Jump to content
Click here if you are having website access problems ×

Fuel pump power supply woes - solved, thrice. (was MFRU - multi-function relay Reverse Power Protection)


anthonym

Recommended Posts

Original subject was:

MFRU - multi-function relay  Reverse Power Protection

but responses showed my subject choice is rubbish and it's all about fuel pump power supply woes

 

this matter originated in my failed fuel pump, but the pump is one only of the items in the MFRU, and this part is often mentioned as having no purpose that cannot be served by four separate relays.

I want to question this notion and seek a deeper understanding of the MFRU. First off it strikes me that "multi function" may refer to more functions than 4 simple relays. 

Now then, my fuel pump when disconnected reveals a short between the power and the earth wires. 

The battery side of the power feed (i.e. from the MFRU) shows 12 volts when the pump is not connected.

Connect the pump and turn the ignition key to activate 12v feed (and usually pump priming) and what we see is 12 volts that instantly drops to zero. Disconnect pump and it's a continuous 12v. No priming at all.

The fuse does NOT blow. Ordinarily with a simple relay, the fuse would be blown by the short.

So we have a short that does not blow the fuse. Perplexed!

This leads to the suggestion that the  multi-function relay unit (MFRU) has Reverse Power Protection, which means the instant it detects a short the relay reverses. Clever stuff. If true it also shows why the use of an MFRU and not a simple relay. I am further told that in electrical engineering terms placing reliance on a fuse for something like a short in a fuel tank internal pump is unsophisticated and not risk averse given the explosive potential. Again, hence the likely presence of said Reverse Power Protection which from what I am literally seeing is very very fast, faster than a fuse can blow.

I also recall that the MFRU sends signals for other devices such as the ECU, though I forget the details. So again not simple relay action.

I took one of these to bits a while ago (Schloss Dyke) and now wish I could have another look. I am told the Reverse Power Protection has the appearance of little springs or coils in addition to relay features.

I am of course "asking google" and can find Reverse Power Protection, but nothing as regards MFRUs as such. 

The first thing my "assistant" did was apply a 12v power probe to the known shorted unpowered 12v feed to the pump, it blew the power probe fuse instantly. No reverse power protection circuit in play and exactly what I expected, so why doesn't it blow the pump fuse on igntion? Must be reverse power protection.  

Any engineers or other experts reading? 

Anthony

Link to comment
Share on other sites

Hi Anthony,

Firstly, apologies, I’ve only just picked up your telephone message, which I guess has now been superseded.

I seem to recall from Schloss Dyck that we thought your car had a higher than expected resistance in the loom in the fuel pump circuit, but at the time it wasn't sufficient to stop the fuel pump working - although I may recall that incorrectly?

In answer to your post, “Multi-Function” simply refers to it operating on several different aspects of the electrical system, nothing more sinister or clever.

So:

1. “my fuel pump when disconnected reveals a short between the power and the earth wires”.
If you are measuring resistance in ohms across a completely disconnected (and functional) fuel pump, then the resistance of the motor winding will be very low.
I seem to recall we measured the current draw of your fuel pump at around 8 amps. This would give a resistance of around 1.5 ohms, so difficult to differentiate from a “short” across the terminals of the pump.

2. “The battery side of the power feed (i.e. from the MFRU) shows 12 volts when the pump is not connected.”
I’m assuming you’re measuring the YG colour wire O/P 4 of the MRFU (or one terminal of the Inertia Cut Switch), with reference to 0v/Chassis, and with the ignition ON. In which case 12v is correct.

3. “Connect the pump and turn the ignition key to activate 12v feed (and usually pump priming) and what we see is 12 volts that instantly drops to zero. Disconnect pump and it's a continuous 12v. No priming at all.”
IF you are measuring voltage at the positive YG input terminal of the Fuel Pump, then that would indicate you have a higher resistance somewhere else in the supply to the Fuel Pump.
The reason it measures 0v is the resistance of the pump is only c1.5 ohms, and it does not blow the fuse is because there is a higher resistance earlier in the supply, across which the voltage is being dropped.

