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Discussion Starter #1
I have a problem with the relay to the fuel pump that I really don't understand. The problem is that the fuel pump does not get any juice, resulting in no starting issue.

I have tested the relay and measured both voltage and ohm on the relay, and it works perfectly fine. I have also tried another relay with the exact same result. As the relay works on the bench with a battery connected to it and the resistance is within the limits, I checked the power that goes to the relay base / connector. I measured 12V supply to the relay, and also 12V from the ECU that goes through the relay and to the fuel pump. But here comes the strange part; If I jump / hot wire the relay connector so that it supply power to the fuel pump without the relay, the pump works. I really don't understand this :thinking: Any suggestions on what may cause this?
 

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I can't see how old your car is and what engine it has but below is the wiring for a 2.0 TS with M1.5.5 Engine ECU.
As you can see 12V is always present at pin 30 of the relay, 12V will be present at pin 85 when ignition is turned on,
If the key is accepted (code light turn off) the ECU will pull pin 86 to ground and enable the relay and 12V will be present at pin 87 and the fuel pump will have power and run.

First think to check is if the code light turn ON and turn off ahain after a short while.
And then measure the voltages on the relay pins
 

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Discussion Starter #3
Thank you for the reply. This is a ´96 GTV 2.0 TB. Code light turn on and goes away shortly, so that is ok.

When measuring between pin 30 and ground on the base connector it shows 12,4 V, but when measuring between pin 85 and 86 it shows 11,6 V. So there is a voltage drop somewhere in the supply to the relay.

I hooked up two cables from pin 85 and 86 on the base connector and to the relay, so that I could measure the continuity when connected to the car. The click from the relay is barely audible when connected to the car, and a continuity test shows that the relay does not close enough - it get´s continuity between pin 30 and 87a, but not 87. But when bench testing directly from the battery the relay click is very audible and get´s continuity to both 87 and 87a.

So from this I think I can conclude that the issue is voltage drop to the relay. So now it remains to troubleshoot what and where causes the voltage drop.

According to the wiring diagram of this particular car the relay get it´s ground directly from the ECU which controls when the pump should start and stop. The power to the relay is supplied from the main relay (S41). So I believe the cause of the voltage drop is either the ground between fuel pump relay and ECU, bad ground to the ECU itself or on the supply somewhere between the main relay, fuel injectors and the pump relay.
 

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Try to measure on pin 85 and pin 86 with reference to ground (chassis) then you can see where the voltage drop might be.
On pin 85 you should measure a voltage equal or close to the battery voltage and on pin 86 you should measure a voltage close to 0V when the ECU engage the relay
 

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Discussion Starter #5
When measuring between pin 85 on the fuel pump relay and chassis, it shows 11,6V. When measuring between pin 86 and chassis, it shows 0,2V.

When I now measured between pin 85 and 86 on the main relay it shows 11,9V, so there is not a huge difference. Can so small amount of voltage drop cause this, or is there something else I overlook?
 

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A 12V relay shouldn't have any problem to engage at 11V.
But as you have 12.4V on pin 30 with reference to chassis I wonder if you have a poor connection from and to relay S41 (Main Relay) because you shouldn't have 0.8V voltage drop in the string to the fuel pump relay coil.
Try to measure the voltage on pin 30 and pin 87 (refernce to chassis) at relay S41.

Furthermore I think you are looking at the wrong wiring diagram.
The wiring diagran for a 96 V6 TB is shown below.
 

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Discussion Starter #7
I agree that it seems to be a poor connection either before the Main Relay (S41) or between the Main Relay and the fuel pump relay. I will measure the voltage on pin 30 and 87 with a reference to chassis on the Main Relay. On pin 30 there will always be power, but pin 87 will only have power when the relay is engaged. So do you mean to measure the voltage on pin 87 when the Main Relay is engaged or not engaged?

Regarding the wiring diagram I am a bit unsure. The diagram I attached previously was according to the manual for a 96 V6 TB. I attach my diagram again here where the headliner also is visible, and it says GTV 1996 TB. However, it says it is for the Motronic ML4.1, and I am not sure which Motronic version I have.
 

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Discussion Starter #11 (Edited)
Sorry for late update, it have been some quite busy weeks lately. Frankly, now I am even more confused. Today I hooked up the battery charger, and started troubleshooting again - and this time the issue was a bit different from last time.

When measuring between pin 86 (+) and 85 (-) on the fuel pump relay connector base I get almost the same voltage as on the battery, approx. 12,4 V. But there are no clicking from the relay, so the relay won´t engage even though there is good voltage. However, if I bench test the relay, connecting the relay to a 12v battery it clicks and closing circuit between pin 30, 87 and 87a.

I jumped / hot wired pin 30, 87 and 87a - then the fuel pump started. I also hot wired pin 85 and 86, but the car is still not starting. So there might be another issue here as well as the car is still not starting with the wires jumped. But I want to attack one issue at a time, so let´s focus on the fuel pump relay issue.

Maybe there is something I do wrong, but I am not able to find any logical explanation for this. Why is the relay engaging when supplied from an external bench battery, but not from the relay base connector?

And another thing, the fuel pump is supposed to prime for only a short limited time when turning on the ignition (MAR), so with that in mind I thought that the power supply to the relay (pin 85 and 86) should only last for a short while when turning on the ignition, but I measure approx. 12,4 V the whole time the ignition is on. Is this correct?

By the way, thank you for the link purple145. I see you had issue with corrosion - that for sure might be the issue here too, but in this case the fuel pump relay do get power supply.
 
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