Fuel pump priming circuit

JD1964 · Nov 17, 2019

I’m still chasing a problem. You may have seen in my other thread about swapping relays. The new distributor did not fix it.

Here is something probably related that I should fix first. With KOEO, the fuel pump won’t stop running. It should run for a few seconds to prime then stop. It doesn’t. When I turn the key on to prime the system, the pump will run constantly until I turn the key back off. Its not the relay as I’ve tried two other relays and the same condition occurs.

What would make the pump continue to run when it should be cutting off after the initial prime?

Somehow I think this is related to my no start condition.

Blown88GT · Nov 17, 2019

You asked this question & it was answered in the other thread in post #7...ECU.

swapping relays

Are the fuel pump and AC relays that are on the fender well near the MAF the same part? I'm having what might be intermittent fuel delivery issues and I think it might be the relay. I was gonna swap the AC one and see if it solves the problem.

www.stangnet.com

jrichker · Nov 17, 2019

For sake of clarity....

Fuel pump runs continuously:
The fuel pump relay contacts are stuck together or the light blue/orange wire (pin 22) has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use an ohmmeter to check out the resistance between the light blue/orange wire and ground. You should see more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t jumpered to ground.
If the wiring checks out good, then the computer is the likely culprit.

Prior to replacing the computer, check the computer power ground. The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972

General karthief · Nov 17, 2019

I will add to the post above, if the hose between the pump and the tube on the fp hanger is split or loose your pump will run on too.

JD1964 · Nov 17, 2019

General karthief said:
I will add to the post above, if the hose between the pump and the tube on the fp hanger is split or loose your pump will run on too.

Are you sure about this? I ask because the system never measures pressure as a reference point for the computer to act on. The initial key on engine off prime is simply timed electronically by the computer. No reference to pressure is made.

At least that’s my understanding

General karthief · Nov 17, 2019

I say this because it happened to me. I will add that there were several issues at the time (most self inflicted) but nothing involving the computer it self.

JD1964 · Nov 17, 2019

I ran out of time this evening to work on it further. I’ll check for blue green/orange wire shorting to ground before deciding to swap computer.

Luckily, I have a spare A9L on the bench. Its a factory unit with an unbroken seal. Hopefully it’s good if I need it. When we bought the convertible we found it in the trunk. The convertible is a 5spd car originally that was converted to automatic. It came with an A9P installed, and I think the A9L we found in the trunk was the original computer when the car was 5spd

a91what · Nov 17, 2019

Ah good you do have a spare, if that one is bad I have an a9l in a box somewhere.

JD1964 · Nov 18, 2019

This car has an SCT chip that’s tuned for the V3. If I do need to swap a new computer in, is it ok to try it without the chip installed first? I would make sure to not push it and keep rpms lower to make sure there’s no critical condition caused by too much spark advance or not enough fuel.

TOOLOW91 · Nov 18, 2019

John Dirks Jr said:
This car has an SCT chip that’s tuned for the V3. If I do need to swap a new computer in, is it ok to try it without the chip installed first? I would make sure to not push it and keep rpms lower to make sure there’s no critical condition caused by too much spark advance or not enough fuel.

No I would swap the chip . I doubt with whatever injectors and stuff you have this it will run well at all

TOOLOW91 · Nov 18, 2019

General karthief said:
I will add to the post above, if the hose between the pump and the tube on the fp hanger is split or loose your pump will run on too.

John is right . The ecu has no way of measuring or knowing fuel flow or pressure . So whatever issues you had going on at the time contributed to that happening . I could see you having fuel pressure / flow issues causing drive ability issues but that’s it

General karthief · Nov 18, 2019

TOOLOW91 said:
John is right . The ecu has no way of measuring or knowing fuel flow or pressure . So whatever issues you had going on at the time contributed rv, to that happening . I could see you having fuel pressure / flow issues causing drive ability issues but that’s it

Oh, you can only imagine the issues, that is what brought me stangnet a few years ago, and I agree with john, after thinking about it I did have to replace my computer!

90sickfox · Nov 18, 2019

With that car being auto swapped I'd unplug the o2 harness before trying the other computer. Or at least check the jumper to make sure its correct. That harness is located on the passenger side by the strut tower and where the carbon canister would mount. It runs down between the rack and oil pan.

