Did I fry something messing with my ignition?

85gtCOBRA · Oct 18, 2025

New member here - on a mission.

'87GT

Was attempting to tap into the ignition coil for a tach signal.
I think i may have shorted something out (stupidly).

Here's where I'm at and what I know:

-Crank, no spark, no injector pulse

-12V at coil connector + (key in run), 8.8V (cranking)
-12V at coil connector - (key in run), 8.8V (cranking), no voltage change, test light does not flicker

-12V at injector harness + (key in run)

-fuel pump primes when key on

T.F.I. module

Module bench tests OK
Put a new one in for good measure (and faint hope).

-12V at power in (key in run)
-2.26V at start signal in (key in run), 1.7V (cranking)
-12V at coil negative (key OFF) - not a typo, .22V (key in run) - not a typo, same to nothing when cranking ... WTF
-12V at ignition ground (key in run), 8.8V (cranking)
-8.8V at PIP (key OFF), .65V (key in run), no change when cranking ... WTF

I don't even know how to interpret what I am seeing here.

Can anyone out there make any sense of this?
What do I even look at next?

General karthief · Oct 19, 2025

See if the computer will talk to ya.

85gtCOBRA · Oct 19, 2025

General karthief said:
See if the computer will talk to ya.

It does.

AeroCoupe · Oct 19, 2025

By it talks to you are you getting codes? At a minimum you should get a Code 11 for all clear on the KOEO test.

85gtCOBRA · Oct 19, 2025

AeroCoupe said:
By it talks to you are you getting codes? At a minimum you should get a Code 11 for all clear on the KOEO test.

Yes, I got code 11 and a fuel pump code (because i had it unplugged). Interestingly enough, after plugging that back in (at the inertia switch) now the pump runs continuously with the key on. Only change made so far is a new TFI module. Didn't really expect it to fix the problem but i put it in there anyway. Do those voltmeter and test light readings mean anything to you?

AeroCoupe · Oct 19, 2025

When the pump runs continuously that is usually an indication of the ECU ting the bed. Take a look here and work through the problem:

Post in thread 'Fuel Pump won't stop running'

Sep 29, 2007

Fuel Pump Troubleshooting for 86-90 Mustangs

Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
It should run for 5-20 seconds and shut off. To trick the fuel pump into running,
find the ECC test connector and jump the connector in the lower RH corner to
ground.

If the fuse links are OK, you will have power to the pump. Check fuel pressure –
remove the cap from the Schrader valve behind the alternator and depress the
core. Fuel should squirt out, catch it in a...

85gtCOBRA · Oct 19, 2025

Don't like the sound of that. I was doing diagnostics, taking readings, key on/off a lot when this first happened. Pump would prime and shut off as expected. decided to unplug it, no need for it to keep doing that every key cycle. only since i put the dist back in with the new module did it start doing that. This is a crap-show.

nickyb · Oct 19, 2025

Most new tri modules are , try putting your old one back on and remember to use the special grease.

85gtCOBRA · Oct 19, 2025

nickyb said:
Most new tri modules are , try putting your old one back on and remember to use the special grease.

stupid thing bench-tested ok. the one in there is not original either but it was working fine. more of a hail-Mary than anything else. distributor has to come out.

85gtCOBRA · Oct 19, 2025

85gtCOBRA said:
OK, fuel pump is NOT running continuously with key on ... it was because i had the scanner plugged in.

85gtCOBRA · Oct 19, 2025

Update ... either the car is healing itself or my connections were not what they should be when i was testing earlier.

1. PIP out - 8.1V (key in run), 6.3V cranking ... this seems about right ... fuel pump primes when i ground the probe (as it should I believe

2. Start signal in 2.3V (key in run), 1.7V cranking ... clearly not what it should be - no change from before

3. Coil Negative - 12V (key in run), 9.3V cranking

For reference;

Could it simply be the coil itself got cooked?

The PIP is clearly sending a signal (of some sort) to the computer (fuel pump primes if I ground pin #6 with the key on)

rednotch · Oct 19, 2025

Check voltages at the fuse links both sides by the starter relay in all key positions, and the one taped up by the firewall ecu plug, then check going into and out of ecu relay. The low voltage is either a failed switch, short or a fuse link that gave out. Car doesnt even need the ecu plugged in for spark. Your pip isn't getting enough voltage so no spark no injectors.

Anyone know how to turn this retarded auto correct off on an old fire tablet. Keeps jacking up what I type out.

85gtCOBRA · Oct 19, 2025

rednotch said:
Anyone know how to turn this retarded auto correct off on an old fire tablet. Keeps jacking up what I type out.

I feel your pain on this one ^^^
And thanks for taking the time to help a guy out.

I know this is lazy as hell, but what do you think?

rednotch · Oct 20, 2025

Coils are cheap, but I've never seen a failed coil lose injection pulse, standard fd478 made in Poland coils is all ill run these days, too many are just repackaged China crap now.

85gtCOBRA · Oct 20, 2025

rednotch said:
Coils are cheap, but I've never seen a failed coil lose injection pulse, standard fd478 made in Poland coils is all ill run these days, too many are just repackaged China crap now.

Well, it's not that it (would have) failed on it's own ... I may have shorted it accidentally. So, potentially more violent than a slow death or wearing out. Pretty sure it was the coil it left the factory with.

85gtCOBRA · Oct 20, 2025

I tested the red/blue start signal wire from the ignition switch to ground and got the same 2.26V reading as I got at the corresponding pin on the module. Does that mean the problem is in the switch?

If I understand, that start signal wire is just an alternate path (vs the run power wire) for battery voltage to get to the module (from the ignition switch). If it's 2.26V directly off the switch to ground ... what else could it be?

rednotch · Oct 20, 2025

Make sure you have 12v into the switch, its fed by the fuse links, should be yellow 12ga wire if I remember right.

85gtCOBRA · Oct 20, 2025

rednotch said:
Make sure you have 12v into the switch, its fed by the fuse links, should be yellow 12ga wire if I remember right.

yes, 12V into the switch. both yellow wires. that start signal wire makes a bunch of stops ... neutral safety switch, ignition module, starter solenoid. starter solenoid is the final stop I think. pretty sure I've been thinking about this in reverse

AeroCoupe · Oct 20, 2025

The ignition switches have been known to fail and there was a recall on them. If its the original one I would replace it but that is just me. What transmission is in the car?

85gtCOBRA · Oct 20, 2025

AeroCoupe said:
The ignition switches have been known to fail and there was a recall on them. If its the original one I would replace it but that is just me. What transmission is in the car?

It's an auto.

Remember, not just a random failure. this is something i did - may have shorted the coil (connector) to ground. this is a super clean car, running near perfect before this crapshow (all me).

ignition switch looks really good (they fail physically i think - literally split apart. internally they are pretty stout, fat copper traces well separated).

here's the stops that signal wire makes:

It looks like that wire's first stop is the neutral safety switch, then it splits off and sends signal to the starter and the TFI module at the same time

Did I fry something messing with my ignition?

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wonder how much it would cost to ship you a pair

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lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

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I gotta say i never painted my nuts, Never Ever

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I'd like to remain having one chocolate starfish

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I'd like to remain having one chocolate starfish

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I'd like to remain having one chocolate starfish

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lube between the nut and the face. I know my lubes

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