The power feed to the fuel pump is as follows (where N = browN, YG = Yellow/Green. P = Purple, Bk = Black):

Battery +ve (N) > Fuse 14 (P) > Engine Loom Power Feed > MRFU Output Conn Pin 6 (N) > MFRU output conn Pin 4 (YG) > Grey engine Plug pin 9 (YG) > Fuse 13 (YG) > Inertia Switch (YG) > Pump (YG) > 0v/Chassis (Bk)

You need to find where the high resistance is occurring in the above chain, and getting at some of the connections with a meter probe is not going to be easy, so a few easy things to try first:

a) bypass (short across) the Inertia Switch terminals to eliminate that switch
b) disconnect, and reconnect the grey 14-way engine loom plug (spray some electrical contact cleaner in it if you can).
c) as above for the 8-way MFRU output plug.

If none of that gives you 12v when turning the ignition on, measuring at the positive terminal of the fuel pump, with the pump connected, then I would suggest a high resistance in the loom between he MFRU and the Pump, or high resistance in the MFRU.

To check if it’s the MFRU: 
with the ignition off and the MFRU output connector unplugged, carefully apply 12v (via a 15A fuse directly from the battery positive) to pin 4 (YG) of the MFRU connector. That should cause the pump to run.

  • If it does, measure the voltage at the positive terminal of the pump whilst it’s running.
  • It should be within 0.5v or so of battery positive voltage.
  • If it’s less than that then there is a high resistance fault somewhere in the loom between MFRU connector and the pump.
  • If it’s near enough battery voltage then it points to a fault with the relay contacts in the MFRU.

If that’s the case get a standard 12v Automotive 4-pin 20A relay and:

  • Connect the wire from Pin 1 of the MFRU INPUT connector to terminal 85 (coil) of the relay
  • Connect the wire from Pin 2 of the MFRU INPUT connector to terminal 86 (coil) of the relay
  • Connect the wire from Pin 4 of the MFRU OUTPUT connector to terminal 87 of the relay
  • Connect the wire from Pin 6 of the MFRU OUTPUT connector to terminal 30 of the relay

Sorry can’t help any more tonight, and I’m out tomorrow morning, but post on here with how you get on.
good luck!

Link to comment
Share on other sites

Anthony,

I can assure you the MFRU contains nothing other than four relays. I've had several apart and I've replaced the one on my car with discreet relays and written up the process for Low Flying. There is nothing clever and no reverse power protection, no clever active logic at all.

What you are seeing is quite simply exactly what you would expect if you had a high resistant at some point in the feed to the pump. Something like a corroded connection or relay switch. Measuring the voltage with things disconnected tells you very little. At the point where you are measuring you are seeing 12V with no load but through a relatively high resistance; as soon as you connect the pump you draw a current from it and the current through the resistance gives a voltage drop, and the voltage falls to pretty much zero assuming the resistance in the circuit is high compared to the effective resistance of the pump.

From your description, I would say there is probably nothing whatsoever wrong with your pump, just a bad connection somewhere along the circuit. The pump isn't a short circuit, it's just trying to draw a hefty but normal current which the circuit can't supply.

The way to track it down is to look where the voltage drops with the pump connected and powered up. As you have found, you will be measuring 0V at the pump and around 12V at the battery and it will drop abruptly at some point around the circuit; that's where your problem lies. You will probably find 12V on one side of a connection or component of some kind and 0V on the other side, and the problem lies between the two.

