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Faulty fuel pump relay - or is it?
twybrow - 17/7/13 at 11:23 PM

My kit died on me last week. I was supposed to be hooning it away from a Pistonheads pub meet, but instead, it spat its dummy, and died just as the traffic lights were about to go green (in front of a gathered crowd - idiot I know). I had a quick look, and found it had two blown fuses - one for the ECU, and one for the ignition circuit. I changed them, and it started fine, ran again, but died 10s later. This time there were no blown fuses, it was turning over just fine, but no fuel pump whirr.... It was getting dark, so it meant a trip home courtesy of the RAC (and they took forever!).

So I have taken the scuttle off to inspect the wiring, but I can't find anything dubious (well - no more so than usual!). I have checked the fuel pump relay (4-pin Omron part), and with the ignition on, i get a voltage at 3 of the 4 terminals, suggesting to me that the relay is not working (there is no audible click when switched on).

So is my relay broken? Would a failing relay have caused my blown fuses, or should I suspect to find some damaged wire somewhere?

Any help much appreciated - it is too sunny to have the car in bits!

johnemms - 18/7/13 at 06:12 AM

Silly question time.. but have you got an impact cut off switch on the fuel pump?

MikeRJ - 18/7/13 at 08:41 AM

quote:
Originally posted by twybrow
I have checked the fuel pump relay (4-pin Omron part), and with the ignition on, i get a voltage at 3 of the 4 terminals,


That sounds like the relay is working as intended. Two of the connection will be the relay coil, of which one side will be ground and the other side at 12v when energised. The other two will be the contacts, and if the relay is closed and connected properly to the 12v supply you would expect 12v on both of these, so three of the four terminals should have 12v, and one should be close to ground potential (assuming low side switch on the ECU).

Obviously you can prove this by measuring the voltage across the pump during the priming cycle, or when cranking.

If the pump is no longer priming, and fuses have blown it sounds more like the pump itself, or the wiring to the pump has burnt out.

twybrow - 18/7/13 at 09:53 AM

quote:
Originally posted by MikeRJ
quote:
Originally posted by twybrow
I have checked the fuel pump relay (4-pin Omron part), and with the ignition on, i get a voltage at 3 of the 4 terminals,


That sounds like the relay is working as intended. Two of the connection will be the relay coil, of which one side will be ground and the other side at 12v when energised. The other two will be the contacts, and if the relay is closed and connected properly to the 12v supply you would expect 12v on both of these, so three of the four terminals should have 12v, and one should be close to ground potential (assuming low side switch on the ECU).

Obviously you can prove this by measuring the voltage across the pump during the priming cycle, or when cranking.

If the pump is no longer priming, and fuses have blown it sounds more like the pump itself, or the wiring to the pump has burnt out.


Mike - that is why when it is energised, you should see 12V at each of those terminals. Yes, when not energised, you should see 12V coming into the switching part of the really, but not back out again. I tested with it energised, and I get 12V going into the switched side of the relay, but not back out the other side to earth.

Just to be clear - when energised, I have 12V into and out of the relay coil, but of the switching contacts, I only have voltage going in - I have no voltage coming back out, which says to me that the relay is not switching.... Or am I being a muppet?!

Johnemms - no impact switch. I have a rollover switch, but it is upright, and glued internally to be always on.

rachaeljf - 18/7/13 at 10:25 AM

Injection ecus pretty much universally provide a ground to the fuel pump relay coil to operate the pump.

Try disconnecting the wire to the ecu and with the ignition on, earth the relay coil directly. If the pump runs, or you at least get 12V on both of the relay switch terminals, your relay is ok and the fault is elsewhere.

Cheers R

[Edited on 18/7/13 by rachaeljf]

twybrow - 18/7/13 at 03:07 PM

quote:
Originally posted by rachaeljf
Injection ecus pretty much universally provide a ground to the fuel pump relay coil to operate the pump.

Try disconnecting the wire to the ecu and with the ignition on, earth the relay coil directly. If the pump runs, or you at least get 12V on both of the relay switch terminals, your relay is ok and the fault is elsewhere.

