MFU relays (no horn)

HI there, I got my first TF a matter of days ago, and with the great information and help on this forum, I've been getting to know it and try to fix a couple of its minor issues.

My horn doesn't work.

I set the alarm off - the sounder and wiring must be okay then. Through my research, I think that it must be the horn relay or the rotary coupler. I get no noticeable relay click when I press the horn buttons. I get no SRS warning light on the dashboard. I just realised tonight from the very useful guide on this forum that the horn relay is in the MFU behind the passenger compartment fusebox. My car was built in January 2003 (pre-Pektron) with a square-ish keyfob.

I'll not get a chance to get under the dashboard again until Saturday evening, Are there replaceable relays in the MFU? If so, where can I get the horn one (part number), so that I can get it on order. Or is the MFU a sealed unit that needs entirely replaced?

Or am I on the wrong track, and it's the rotary coupler that needs replaced?

So your car has the Lucas system, as Cobber is never tired of saying “it’s the only case where the Lucas electronics is preferable to its alternative”. The MFU is a red box that mounts on the back of the fuse board under the steering wheel. So that should give you an idea of how much fun your going to have. I’ve only come across one of these units failing with a horn malfunction & that was mine; before replacement no horn after replacement horn & horn ever since. I would look on eBay for a replacement, one out one in & away, they were bobbins of a price then I bet they’ve inflated since then.
The SRS is a bit more of a worry. It has no direct connection to the rest of the network, being a self contained stand alone system. The controller is mounted at the bottom of the centre console & can, with a little bit of dexterity, be accessed by removing the to two side footwell panels. I’ve arsed about with the controller a bit trying to get the passenger airbag to be recognised, so I’ve form in that area.
If you’ve no dash indication it could be the warning light out (or missing) or both maladies could be tied in with the clock spring in the steering wheel. Again I’ve got form in that area too & I’m always impressed that a mechanism as delicate as this can withstand the constant movement without work hardening. My car has done 125000 miles & there’s a lot of corners involved in that & mine is the one installed in 1998 & I’m unaware of any failures. Happy rummaging, the horn was the first job I did on buying the car & although I’ve not stopped faffing about with it it’s been a wonderful piece of kit, let’s hope yours is the same once the horns hooting again.
M

Thank you Airportable, that's very useful indeed.

I think that I mustn't have been clear about the SRS light though. I seems to work fine - I have no reason to suspect that the airbag is malfunctioning. The reason I mentioned it is that I understand that usually if the rotary coupler is broken, the SRS light will show.

I'm off to hunt for a MFU now...

MFU = Massively F@$ked Up.

Hello again. I ordered a replacement MFU. It arrived very quickly this afternoon, and I had it fitted within the hour. Still no horn.
Can it still be the rotary coupler, when the SRS light comes on and goes off exactly as you'd expect?

Hum, I wasn’t expecting that, sorry & I’ll have a look at the circuit later & report back.
At least you know the MFU is all functioning & you now have a spare (cold comfort).
M

Ask me that same question & I would give you the same answer. From fuse F7 you're straight into the MFU (PN), the output from that goes to the horns (PY). The relay coil is energised by the horn push (PB) via the rotary coupler & in the wheel itself the two buttons link to the earthy side (black) & red to the coupler. Push the button, current flows from the fuse through the relay coil to ground through the coupler. the contact on the relay is pulled in & current again flows from the fuse through the horns to ground. PAARRRPPP ! The circuit is so basic there's bugger all to go wrong, why they couldn't hang the relay outside of the MFU is a mystery that Joseph Lucas took to his grave. Next step, obviously is to take the airbag out of the wheel, taking all the care that is advised by just about everyone who has taken a screwdriver near it. I think the coupler can be tested in situ but it's not a job I've ever needed to do.
M

Hi again everyone. The mystery of the non-functioning horn continues. I bought a replacement rotary coupler. I bought the wrong rotary coupler! :oops: But re-looking of the photos this evening, the part number of the part that was already fitted to the car (on the right of the photo?doesn't seem to match those available on eBay. I'm confused. What part number do o I actually need? Is it possible that the previous owner was in there before me and fitted the wrong part, meaning that the airbag (appears) to work properly, but the horn doesn't?

The one you have fitted looks like a patern part & comparatively new, the left hand one is more encouraging, it looks like it's worked for a living, it a Lucas device & was made in UK. These are clues to its heritage, get it stuffed in, it very unlikely that harm would be done. You could verify continuity through the spring by metering up the connectors front to back of the coupler.
M

Who would’ve ever have imagined the day, when they’d hear that the Lucas part was superior to the alternatives…….the world has gone mad!
:omg:

The part number of the one you have fitted ( XRC100390 ) seems to be for a Landrover Freelander Mk1.

Part No for MG TF is YRC100300

I don’t recall the blue appendage on the device illustrated from my last delvings into the clock spring whilst fitting the wheel mounted audio controls.