The awkward part of doing this with the fuel pump circuit is that the pump only powers up for a couple of seconds when you first switch on, then powers off. One thing you can safely do is unplug the inertia switch (assuming you have one) and use a length of wire from the positive battery terminal and touch it to the pin on connector that is connected to the pump. You can do this with the ignition switched off. You can identify which terminal is the pump side with a multimeter; it will be at 0V when the pump should be priming and the other will be around 12V (with the inertia switch unplugged the circuit will be broken so there will be no load current flowing). If you accidentally jump 12V to the other terminal it won't do any harm, it's only connected to the output switch of a relay in the MFRU which will be switched off. If you are worried and want to play it really safe, unplug the larger connector from the MFRU while you do it. If you're really worried that there is a short in the pump, you could put an inline fuse in the jump wire for protection but if I was you I would just briefly "blip" the wire on the battery. If you get the sound of a fuel pump, hold it on for a bit longer and it should run properly. If you just get a shower of sparks then think again! Just remember the standard pump can draw around 7A when running, I'd guess the uprated one could draw more, and the inrush current as it starts will be a lot higher, so you will get a healthy spark anyway and any fuse in the line needs to be 15A+ or it will probably just pop with the normal pump operation.

If the pump runs when you jump it off the battery i) The problem lies upstream of the inertia switch and ii) if you're really stuck you have a "get you home" solution, you could make up a slightly more permanent wire and run the pump straight off the battery this way until you get home, you would have to remember to lift the bonnet and unplug it when you shut down. It's a little less safe than running it wires properly as it won't stop automatically if the engine fails or in case of an accident, but it shouldn't do any harm.

If it doesn't run, you need to follow the wiring that runs down from the inertia switch to the pump. Somewhere the 12V will drop to 0V but to be honest there isn't much other than a single wire in this bit of the circuit so unless there's a poor connection at the pump itself, which I doubt as you say it drew enough current to blow the fuse on your test supply, I doubt there's much wrong downstream of the inertia switch.

Upstream of the inertia switch the most likely culprit would be the relay inside the MFRU. Maybe somebody could get a spare out to you to try? Just be aware that EU2 and EU3 MFRUs are different and not compatible so make sure you get the right type.

I'm on holiday in Indonesia at the moment and struggling a bit with Internet access on my mobile, so I don't have access to a full wiring diagram to give you any more details of the first part of the circuit ahead of the inertia switch.

Just to reiterate though, the symptoms you describe DO NOT initially suggest a short circuit in the pump and until you've tried running the pump straight off the battery I wouldn't start trying to empty the tank and pull the pump out on tour, you're more likely to introduce a problem than you are to fix one.Hope that helps, I'll try to keep my eye on this thread if you post any replies but if you want you can BM me and I'll send you my direct contact details. Andrew

Link to comment
Share on other sites

Thinking about it, the "Reverse Power Protection" you may have heard about is probably just diodes across the relay coils. I'm not sure whether the relays inside the MFRU have these or not from memory, but they won't be relevant to your problem here. When you apply a voltage across a coil the current builds up slowly due to the inductance of the coil. You can think of this almost like "inertia of the current", the voltage gives a steady push and the current builds up. When you then switch the coil off abruptly, the inductance means it takes a very large voltage to stop the current suddenly, so you get a large voltage spike of reverse polarity induced across the coil when it switches off. This can be several hundred volts and can damage the electronics driving it. Diodes pass current easily only in one direction, so putting a diode across the coil allows the current to decay more slowly without a large reverse voltage spike without conducting when the relay is powered with the normal polarity. They act to absorb potentially damaging voltage spikes and nothing more clever than that.
Link to comment
Share on other sites

I’ve mentioned before, my car wouldn’t start when I built it. Fuel pump wouldnt run. I used a jump wire and it ran.

Classic carriage (pre Caterham Midlands) found a crimped joint in the loom in the engine bay had come apart. Not exactly sure where it was but was but think it was downstream of the inertia switch.

Ian

Link to comment
Share on other sites

OK, some bits of info. The wire pair from the loom plug to the pump is fried. By this I mean the Swiss rear exhaust box has heated the wires and they have become stiff, stiff enough possibly to snap. Same as "click of death" heat soak to the starter wires. I have replaced the stiff wires and all the above is AFTER having done that. If I have a point, it is that excessive heat in the region of the pump is an issue. Doesn't affect any of the above I don't think. 

At Schloss Dyke we found fried Y/G across the engine bay bulkhead and a BROWN fried wire which i i r c we replaced. There was also as a suspect spade connection in that series of connections to the MFRU. 