Cheers R

[Edited on 18/7/13 by rachaeljf]


I see what you are saying - bypass the ECU switching the earth, and switch it myself to check if the relay is faulty or not. That is tricky, as it would mean cutting the wire to the ECU and then reconnecting....

Does this diagram help? This shows how the bike/my car is wired up. For clarification, looking at the diagram and specifically at the relay (working from left to right):

1. - 'Fuel pump on and off' - I think this is the switched earth via the ECU to trigger the fuel pump relay coil.
2. - +ve feed from the ignition switch.
3. - Not sure. Is this a 12v feed from the ECU? Or an switched earth?
4. - 'Fuel pump feed check' - this is connected to the fuel pump, the relay and the injectors, as well as to the ECU. I cannot see this being anything other than a +ve supply from the ECU.

Can anyone explain how this should work, and therefore how I should be testing it?

[img] ZX12r fuel pump and relay
ZX12r fuel pump and relay [/img]

rachaeljf - 18/7/13 at 05:55 PM

The top connection is an odd one. I can only think the ecu monitors the fuel pump relay. It's common for the fuel pump relay to feed both the pump and the injectors, and lambda sensor too if you have one.
The second connection is odd too, it must be the permanent live feed, via the ecu for some reason.
The third connection is how your ecu controls the fuel pump, by grounding the relay coil.

twybrow - 18/7/13 at 11:02 PM

I took the relay off, and tested it. It clicked nicely, and I had continuity on the other two terminals when energised.... I then put it back onto the car, and I get 12V at three of the terminals, and a low voltage (0.03-0.06V) showing on the 4th terminal which is what should go on to power the fuel pump etc. I can't hear a click either... Grrrrr!

If I had a short on this wire - how can I test for this? How can I find it, or is it a case of just pulling all of the loom apart until I (maybe) find a chaffed wire?

rachaeljf - 18/7/13 at 11:34 PM

If you have 12V on the relay coil-ecu terminal when trying to start the engine, the ecu isn't grounding the relay coil. This could be because the ecu isn't getting the required signals from its sensors to run the engine, or the coil grounding transistor in the ecu has blown. If at any stage 12V has inadvertently been applied directly to the ecu's grounding transistor while the ecu has been operating, it will blow.

You can try grounding the coil-ecu wire, which won't harm the ecu's grounding transistor but will allow the pump and injectors to operate, and see if you can get the engine to run.

To test for a short on the wire that powers the pump etc., unplug your injectors and pump and simply measure the wire's resistance to earth with a meter. You will get a lowish reading because you may still have a couple of items connected, e.g. lambda heater but if it is less than around 0.5 ohm you have a short.

twybrow - 19/7/13 at 12:15 PM

quote:
Originally posted by rachaeljf
If you have 12V on the relay coil-ecu terminal when trying to start the engine, the ecu isn't grounding the relay coil. This could be because the ecu isn't getting the required signals from its sensors to run the engine, or the coil grounding transistor in the ecu has blown. If at any stage 12V has inadvertently been applied directly to the ecu's grounding transistor while the ecu has been operating, it will blow.

You can try grounding the coil-ecu wire, which won't harm the ecu's grounding transistor but will allow the pump and injectors to operate, and see if you can get the engine to run.

To test for a short on the wire that powers the pump etc., unplug your injectors and pump and simply measure the wire's resistance to earth with a meter. You will get a lowish reading because you may still have a couple of items connected, e.g. lambda heater but if it is less than around 0.5 ohm you have a short.


The wire is showing a resistance of 40 k.ohm between itself and earth - so no short there then....

So that leaves me thinking that either I have failed sensor, or my ECU is faulty. I just dont know where to begin trying to find out where the problem is. I am reluctant to buy a replacement ECU (£100-£150 second hand) without being sure that is the problem. I would like to to plug in my fault code reader, but alas these bikes dont have a connection for one. I will try earthing the wire that activates the fault code reading mode, and see if that throws up anything.

