for TFi, from a friend' of mine, I've tried his TFi from a working car and issue is the same. This was after I test my BNIN motorcraft one and still getting issue.
One side note a bit irrelevant. When testing different alternator, to reproduce the issue, its not at the same RPM.
Here's an exemple.
Alternator A - misfire at 1,8k or higher
Aternator B - misfire at 2k or higher
Alternator C - misfire at 3k or higher
All of them were charging the same voltage.
What is PNP? I live Canada and checked.on Amazon and Rockauto, and dont find. Still looking. Any part number?
CassieL
Active Member
PNP is pick n pull wrecking yard.
SO, what this has come down to is a bad pip signal.
You see this by scoping the pip at the tfi module.
When i look at the scope capture I'm seeing a failure of the pip signal. The signal is losing its proper waveform and giving just enough of a signal to make the ecu think it's a higher speed.
This seems to be aggravated by a high electric load.
You said if alternator was disconnected the symptom goes away, but you've tried other alternators.
I fail to see anything causing a corruption of the pip at the output of the module. I know that scoping the power to module you get noise. But what's the sweep of that voltage? The system can handle quite a bit of that and still run fine.
I had my truck start misfire and stall after about a half hour of running and the hotter it got the worse it was, leaving me stranded for hours. I replaced the tfi module, the distributor and coil and nothing helped.
When i scoped the ignition i captured the voltage, pip and coil trigger.
What i found was voltage was dropping as it warmed up and eventually dropping to near zero. Turned out to be a coroded wire and the ignition power splice.
Around 9 volts the ignition started cutting out. The pip would randomly drop out from lose of voltage.
My point is i learned what the system can handle in terms of voltage.
If you remove circuits from the ecu, don't cut them just remove the pins from the connector. You might want them later.
When the symptom happens and you get a bad pip and noisy power, is this also at the positive battery terminal?
SO, what this has come down to is a bad pip signal.
You see this by scoping the pip at the tfi module.
When i look at the scope capture I'm seeing a failure of the pip signal. The signal is losing its proper waveform and giving just enough of a signal to make the ecu think it's a higher speed.
This seems to be aggravated by a high electric load.
You said if alternator was disconnected the symptom goes away, but you've tried other alternators.
I fail to see anything causing a corruption of the pip at the output of the module. I know that scoping the power to module you get noise. But what's the sweep of that voltage? The system can handle quite a bit of that and still run fine.
I had my truck start misfire and stall after about a half hour of running and the hotter it got the worse it was, leaving me stranded for hours. I replaced the tfi module, the distributor and coil and nothing helped.
When i scoped the ignition i captured the voltage, pip and coil trigger.
What i found was voltage was dropping as it warmed up and eventually dropping to near zero. Turned out to be a coroded wire and the ignition power splice.
Around 9 volts the ignition started cutting out. The pip would randomly drop out from lose of voltage.
My point is i learned what the system can handle in terms of voltage.
If you remove circuits from the ecu, don't cut them just remove the pins from the connector. You might want them later.
When the symptom happens and you get a bad pip and noisy power, is this also at the positive battery terminal?
CassieL
Active Member
Yes if you turn on eec relay the ignition is also powered. What this test is doing is isolating the engine system from other key on accessories
CassieL
Active Member
Ok for PNP, will look into.
For bypassing ignition, other thing is dash wont be powered so alternator wont be powered to by the exciter. I'll have to bypass it too if I want to that test.
For pin, yes I will just remove them, not cut them because for sure, I will reuse them.
I'm not sure if I have done that check at the battery post but I could clip the scope there and see.