Given that the pump is unscrewed from the tank (and because I had to hacksaw off one screw there's a rivnut to replace), but not yet removed, assuming it just lifts out, I will test it away from the car/tank to see if it runs. Half hoping it doesn't because at this juncture replacing the pump is easier than tracking down the bad wiring. However, all the above indicates it's not the pump and further sleuthing is required. 

The MFRU is new (famous last words of course). Maybe a year or so, because of the old one being suspect, but that's been running in another car without a problem.

Need to think some more.

Anthony

 

 

 

Link to comment
Share on other sites

a few seconds enough? hopefully. Will really do my head in if it works. Bringing back the memories of Schloss Dyke that IanB is mentioning. On that occasion it was "intermittently the car won't start" traced to lack of fuel due to lack of volts to the pump .. (sighing here), which turned out to be I think (the famous) "brown wire" that brings power from the loom to the MFRU, also via a suspect spade connection which I should go and test. 

How do I test these connections and wires ? By which I mean this business of "under load" and "not under load" is throwing me. I thought reduced volts shows up regardless of load, subject to heat increasing resistance. And by all this I mean must I have the pump connected so as to create the current draw for the tests to be correct? 

If there is nothing wrong with the pump when I test it that means rebuilding the wires from it which I have cut out as they are heat fried so solder (?) and heat coat (forget name) - this was a MINOR advantage of new pump. Indeed access to repair these wires was well nigh impossible in situ.

This would be a pleasant little project if I were not on a sub trip from a holiday 5 hours from the tow car and trailer. Must be making me stronger :-) 

Link to comment
Share on other sites

I have the wiring diagram for this, annotated with this circuit. Yes I have "shorted" the inertia switch, using the same bit of wire we used at Schloss dyke. So what I haven't been examining is the feed TO the MFRU and I think I know where to look for that (the brown wire etc)

Link to comment
Share on other sites

OK. May struggle to answer quickly. I'm 6 hours ahead of UK time at the moment, so probably 5 hours ahead of you. You need to follow a logical path trough this to diagnose it. Firstly try powering the pump off the car. If it it doesn't work, it's the pump. If it does, reconnect it to the car and try powering it by jumping the battery positive to the inertia switch terminals. Not jumping across the two switch terminals, but from the battery positive to the terminal (supposed to be) connected to the pump. If in doubt try them both, it won't damage anything. If that still runs the pump, there's a problem somewhere BEFORE the inertia switch. If it doesn't there's a problem somewhere AFTER the inertia switch. That will narrow down the possibilities a lot, we can take it from there when we know the answers to those questions.
Link to comment
Share on other sites

all fixed.

Very grateful thanks to you above - appreciate all that typing. It made me think straight.

I followed the 12v supply (or lack of) as directed back up from the loom connection at the pump, and it was a (previously noted as doubtful at Schloss Dyke) spade connection between the battery (we (meaning Ian & Mick) cut out the brown wire supply at Schloss Dyke, with the comment this connection was dodgy) and the MFRU - connects to the (quite thick) Brown/White  wire that splits itself in to two for two entrances to the MFRU - so something else also was not getting the power the MFRU needed. 

The pump when tested off the car worked fine. So if I had tested it on the car BUT without a fuse or with a suitably large fuse, I would have not removed it at all. That was a lack of courage/confidence on my part. What misled me was there appearing to be a short through the pump, which as Ian/Andrew have described above, what looks like a short, is not.

Also very grateful thanks to EEK Malcolm who spent hours with me at the car over these days - both moral and very practical support, we couldn't remember why soldered joints are arguably deprecated so w/he soldered/heat shrunk the joint and removed the bad connector. Likewise the heat hardened wires in the loom-to-pump link, except when taken to bits it seems the wires were ok, just the outer casing was hardened by the heat. I have relocated all boot area wires up and away from the rear exhaust box. 

Hindsight, the fix would have taken about five minutes roadside fix (snip/snip, wind wires together and tape) if I had gone straight to it based on Schloss Dyke work last year or whenever it was. Seems ages ago.