After that, I am rather stuck as what to do next....

owelly - 19/7/13 at 01:41 PM

Sorry if this has already been mentioned as I'm trying to read this on my phone but:
The ECU won't give the fuel pump relay a ground until it sees the engine turning over so check to see if there is a tacho signal. If the tacho isn't reading anything as you're trying to start the engine, it ccould be a good place to start loolking!

MikeRJ - 19/7/13 at 02:09 PM

quote:
Originally posted by owelly
Sorry if this has already been mentioned as I'm trying to read this on my phone but:
The ECU won't give the fuel pump relay a ground until it sees the engine turning over


You normally getting a priming pulse of a few seconds on the relay when you turn the ignition on.

Symptoms suggest either:

1) The wiring on the coil side of the relay is open. Since you are seeing 12v on the coil then the problem is on the ground (ECU) side so check continuity between the ground side of the relay coil and the appropriate pin on the ECU.

2) The ECU is poorly. A much less common fault than many people seem to believe in my experience.

twybrow - 19/7/13 at 02:24 PM

quote:
Originally posted by owelly
Sorry if this has already been mentioned as I'm trying to read this on my phone but:
The ECU won't give the fuel pump relay a ground until it sees the engine turning over so check to see if there is a tacho signal. If the tacho isn't reading anything as you're trying to start the engine, it ccould be a good place to start loolking!


It shouldnt need to turn over to prime the fuel pump - it normally primes when you first switch the ignition on to buil dup te pressure ready to start.

rachaeljf - 19/7/13 at 03:06 PM

Are you saying it isn't giving the priming pulse you usually get?

If it isn't priming on first switch on, in rough order of likelihood:

1) the earth wire and/or the supply wire to the pump has come adrift
2) the pump itself has died
3) the ecu isn't getting an ignition 12V supply,
4) the ecu earth wire(s) have come adrift, or
5) the pump relay grounding transistor in the ecu has died (as Mike says the ecu electronics rarely fail unless abused)

Hope that helps!

twybrow - 19/7/13 at 03:44 PM

quote:
Originally posted by rachaeljf
Are you saying it isn't giving the priming pulse you usually get?

If it isn't priming on first switch on, in rough order of likelihood:

1) the earth wire and/or the supply wire to the pump has come adrift
2) the pump itself has died
3) the ecu isn't getting an ignition 12V supply,
4) the ecu earth wire(s) have come adrift, or
5) the pump relay grounding transistor in the ecu has died (as Mike says the ecu electronics rarely fail unless abused)

Hope that helps!


Exactly - no fuel pump operation at all, and that whole side of the circuit (the 12V feed via the fuel pump relay) does not become live when the ignition is switched on.

1. there is no voltage getting as far as the pump
2. see above
3. possible - one to check!
4. also possible - one more to check!
5. also possible - but I cant think of a way to test it without knowing that everything else is ok

I retested the resistance between the fuel pump wiring 12V feed, and earth. I am getting a 2 ohm reading, which suggests to me that there is a short circuit (leakage to earth) on that wire at some point.... So I will strip it down this weekend, as well as checking points 3 and 4 above.

twybrow - 22/7/13 at 11:31 PM

I have checked for shorts and cant find any, and the pins in the ecu have been checked for continuity to the relay. There is current going into the ecu from the relay coil, and coming out of the ecu to go back to the relay switch, but it seems the ecu is not grounding the coil of the relay, as there is no click.

What would cause an ecu to do that?
Are there certain essential sensors giving readings when you first switch on the ignition

Volvorsport - 23/7/13 at 01:07 AM

got a paper clip handy ? switch ignition on ....

short across the relay to make it live all the time , and check for the pump running , if it doesnt its the wiring/pump .

twybrow - 23/7/13 at 11:44 AM

quote:
Originally posted by Volvorsport
got a paper clip handy ? switch ignition on ....

short across the relay to make it live all the time , and check for the pump running , if it doesnt its the wiring/pump .


It is not the pump or wiring to the pump.... there is no current coming back out of the relay. So yes, shorting the relay to ground would make the circuit live, and switch the pump on, but that does not really solve the problem!