That harness has smoked more than a few computers.

JD1964 · Nov 19, 2019

90sickfox said:
With that car being auto swapped I'd unplug the o2 harness before trying the other computer. Or at least check the jumper to make sure its correct. That harness is located on the passenger side by the strut tower and where the carbon canister would mount. It runs down between the rack and oil pan.

That harness has smoked more than a few computers.

The black LX which is the car with the possible bad ecu is not auto swapped. Its 5spd and always has been. So there is no reason for me to check the pin configuration on the o2 harness.

The red convertible is the one that had the trans swapped from 5spd to auto.
Anyway, what else can cause ecu failure? What else can I check and possibly correct to reduce future ecu failure potential?

TOOLOW91 · Nov 19, 2019

John Dirks Jr said:
The black LX which is the car with the possible bad ecu is not auto swapped. Its 5spd and always has been. So there is no reason for me to check the pin configuration on the o2 harness.

The red convertible is the one that had the trans swapped from 5spd to auto.
Anyway, what else can cause ecu failure? What else can I check and possibly correct to reduce future ecu failure potential?

Bad caps , bad solder joints .

Boostedpimp · Nov 19, 2019

had same issue in a 90lx of mine... fuel pump wouldn't stop priming and end result was the a9l was the problem

Blown88GT · Nov 19, 2019

John Dirks Jr said:
The black LX which is the car with the possible bad ecu is not auto swapped. Its 5spd and always has been. So there is no reason for me to check the pin configuration on the o2 harness.

The red convertible is the one that had the trans swapped from 5spd to auto.
Anyway, what else can cause ecu failure? What else can I check and possibly correct to reduce future ecu failure potential?

Capacitor Repair: Ford A9L ECM « Moates Support

JD1964 · Nov 19, 2019

Awesome link to capacitor repair! I’ll be all over this for sure!

thanks

jrichker · Nov 19, 2019

John Dirks Jr said:
Awesome link to capacitor repair! I’ll be all over this for sure!

thanks

@Blown88GT
Capacitors come with some ratings that people often overlook, One is operating temperature (105°C is preferred for automotive work) and operating hours at a specified temp. Look for the longest lifetime at 105°C . Cheap out here (usually less than a dollar difference between the best and the cheapest) and you might get to replace the caps again if you drive the car a lot.

JD1964 · Nov 23, 2019

Here’s the donor A9L. Is there any way to tell by the numbers what year car it came from?

Fuel pump priming circuit

JD1964

there is enough sticking out to grab on to

Blown88GT

swapping relays

jrichker

StangNet's favorite TOOL

General karthief

wonder how much it would cost to ship you a pair

JD1964

there is enough sticking out to grab on to

General karthief

wonder how much it would cost to ship you a pair

JD1964

there is enough sticking out to grab on to

a91what

SendMeUrDataLog

JD1964

there is enough sticking out to grab on to

TOOLOW91

If you're the village idiot what's that make me?

TOOLOW91

If you're the village idiot what's that make me?

General karthief

wonder how much it would cost to ship you a pair

90sickfox

Wasn't a pretty sight...and I've got big hands

JD1964

there is enough sticking out to grab on to

TOOLOW91

If you're the village idiot what's that make me?

Boostedpimp

Blown88GT

JD1964

there is enough sticking out to grab on to

jrichker

StangNet's favorite TOOL

JD1964

there is enough sticking out to grab on to

Similar threads

Fuel pump priming circuit

there is enough sticking out to grab on to

StangNet's favorite TOOL

wonder how much it would cost to ship you a pair

there is enough sticking out to grab on to

wonder how much it would cost to ship you a pair

there is enough sticking out to grab on to

SendMeUrDataLog

there is enough sticking out to grab on to

If you're the village idiot what's that make me?

If you're the village idiot what's that make me?

wonder how much it would cost to ship you a pair

Wasn't a pretty sight...and I've got big hands

there is enough sticking out to grab on to

If you're the village idiot what's that make me?

there is enough sticking out to grab on to

StangNet's favorite TOOL

there is enough sticking out to grab on to

Similar threads

Privacy & Transparency

Privacy & Transparency