I think in all this the moral support was very much appreciated by me - this roadside stuff really does suck and makes it more difficult to think straight (granted this particular "roadside" is at a holiday location I was about to depart, but then could not.)

Special mention, Redline delivered part unbelievably quickly, practically next day UK to France.

Anthony

Link to comment
Share on other sites

http://www.elisewiki.de/wiki/images/c/cc/Relais_1.jpg

Looks like they've used resistors to suppress the reverse spikes rather than diodes. You can just about see they are wired across the coils. Other than that... Just boring old relays I'm afraid! 

Link to comment
Share on other sites

What do the relays do? (aside from what relays do) One is clearly the fuel pump. The Brown/white provides 12v to two inputs, I'm guess these in turn power two relays each? So what other bits are serviced from here and more importantly why? Why not just have 4 separate relays? Are those resistors crucially important, meaning are the items serviced from here vulnerable to reverse spikes?

Went for a very nice blat with Malcolm EEK this evening, very pleasant to follow a quick local. No issues - aside from a daylight running light fell off because the screws both vibrated loose! Cable ties to the rescue as usual. I think nyloc nuts are needed.

Thanks for your concern, I was just exhausted after six hours fixing the car and 12 family members arrived at the house so it was non stop until bed. 

Anthony

Link to comment
Share on other sites

Activate the starter motor (hence the click of death when the realy fails)

Supply the lambda sensor

Supply the injectors / ignition coil

Supply the fuel pump

This could as well be separate relays. I suppose - but it's a guess - that the MFRU has been chosen to simplify the wiring and because it's fairly weatherproof.

 

Yes. protection is always important. I don't know if the ECU has diodes inside, regardless of this, the less there is to supress the better.
 

Link to comment
Share on other sites

thanks rj - they use resistors per revilla's picture post above.

GUESS WHAT CHAPS? Broke down AGAIN, and again it's the fuel pump 12v supply. This time only ten minutes drive from the trailer after driving all day and evening (fun!). I fixed it using skills learned in this thread, plus a young auto electrician called Christopher showed up - luck? You're telling me! :-)  This was about 11pm at night. No traffic, just one auto electrician happens by, says he has all the time in the world - and likes Caterhams.  

There I was working my way through the extensive destructions earlier in this thread (Ianb/revilla) using my 12v probe (brilliant gadget has to be said, tells earth/12v, tells short/open circuit delivers 12v if desired ) and other things.

My real problem was having the guts to remove the probe from the fused accessory (fag lighter socket) which is fused and I know from last time will blow the small fuse (I don't have any more of that glass tube type) when powering the pump and connect it with it's clamp accessory direct to the battery. So I did. It really is/was roadside (a petrol station, with lights and overhead cover not blocking any pumps) And touched the 12v inward wire to the pump at the pump loom connector, applied 12v power; zzzzzzzzzz pump whirs. Bemused relief. Not again shirely? Test 12v at the replaced connector, 12v shows. So that fix still works. Try inertia switch, Chris switched ignition, no 12v on either side of the inertia switch. Still incredulous. Go to MFRU 12v outward, YES! 12V shows, so it's not the MFRU. tfft. Go to fuse "box", look for 12 v; nothing, nada. So somewhere between the MFRU and the fuse box, the Y/G wire is cut. Give the loom a wiggle and try, she nearly starts. Done that once or twice. 

Christopher splices a wire in to MFRU end, in through bulkhead (get you home only link, no grommets) and splice to same wire where it enters the fuse box. VROOM! She starts and is still running. Ten minutes and she's in the trailer. 

I cannot help wondering if this has been haunting all the other problems as well. 

I think I will solve this with a wire through an inline fuse direct to the inertia switch. About 18 inches long, using existing wire I know isn't cut, so cutting out yards of loom wiring which somewhere is cut. 

I will try to track down the cut because it may show other wires are vulnerable, but for now please let this be the end of my fuel pump 12v supply failures. 

Thoughts welcome.

Anthony

 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...