Engine Fuel pump not running

NitrusReigns

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Dec 13, 2021
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Hey all,

I have a problem that I probably created for myself and I'm hoping you folks here can help me figure out what I did (other than take on a project that was over my head...). Here's the situation:

I bought a 1991 Mustang GT Convertible that had been converted from a 5-speed manual to an automatic that the guy drag raced it with. I wanted the 5-speed back so I bought the car and the old transmission and had the transmission rebuilt. I probably shouldn't have attempted something a transmission swap myself with my level of expertise/experience, but it just looked so straightforward on the internet :)

Anyway, here's what I did:
  • Used the videos LRM produced to pull out the automatic transmission and put in a T5 manual
    • Did everything exactly as in those videos (I believe)
  • Completed that process and realized the car didn't start - turns over fine, but no firing at all
  • Made sure I was getting spark at the plugs (by pulling one and putting it against the block)
  • Also loosened the air intake hose clamp and sprayed ether in - fired right up on ether without an issue
  • Tested the fuel pump relay and didn't get the voltage I expected so replaced it
  • Tested the inertia switch and it check out fine (basically just allowing current through)
  • Use the little test box thing with the jumper (in driver side rear of engine bay) to force the fuel pump relay and still the fuel pump wouldn't run (even with a brand new inertia switch)
  • Dropped the fuel tank and pulled the pump thinking it was bad, but it runs fine when I put 12V directly to it
  • Basically hot wired the fuel pump to the ignition so it got 12V when the ignition was on and it kinda ran, but not well (assuming that's because of the lack of ability of the computer to regulate it) - but it confirmed that the fuel pump running is my problem
Here's what's going on now:
  • Fuel pump isn't running and therefore engine isn't running
  • Fuel pump, relay, and inertia switch are good (or should be since they are both new), but I'd love more info on how to confirm the relay and inertia switch (electrical stuff has never been my forte)
  • Still cranks just fine, starts on ether, and starts kinda roughly when I force 12V to the fuel pump
So here's the question, of course: what now? What do I do next? I'm totally at a loss for why I can't get the fuel pump to run in it's normal electrical configuration and what else is going on here.

My assumption is that I screwed something up in my transmission swap process because it ran and drove fine with the auto trans in it, but I just can't figure out what I did that messed it up this badly...

PLEASE HELP!
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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Here is a checklist that will get you up and running, read the instructions and go through the list step by step, this verifies things are working because there could be more than one issue for the no fuel delivery that you have, don't skip around, sometimes it is something simple that can be missed when you skip a step.
Keep us posted on your progress here and if you have any questions post them here also so members don't have to ask about the car ie: the auto swap to manual or any aftermarket parts that can also be an issue
 

NitrusReigns

Member
Dec 13, 2021
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Beer City, USA
Here is a checklist that will get you up and running, read the instructions and go through the list step by step, this verifies things are working because there could be more than one issue for the no fuel delivery that you have, don't skip around, sometimes it is something simple that can be missed when you skip a step.
Keep us posted on your progress here and if you have any questions post them here also so members don't have to ask about the car ie: the auto swap to manual or any aftermarket parts that can also be an issue
I appreciate the help. This was actually one of the first things I did after having the issues since I'm new to these cars. I was delighted when I found that thread and worked my way through the list. The only thing I didn't get to yet was the fuse links - mostly because I can't find them. I'm glad you brought this back up because in my other troubleshooting and looking around, I had kinda forgotten about the fuse links.

I know where the starter solenoid is, but I'm not really sure what I'm looking for regarding fuse links (it's a new and foreign concept to me). Anyone happen to have pictures of what they look like and how to identify them?
 

John Dirks Jr

there is enough sticking out to grab on to
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I can’t remember the full details off the top of my head but maybe someone will chime in and elaborate if my idea here might be relevant.

There was a difference in o2 harness between the automatic and manual transmission setups from the factory. When swapping transmission from one to the other, sometimes a re-pin of the harness connector is needed. If it’s not done where needed it can damage the ECU. A damaged ECU can prevent the fuel pump from running.
 

KRUISR

5 Year Member
Apr 16, 2015
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There was a difference in o2 harness between the automatic and manual transmission setups from the factory. When swapping transmission from one to the other, sometimes a re-pin of the harness connector is needed.
I think the re-pin is needed if you swap to a manual ECU with an auto harness. If you use auto ECU with auto harness with a manual trans, only need to jumper a wire for neutral safety switch (at least in my case).

