Fox Please help: Electrical load creates missfire ?!

M

martroy

Member
Sep 29, 2024
73
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18
Canada
#1
Hi guys,

I do have a 91 Mustang and I do have a random missfire I can't put the finger on it but found a way to reproduce the issue all the time. When tuning on blower motor over 2k RPM, it creates a missfire. Tachometer drops few hundred RPM and than recover a millisecond later. If I remove spout connector, I can't reproduce while tuning on the blower but will happen randomly.

What I've done is to remove unnecessary fuses to testing it on the road and issue remains.

Battery is in the trunk. All ground are solid.
I've pretty sure it's wiring related but not quite sure how to troubleshoot that one and would love help from you ?

Thank you,

Martin
 
#2
You need to check your coil saturation. I'm guessing that it's on the low side. You [should] get the same voltage or very close to it, when you test voltage at the coil against voltage at the alternator/battery with the engine running. When you do this test, ensure you are using the coil's ground and not battery ground when testing voltage at the coil. Use battery ground when testing voltage at the alt/batt.

If not, improve or replace your wiring from the battery to your coil then perform the test again.


My other thought is that you may have some component or set of wires that is robbing you of available current. Check all of your ground connections (by that, I mean find them, disconnect them, clean them, and reinstall them).

I'm curious if turning the headlights on and off, under the same conditions will produce similar results.


Check for these things as you have time and let use know what you find. I have a couple of possible fixes in mind but that's putting the cart before the horse. I've been in your scenario before with my 86, if your issue is the same.
 
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#3
First of all, thank you for giving me your feedback.

As for coil, I can do that test but wondering how turning blower is affecting coil? If that helps, its a megasquirt mustang and it logs, when issue arrives, RPM goes to the moon (20k RPM) witch I guess is on the PIP wire?

As for headlight, I have done that test like defrost too and issue is not repetable that way. Dont forget turning on blower is only a way I've found to repeat issue on demand.

If coil test still valid, just to make sure, while engine running, black probe to coil harness and red probe to coil harness and write it down number. Than, red probe to alternator stud and black probe to ground post battery and compare?

While running, coil voltage wont flicker?

Thanks
 
#4
martroy said:
While running, coil voltage wont flicker?
Click to expand...

It may jump around a little bit but that part is not important.

If you see 14.5 volts at the alt/batt with car running then you see be very near that reading at the coil.


martroy said:
If that helps, its a megasquirt mustang and it logs, when issue arrives, RPM goes to the moon (20k RPM) witch I guess is on the PIP wire?
Click to expand...

This is new info.
 
#5
Noobz347 said:
It may jump around a little bit but that part is not important.

If you see 14.5 volts at the alt/batt with car running then you see be very near that reading at the coil.




This is new info.
Click to expand...
Do you still think coil testing should be done?

When it happens, stock tach goes down for few millisecond and recover while in logs, it goes to the moon....

I hope I will find the issue soon....
 
#9
Still doing some test tonight and still no clue. I have done:

---------------
1. For the wiring check, measure continuity from pin 36 at the ECM connector, ECM and TFI module disconnected, to the SPOUT pin at TFI connector, if it reads @/over 5 ohms, need to trace the wire for an open.

2. TFI and ECM disconnected, key on, check pin 36 for voltage to ground (short to VPWR if you read any voltage)...you can use pin 40 or 60 as your ground reference.

3. ECM and TFI disconnected, check resistance from pin 36 to pins 16, 20, 40, 46 & 60 (short to ground if you get anything below 10k ohms).

4. Same as 3., check resistance from pin 36 to 26 (short to VREF if you get anything below 10k ohms).

5. Same as 3., check resistance from pin 36 to 56 (short to PIP if you get anything below 10k ohms).
---------------

Everything checks in. I was trying to find out the wiring for blower in case something will jump in my face and based on wiring, 2 grounds on left center console are for blower and resistor. When I unscrew them, blower still working and seems "normal" cause I've verified with a friend's car and same behavior. Where is getting the ground than?

All grounds I know of are good since the past years and wiring is as follow.

Battery negative to rear torque box bolt.
Driver swar bar bolt to engine timing cover bolt.
Engine compartment firewall to rear driver cylinder head

Battery positive to driver fender solenoid.
Driver fender solenoid right side to alternator and all relays, devices, door lock, and starter solenoid
Driver fender solenoid left side to starter

I dont have stock ECU to switch back. I guess something makes a spike on PIP/TFI/Spout line when I'm tuning on blower and it happens but not sure how and where to put my probe with multimeter ?

