Fuel pump permanently on

[djwhittaker14]

I've also been researching and found this diagram which I am struggling to decipher.

http://www.tom-seven.stillgame.ch/Main/Fuel_pump_l...

Not sure if it helps?

[DCL]

Yes, I thing it's part of the ignition senor circuit (33) - what it is doing is stopping the fuel pump feeding back to the relay and latching it on.

[djwhittaker14]

DCL thanks for all the info much appreciated. I have just been to the garage and put the plug back together, the pump is still coming on with the master cut off switched on. I am assuming now the diode is bust so will source another one. Hopefully that solves it.

As an aside having just been out there to check there is a significant (circa 2-3 second) pause before the pump turns on after switching the master cut off on. However when the ignition is switched off there is no pause the pump just runs straight away! So I am assuming that relay closes after the power is initially supplied to the circuit and then remains closed (only interrupted by the ignition and immobiliser). This could potentially point to the diode being unserviceable as I would assume that it should stop the initial signal from the battery (post making the master cut off ON).

Not sure what your thoughts are on my simplified thought process?

[Penelope Stopit]

Helped someone here at PH with a problem in the past, got involved with a MFRU

The car was an Atom with Honda engine

Looks like you could have a similar MFRU fitted

Ignore the wiring colours in the diagram

Is this it? If so, have you the part number?

[Penelope Stopit]

Can't see an open circuit diode causing a problem

A short circuit diode could cause a problem

Disconnecting the diode is the same as having an open circuit diode

If the diode is short circuit, removing it should stop the pump if the diode is causing the problem

Part number of the MFRU may help

Know of 2 types, there could be more

EU2 MFRU Internal Schematic (YWB10022)

Edited by Penelope Stopit on Monday 19th October 13:13

[Penelope Stopit]

Must go, will visit back later

[djwhittaker14]

[djwhittaker14]

Looks like the lower number is different? With the diode removed and nothing inserted into the aforementioned plug to replace it the fuel pump will not turn on at all (including ignition on).

[Penelope Stopit]

Ok then

Removing the diode turns the pump off, that's good

As far as I can make out, the diode is in series with the Green/Pink fuel pump relay ignition supply, the supply will probably be fused

If I'm correct with the above, the diode is in the circuit to prevent a faulty MFRU from keeping the ignition on when it's turned off.

If water got into the MFRU or a relay shorted out, it is possible that the MFRU would supply the ignition

[Penelope Stopit]

This is your MFRU


Here's the 6 way plug


Here's the 8 way plug


Ignore where it mentions Pin 8-1 is not used
The main fuel pump fuse should connect to Pin 8-1

The Caterham MFRU might operate in a similar way to the below Honda circuit (Don't know what ECU you're running)


That should be all the information needed to trace the fault

Need to re-read your earlier posts about the fault and have a think

Will visit back tomorrow if not sooner

Edited by Penelope Stopit on Monday 19th October 20:20

[Penelope Stopit]

Fuel pump circuit simple version



Pin 8-1 to Main Fuel Pump Fuse

Pin 8-4 to Fuel Pump

Pin 6-2 to ignition controlled supply (possibly from MFRU Main Relay) @ Green/Pink Cable with safety diode wired in series with it

Pin 6-1 to ECU timer switched negative or fuel pump relay timer unit (includes 3 second priming and run)

[Penelope Stopit]

As was posted earlier, logic says it's something that's been disturbed

If I've got it wrong about the diode......it could be stopping a reverse voltage that holds the fuel pump relay in, someone mentioned all of this earlier

Have you definitely fitted the diode the correct way and have you tested it?

???Does disconnecting the ECU stop the pump from running???

[Penelope Stopit]

djwhittaker14 said:

I stupidly thought it was the spare connection for the oil temp sensor (not fitted to my car, that I was fitting). So I stripped it down and fitted the cable and sensor to check at which point I realised it wasn’t

Mmmm, hadn't taken this in yesterday

Blown a fuse????

[djwhittaker14]

Penelope thanks for all the help sorry in my delayed reply been working nights. I’ve checked all the fuses that I think could be related and they are fine.
The only thing I fiddled with was the plastic connector and diode, the MFRU could have been damaged by my actions I suppose although it seems to be functioning and I can here the relay making initially when the master cut off is supplied.
I will check to see of the pump works with the ECU unplugged but I am 99% confident it will not run. The ecu is an MBE type standard Caterham fit.
I have some new diodes in the post and am hoping this will cure it, I have been unable to test the current one as I don’t have the right equipment. I believe it should be installed as DCL mentioned with the silver band toward the wire going into the MFRU, but I have tried it in both directions and it makes no difference. My hopeful belief is that it’s failed short and is now just acting as a wire.

[Penelope Stopit]

Don't understand why you are confident the pump will run with the ECU disconnected

The MBE ECU switches a negative to the fuel pump relay when the ignition is turned on

Will be interesting to know what happens when the ECU is disconnected

[djwhittaker14]

Penelope you are probably right to be honest, my knowledge of the circuitry is limited.

In good news I have just fitted a new diode and it has cured the problem so I must have damaged the last one when removing it.

Big thanks to DCL and Penelope for helping me out with this one and saving some cash (£2 for 10 diodes was the total cost), if you are in or visiting the Reading area anytime send me a message I owe you both a beer.

[DCL]

Well done, I'm glad it fixed it. The diode is probably one of those Caterham quirks that's has evolved to allow the use of the MBE ECU with minimal change from the earlier cars.

[Penelope Stopit]

There might be something connected to the MBE ECU fuel pump relay negative switching circuit that goes positive when the ignition is turned off and it holds the relay in as the other side of the relay goes negative with ignition off, doesn't take many volts to hold a relay in

Unless the MBE ECU fuel pump relay switching circuit goes positive with ignition off, can't see it though

The diode will stop the above from happening

DCL gets the Blue Peter Badge

DCL said:

I would simply check the diode with an ohm meter (replace if required), and put it back together as it was - the silver band on the diode goes to the round end of the plug which is the wire that goes to the MFU, and not one that goes further into the harness.

Nice result

The diode must have gone short circuit, doesn't happen so often, diodes tend to go open circuit far more often

All that's now needed is a diagram

Would enjoy finding out why a diode was used in the first place

Bombarded you with information as it may have saved an electrician some time in working out the circuit, time is money

Would have been nice to one day visit Reading, see the car and have a beer, not been to the UK for many years and won't be visiting in the future

Thank you for the offer

If anyone does have a proper schematic diagram please post it here

Edited by Penelope Stopit on Wednesday 21st October 15:07

[DCL]

I think the key to it is that the MBE ECU has a permanent 12v to it and all the switching is solid state. When the ignition is off all the outputs are effectively in their 'on' (ground) state. The sensors are comparator circuits that use a reference voltage which probably exists on the sensor input when the ignition is off.

[Lewboy95]

Sorry to jump on post, looking for some urgent help! I bought a caterham 1.6 road sport three weeks ago it’s a 2019 plate, been running fine till Sunday when it felt a bit jumpy driving around, refuelled after been driving for around an hour then tonight when I started it, it blew a massive ball of grey smoke out the exhaust then when I went to rev it, it stalled and now I have an engine warning light on.

Would appreciate any thought on what this could be as I’m not much of a mechanic??

Again sorry for jumping on post