Replacement for the MFRU

[myothercarsa2cv]

So Caterham are out of stock, even when they aren't they're really expensive, and they are a common cause of the click. Has anyone ever made a replacement unit with proper, replaceable relays like the type we use for the bypass mod? If not, why not? A nice little plastic box with a sealed but removable lid, and the same plug fittings can't be too difficult, right?

[Jonathan Kay]

Can anyone write a brief spec of what the MFRU does? How many relays, load, other functions?

 

Would something like this with a couple of wiring adaptors work?

 

Thanks

 

Jonathan

[simonpa]

In 17 years and 9 cars, I have never had a starter click issue that was really down to the MFRU.

They are also cheap enough to be easier to replace than make the mod IMO..

 

I have also seen quite a few relay mods that looked awful and were probably just mod waiting to go wrong - but I am not an electrical engineer :)

 

I would suggest that if you really feel the need to bypass it for the starter, and want a neat solution, then use one of the spare relay holder slots in the existing Caterham fuseholder - AFAICR, there are 2 unused spaces there.

 

Be interested to know if you find a neat solution.

 

Cheers - Simon

 

 

[SLR No.77]

I have to agree with Simon. I'm only on my 2nd car but in 15 years of ownership I've found 99% of incidence of the infamous k-series click have been solved by the simple addition of a separate 30A relay. The existing MFRU can be left in circuit or by-passed depending on how bad it is.

I'm not aware there's any spare relay slots in the existing fuse block - I added an extra relay holder to the end if the existing row (polevolt can supply the o/e ones) and routed the extra wiring in with the loom.

 

Stu.

[myothercarsa2cv]

Thanks all. JK, I was thinking a direct replacement ideally using the same plugs.

 

I didn't think about the spare slots... Why use an MFRU though if simply adding a few relays will do the trick? Does it do something else I'm not aware of? I'd like to do away with the need to add a simple extra relay - replacing the existing one would be a million times better.

 

Really I was just musing. Looking at Angus and Tessa's site (here) the contents just seem really simple, and I don't see why they're so expensive (supply / demand / monopoly?) and why someone hasn't just made a cheaper easier to service replacement - but if very few people have issues with it, then I guess there isn't the need?

 

Edited by - myothercarsa2cv on 2 Aug 2013 13:31:39

[Mankee]

You could use something like this from Simtek. Probably massive overkill unless you are building a loom from scratch and want to get rid of the combined 4-relay unit and move it all into the cabin.

 

I forget the chap's name, but below is a very useful resource for the MFRU:

 

http://www.tom-seven.stillgame.ch/What%20is%20it/MFRU_wit.htm

[Phil B]

Not having a K (Zetec in an old crossflow car ), I haven't experienced the dreaded click.

But wasn't there an article in Low Flying this month or last which described a means to address this problem?

 

[TJG]

Page 14 in July Low Flying. Excellent contribution by Andrew Revill.

 

Tony

[brookside2004]

As far as I can see the MFRU is just a compact, tidy, and waterproof method of mounting a set of independent relays within the engine compartment. IF and it's a big if, the click is really caused by pitting and then high resistance across the (under-rated) starter relay, then the true engineering method of resolving it would be to uprate the starter relay within the MFRU. The best compromise will be the one most commonly used - add a secondary relay powered from the MFRU to reduce the current through the MFRU starter relay, and remove the issue once and for all.

[SLR No.77]

Correct (and very eloquently explained 😬).

 

Stu.

[myothercarsa2cv]

then the true engineering method of resolving it would be to uprate the starter relay within the MFRU

Hence the thought about having something that you could actually service rather than have extra bits floating around the engine bay (which I do).

[brookside2004]

Since we are only talking about relays it would certainly be possible to build a complete replacement for the MFRU with uprated relays which would fit into the existing loom and connectors. There would always be concern around durability of a non-standard part, particularly as it would mean a change in design of the relays in the MFRU which are not problematic (ie those not related to starting). A potential can of worms !

In terms of design and implementation the main challenge will be calculating what actual rating and performance is required for the replacement starter relay, if we are assuming that Rover/Caterham have got it wrong in the first place. The current drawn to energise the new relays would also have to be considered, to ensure the problem is not moved further up the chain.

The case for change would surely be proof that that is in fact the problem !

[Paul Deslandes]

if the failure of the starter relay is due to arcing, I wonder if a diode across the solenoid to kill any back emf would prevent the problem happening in the first place.

 

Just a thought............

[brookside2004]

If back EMF is the cause then a diode could be an effective solution, however given that some K owners seem to have this as a recurrent problem, and others (me included) have never had it happen, there is probably some more research required to understand whether it really is a weakness in the relay, whether there are different variants or relay in use, some of which are under-rated and prone to it, or whether there is another external cause that in some cars is damaging an otherwise perfectly adequate relay design.

If there is an appetite for develping a once and for all fix, then in the first instance I would be looking for differences in the build of MFRU's and starter solenoids on cars that suffer and those that don't, and measuring solenoid energising current in the different cars to see if a definitive fault could be isolated.

[SLR No.77]

Simply replacing the MFRU with an uprated version may not be the correct answer. The MFRU is part of the original Rover setup, engineered by Rover for the K-Series installation in a variety of vehicles. As far as I'm aware it has proved to be perfectly reliable in these installs ... but not in the Caterham. The Caterham however is not a Rover install beyond the engine itself - the gearbox, bell housing and starter are all Ford-derived reworked to mate to the K-Series block, but using a Rover-derived loom etc.

The common modification is to piggy-back or bypass the MFRU starter relay with a larger 30A relay and in doing so enable a direct feed from the battery to the starter solenoid. It's not rocket science but it works just fine.

My first car I bypassed the MFRU starter relay completely with a relay fairly scruffily mounted next to the battery, my current car still has the MFRU in circuit, but rather than driving the starter solenoid it simply latches the added 30A relay which then in turn drives the starter solenoid. It's wired into the loom and relay/fuse block as if o/e and it's proved to be perfectly reliable. So why go to the trouble of replacing the whole MFRU?

 

Stu.

[brookside2004]

Personally I wouldn't see a case for it, and agree with your post entirely. I am aware that some owners may want to look into it purely from an engineering perspective. Mine has been fine in original spec for over ten years which is why like you I suspect it is a caterham issue, and probably only affects certain installations. If it became a problem I would use the usual external relay mod :)

[SLR No.77]

 

I'd actually consider doing the relay mod now and take the load off the MFRU starter relay, but keep it in circuit thereby securing some longevity from it. Simply a case of taking the brown/red wire that runs from the MFRU to the spade connector at the starter solenoid, and inserting a bigger relay

 

Stu.