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What sensors and signals ECU to send trigger to trip relay to start fuel pump?

XJ XJ6 / XJR6 ( X300 ) 1995-1997

What sensors and signals ECU to send trigger to trip relay to start fuel pump?

 
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Old 06-05-2019, 09:23 PM
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Exclamation What sensors and signals ECU to send trigger to trip relay to start fuel pump?

Following the recent seeming solution to my long suffering "dies while stopping" issue, which was the replacement of the IACV (Idle Air Control Valve).... the XJR ran great for about two weeks with no trouble. Then one morning two weeks ago while driving to work, I lost power and had to coast to the side of the highway.

The car wouldn't restart, although it would seem to occasionally nearly catch, so I had it towed back to the mechanic.

He's done a lot of troubleshooting, and eliminated the typical suspects such as the crank position sensor (new recently replaced), camshaft position sensor (new recently replaced) and a few other things.... and has narrowed the issue down to be that the primary fuel pump (XJR has two pumps, second kicks in around 3500-4000 RPM) is not getting the ground trigger from the ECU either at all or intermittently. The pump relay is getting supply power reliably, just not trigger.

So maybe I had two issues all along, IACV and intermittent fuel pump


He's tracing the wiring harness now to verify there is continuity from ECU to the relay for the primary fuel pump, so we haven't ruled out a broken wire or bad connection yet.

We haven't ruled out a faulty ECU either, but we're not making that determination yet. I have two spare ECUs we can test with if we go that route.

But the other side of the equation which so far he hasn't found reference to in the service manual is what signals the ECU is expecting before it will trigger the fuel pump so we can check that those signals/sensors are happening properly.


So, here's the question: What signals must be received and from which sensors before the ECU will trigger the fuel pump to run?
 

Last edited by al_roethlisberger; 06-05-2019 at 09:28 PM.
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Old 06-05-2019, 09:49 PM
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There is one fuse in the 2 heelboard halves that provide power to control or close both relays to contact main power to the pumps

The ECU provides a command ground

This one fuse also provides power to the OBD 2 port that has been found to have bent pins before

To directly power the # 1 pump would be to bring power from a terminal post like the trunk fuse box to the fwd most socket in the relay base

Or you can bring power from the aft most socket to the same fwd socket

A consideration is to not stuff a wire and hope for proper amperage to run the pump properly

use a blade type connector for better contact

See page 36 as fuse # 10 / 5 amp RH heelboard fuse box

You can swap the king relay ( ignition positive relay by name ) that controls that fuse box with the one at the LH engine fuse box which in reality is only the horn relay ( I hear a test toot )

http://jagrepair.com/images/AutoRepa...f:icon_banana:

Researching on my new computer
 

Last edited by Lady Penelope; 06-05-2019 at 10:13 PM.
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Old 06-05-2019, 10:02 PM
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Thanks, yes he's verified all of that so far, other than the OBD port short info.

He printed out the wiring diagram and has apparently taped it together to trace So I have to give him credit. He's a smart and diligent guy.

But while doing that, he wondered what signals (and sent from which switches or sensors) the ECU must receive before it commands and delivers ground to the relay. We want to be able to check those sources too. Does anyone have the list of what must happen before triggering the fuel pump to start/run?
 
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Old 06-05-2019, 10:17 PM
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As you rotate the key to the on position a command ground is provided to the ECU so it in turn provides a short 4 second fuel rail priming then off by design

On page 38 the ignition switch has a ground on pin 5 and the rotation position closes pin 5 to X

You can check for corrosion on the pins an a good # 5 car ground

I'll be out most of tomarro so let me hurry tonight

I have a pic of the connector map numbers but is on my old computer that only opens in safe mode so the pics are safe but will take some time to retrieve

The # 10 fuse travels through the large connector above the fuel tank which can be tricky . This is the control power to the relay and not the control ground provided by the ECU







Editing
 

Last edited by Lady Penelope; 06-05-2019 at 11:26 PM.
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Old 06-10-2019, 02:42 AM
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Originally Posted by Lady Penelope View Post
There is one fuse in the 2 heelboard halves that provide power to control or close both relays to contact main power to the pumps

See page 36 as fuse # 10 / 5 amp RH heelboard fuse box

You can swap the king relay ( ignition positive relay by name ) that controls that fuse box with the one at the LH engine fuse box which in reality is only the horn relay ( I hear a test toot )

Researching on my new computer
So glad you got a new computer Parker!

Your advice is a +1. The RH heelboard king relay must be reliable in order to keep the fuel pumps running. In 2013, my old VDP had an intermittent relay there and bewildering occasional failure to fire up (no fuel pump power). Once running, there were no troubles. Quite a simple solution, once I discovered the problem. Just took a couple of months of research and trying all sorts of things that didn't solve it!

The connection at fuse #10 when switched on by the RH heelboard relay, is the signal relay for the main fuel pump relay in the trunk. Like a double-relay system.

Al, it is hard to imagine that the RH heelboard relay would lose contact just as you slow down when the engine is already running (relay engaged). But it could very well cause a lack of ability to fire up again.

Being a dual pump system, the secondary pump should run the engine if the first one fails for some reason. Unless... the RH heelboard relay has gone out.
 
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Old 06-10-2019, 08:53 PM
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A alternative way to get a fuel pump on only when the key is in the on position is through the # 3 ACC relay near the # 1 fuel pump relay

This was how M' Lady P was wired as I purchased her

See page 19 of the Doc below

The original problem with my car was a damaged RS3 connector the carries that command ground wire to close the # 1 relay provided by the ECU

The repair worked until the black electrical tape glue on the inferior twisted splice migrated into the multi stranded wire losing connection

This will only get your # 1 back and the # 2 may still work as it has a different command ground

The ACC relay can be found on page 168 below

http://www.jagrepair.com/images/Auto.../jagxj1997.pdf

Editing

Previously someone plotted out your ELM - 327 graph where it could be seen that your voltage regulator was failing and the car voltage was instantly falling to the battery voltage of 12.75 volts

If you had a weak or floating relay this would be the instant the relay would lose contact

Want to borrow a voltage regulator to try ?
 
Attached Files
File Type: pdf
X300 1996 LWB.pdf (2.14 MB, 1 views)

Last edited by Lady Penelope; 06-10-2019 at 11:16 PM.
 
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