Thanks
 
#10
I am not an expert in this but you may be dealing with an electrical noise issue.
Could be excessive noise created by a problematic device such as the blower motor. You may try putting a radio noise suppression capacitor across the 2 motor power leads to see if it helps.
Another possibility is sometimes shielded wires are used to protect wires from picking up the noise. For them to work, the shield needs to be grounded. I have seen shielded wires used on distributors in the past. If you car has them, check the shield to make sure it is grounded on both ends of the foil shield (verify there are no internal breaks).
 
Last edited:
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#11
#13
dcm0123 said:
I am not an expert in this but you may be dealing with an electrical noise issue.
Could be excessive noise created by a problematic device such as the blower motor. You may try putting a radio noise suppression capacitor across the 2 motor power leads to see if it helps.
Another possibility is sometimes shielded wires are used to protect wires from picking up the noise. For them to work, the shield needs to be grounded. I have seen shielded wires used on distributors in the past. If you car has them, check the shield to make sure it is grounded on both ends of the foil shield (verify there are no internal breaks).
Click to expand...

Hi,

Just to make it clear; The blower thing is only a way to reproduce the issue but my main concern is it's happening randomly. I do share your thinking on electrical noise but can't figure it out.

Regarding shield wire, Based on that diagram (https://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif) I do have one but can't figure it out visually where is it. It's the stock foxbody harness.

Thanks
 
#14
Blown88GT said:
Bad connection somewhere.
Your MS-2 is one of the originals.
Unplug & replug this adapter harness.
Then check TS settings.
https://www.diyautotune.com/support...our-fox-body-mustang-5-0/5-0-mustang-adapter/
Click to expand...
Hi,

I'm pretty sure I do have a bad connection somewhere but can't figure it out. I do have a MS3 but original adapter harness. All settings proposed in that article are for a new configuration, witch isnt my case. Anyhow, I double check what's in the article and all settings checks in.

Thanks
 
#15
I wonder if it is the switch itself:

1728234373364.webp


This is out of the 1989 EVTM manual.
 
#17
martroy said:
Hi,

Not sure if that helps but if I remove blower fuse, I will have the same behavior randomly?!
Click to expand...

I don't know what it means, "to have the same behavior randomly".

What you are looking for is short at pin 754, 752, or 57.

The other thing you should be looking for is any stray power at those pins or broken grounds. There should be no stray power at those pins and they should [all] have continuity to ground.

If for instance, one of those grounds broken, or corroded, or even connected incorrectly, then it can cause the kinds of issues that you're seeing.
 
#18
Noobz347 said:
I don't know what it means, "to have the same behavior randomly".

What you are looking for is short at pin 754, 752, or 57.

The other thing you should be looking for is any stray power at those pins or broken grounds. There should be no stray power at those pins and they should [all] have continuity to ground.

If for instance, one of those grounds broken, or corroded, or even connected incorrectly, then it can cause the kinds of issues that you're seeing.
Click to expand...

Here's tests I've done.

Key off,

Connector removed from panel and

probe in pin 57 of connector to seat bolt = 0 ohms
probe in pin 261 of connector to seat bolt = 2,6 ohms
probe in pin 752 of connector to seat bolt = 0,5 ohms
probe in pin 754 of connector to seat bolt = 0,5 ohms

Key on and selector to floor
Connector removed from panel = fan remains on in low speed.

probe in pin 57 of connector to pin 261 = 7,75v
probe in pin 57 of connector to pin 752 = 2,91v
probe in pin 57 of connector to pin 754 = 0,92v

Key on and selector to floor while backprobing connector (still connected) with testlight

probe in pin 57 and 261
Low speed = testlight is really bright
Medium 1 speed = testlight is dimming a bit
Medium 2 speed = testlight is dimming more
High speed = No light ?!?!
 
#19
I'm still in the middle of nowhere cause I'm still not sure what is causing this random issue. Every wiring and grounds I've checked seems ok and blower switch too.

When spinning a distributor connected in the harness and key on, I cant reproduce the "blower thing".

Wondering what is different than running ?! Or how/where should I use multimeter to get results and direction to follow.

Thank guys
 
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