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Fuel pump relay clicking but car runs fine?

12 April 2021
Hi all,

I have a 97' Targa with a fuel pump relay problem. The relay began to click after 2 months of not driving the car.

Sometimes it'll be an occasional click every minute or so, or at times when it gets severe it will sound like a type writer right behind the driver seat.

When I removed the rear bulkhead trim, I traced the sound to the fuel pump relay. I can also feel the relay vibrate when it clicks so the sound must be coming from there.

I've noticed there's an identical Denso relay, behind the passenger seat, looking at the service manual it says it is the "Throttle Valve Control Motor Relay" and the part number are identical. So I did an experiment by swapping the two relays and no matter which relay I plug into the wire connector, the relay will start to click when paired with the fuel pump connector.

I then checked my car's battery and it is an Optima Redtop. After charging it for half a day the battery is showing a healthy 12.5V , dipping to 11V when I start the car as my friend checked the voltage.

The last test we've done is to test the voltage coming out from the wire connector, and as predicted it is not giving a stable 12V. It is only giving 3 - 4V and will fluctuate to 0V at times, which I suspect is the cause for the relay to click repeatedly due to insufficient voltage.

However, the car drives fine and there's no drop in power or stalls. I'm just annoyed at the fact that the cabin will have this noise as I drive.

Is there anyway to fix this problem? I've tried to disconnect the relay to see if the car will start and the engine will start briefly and then stall out.

Any tips or advice would be greatly appreciated as I am not too good with electrical issues.

Thank you!
The oddly named fuel pump relay does not control the fuel pump - the main EFI relay does that. The fuel pump relay controls the fuel pump resistor. Unless you are entering into the V tec operating range on the engine, the relay is normally de energized. When you enter the region of high engine load the relay is energized and the relay contacts close to by-pass the fuel pump resistor. This applies full voltage to the fuel pump so that it is able to maintain adequate fuel pressure under high flow conditions. If you are just driving around in a relaxed manner the relay should not be operating.

As you can see from the diagram, the relay is operated by the ECU. The ECU switches the relay on by applying a ground to the red wire. If you are hearing the relay click on and off when you are not at high throttle opening chances are that you have a wiring problem that is applying an intermittent ground to the red wire.

From the schematic, you can see that the engine should run fine under low load conditions with the relay removed. With the relay removed the only thing that should happen is that the ECU will probably block Vtec operation because it does not sense full voltage being applied to the fuel pump. The fact that the engine starts; but, then dies when the relay is removed is unexpected. I would start by checking the socket for the relay and the connections to the fuel pump resistor looking for a wiring problem.

Main FI relay.JPG
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Hi Old Guy,

Thank you very much for the diagram and explanation to troubleshoot.

Situational update:
- Tried to clean the connection between the ECU and electrical wiring with contact cleaner but the problem did not go away.
- I tested the car again with the relay unplugged and the engine did not stall, perhaps my initial observation was incorrect. The car runs with it disconnected, but does that mean VTEC operation will be disabled with the relay discon'd?
- Measured the voltage from the relay connector at low RPM/idle and it's giving the same voltage as the battery (13.x V), I believe that is normal behavior based on the service manual?

- Under high throttle, the service manual says the voltage should be 0V, which would cause the relay to energize and by-pass the fuel resistor correct?Fuel Pump Relay.jpg

The clicking is still happening at random intervals, I cannot pin-point the cause. I would like to troubleshoot it a bit more before I try to go the resistor by-pass route. Any tips to troubleshoot would be appreciated, thanks a lot!

Unfortunately, the problem occurs periodically. This year I only did 200 km. The last time he was there was in the summer. Hence my suspicion about the ECU. I bought new ones and if the problem occurs, I will replace them. Then I will write.
As noted, the fuel pump relay should not be energized until the engine is entering VTEC operation. If you have confirmed that it is the fuel pump relay that is clicking when the engine is out of VTEC operation then the relay is being momentarily switched on. The relay is switched on by the ECU connecting the red wire from the fuel pump relay to ground. This ground switch is done through a transistor in the ECU controlled by the ECU firmware. Since the ECU firmware will not switching the transistor on in non VTEC mode, it is probably safe to assume that the relay is being operated by an intermittent ground. This intermittent ground could be caused by an insulation failure on the red wire between the fuel pump relay socket and terminal F4 (on 1997 and later cars) on the ECU. Or, it could be caused by an insulation failure in the terminal plug or on the terminal on the ECU side that connects to the switching transistor.

I suggest that you try de pinning the red fuel pump relay control wire from F4 on the ECU and wrapping the de pinned terminal with tape. Drive around and if the fuel pump relay continues to click then you know that the intermittent ground is caused by some kind of insulation failure on the red control wire. If the relay stops clicking then it would appear that the intermittent fault is originating at the ECU end. This fault could be in the plug body or on the connection from the plug body to the circuit board inside the ECU.
Hi, to update, we were able to fix the clicking relay issue by repairing the alternator, the diodes were busted thus the voltage the battery drops to around 12v, which is the reason for the intermittent clicking. Glad to have figured out the cause. Hope this helps!