I was fighting a fuel pump not running as well (at least intermittently). I thought it was the relay and swapped that a couple times, still not long term fix. Turned out to be a connector behind the driver kick panel where the pins were not making full connection properly.

20211213_200832.jpg

Here you see the fuel pump relay and the connectors behind the kick panel. I would recommend to check them out for corrosion or loose connections.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
22,468
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203
polk county florida
I appreciate the help. This was actually one of the first things I did after having the issues since I'm new to these cars. I was delighted when I found that thread and worked my way through the list. The only thing I didn't get to yet was the fuse links - mostly because I can't find them. I'm glad you brought this back up because in my other troubleshooting and looking around, I had kinda forgotten about the fuse links.

I know where the starter solenoid is, but I'm not really sure what I'm looking for regarding fuse links (it's a new and foreign concept to me). Anyone happen to have pictures of what they look like and how to identify them?
It's early, (for me) only one cup of coffee so far so I'm typing slow.
You're looking for the fuse links, ok,
look for a yellow wire at the starter solenoid (relay) follow it, it will join up with other wires, the orange/lt blue wire is the fuel pump fuse link.
Oh, and yes, there can be an issue with O2 jumpers and computer damage, I'll have to get more coffee for that explanation.
I've said it before, I would send the computer (technically it's a 'electronic engine control unit' ) out to get checked, they are 25+ years old, there are better made capacitors, power supplies and other techno junk that we don't think about needs updated,
Disclaimer: I am not a 'mechanic ' but there are a lot of trees in my back yard to provide shade.
 
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Mustang5L5

Put lubricant all over the balls
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I wouldn't blame the ECU just yet. Yes, it should be checked, but he didn't state what ECU and wiring in the car so that queston needs to be asked.

#1) What is the ECU in the car
#2) How is the o2 sensor jumpered?

If you can, pull the ECU out and test continutiy from pin 46 to pin 40/60. You should have it.

The fuse link that powers the pump circuit comes off the stater solenoid on the big yellow wire, and is the brown fuse link D which feeds a pink/black wire. That wire runs to the fuel pump relay to feed the pump.

1639492164240.png



Down at the fuel pump relay, you should have power at the fuel pump relay on the Red wire with the key on, and the pink/black wire all the time.

You stated you jumped out the pin 22 ground at the test connector and it didn't work. That makes me think you do not have power on the red wire 351 with key on. The power for that wire is sourced through he EEC relay next to the ECU. The relay is triggered by the ECU and fed by a different fusible link. I would check that red wire first and the EEC relay and start there

1639492237791.png
 
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NitrusReigns

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Wow - thanks for the replies! You guys are awesome! I'll have to take an evening soon and sort through some of what's being asked/suggested here and get back to you all with some more intelligent responses.

Initially, however, I can say that I'm confused and disappointed. I thought the video series produced by LMR on how to swap from an AOD to a T5 was pretty detailed and well done. However, if they skipped something as pivotal as potential ECU issues.

The short version of what I've done is this - I haven't done anything major to the electrical system. I added the clutch switch, made sure the brake switch worked while I was under there, and reconnected the neutral sensor. I will have to go back through my notes, but I think that's all I did electrically because that's all the LMR videos suggested I do...

Again, I truly appreciate the replies! I'll look back into my notes, read the replies in more detail, and take a look at some of the things you asked for info about before responding again.
 

Mustang5L5

Put lubricant all over the balls
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I thought the video series produced by LMR on how to swap from an AOD to a T5 was pretty detailed and well done. However, if they skipped something as pivotal as potential ECU issues.

Yeah, that's a big gripe of mine. They leave out some info that is often debated as being unnecessary. I feel it is necessary, but i know some folks will disagree with me.

I have done an AOD-to-T5 swap on my car. I originally left the AOD computer in and it drove fine, but i would occasionally have a hanging idle. I made the changes to swap to a manual trans ECU and the issue resolved.

To do that you need to:
  • Swap out the AOD transmission harness to a manual trans harness so you can plug the NSS on top of the T5 in. Transmission Harness
  • Repin the O2 sensor harness for manual transmission based on the correct year. O2 harness repin
  • Install A9L or equivalent.

Don't me wrong. Even with the AOD computer on a 5-spd, you should still be able to drive the car around fine. I did so for about 10 years before finally going all manual wiring.
 
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