MFRU relais specs for replacment

[Hanns Per]

Hello, I want to get rid of the MFRU the relais int the MFRU are controlled by 5V current from the ECU. So I assume I need special relais which can be controlled with 5V or would any normal  car relais which works with 12V control current also work with 5V control current?

Thks for Your help

Hanns Per

[rj]

I don't think the MFRU relays are 5V - the starter relay is operated by the ignition key.

 

[rj]

(I have an MFRU in the garage 10 miles away and if I remember I can check tomorrow if it'll operate at 5V)

Please remind me which engine you've got althoug with an MFRU I assume it's a K?

[Wrightpayne]

Revilla is your man on K series electrics - send him a PM - if anyone can help its him!

Regards

Ian

[Hanns Per]

thks it is a CSR with the Rover MFRU the control current is 5V from the ECU and  the operating current is 12V. This is what I see from the cables and the loom  plan. will pm Revilla thks Hanns Per

[rj]

No doubt Andrew will know.

[revilla]

Have replied by PM. Maybe I should post here when I get a chance.

[Hanns Per]

thks have read and partly understood :-), here are 2 connections to the MFRU in my installation. The reason for this rebuild is to move all the electrics under the scuttle and to make it as simple and straight as possible. ECU MBE9A4 fuses and relays (without the MFRU)  will be there on one spot. behind the middle part of the swoopy dash. I also would like to control the 2 fans of the radiator individually which is not possible with the MFRU. With the 2 pics. below does anything additional come to Yourmind, thks Andrew

[revilla]

Hanns Per I can't see any pictures!

My original PM to Hanns Per read (after corrections!) ...

Hi Hanns Per,

The relay drivers inside the ECU are actually the collector of a transistor which switches them to ground when turned on. The ECU does not supply 5V to the relay coils, they are supplied by 12V on one side and the other side is switched to ground by the ECU. So standard automotive relays are fine, but you should use ones that have an internal diode or resistor to suppress the back-EMF spike generated when they are turned off to protect the drivers in the ECU. Both types are commonly available. If you do get the type with a diode which is preferable, make sure you drive the coil the right way around. Terminal 86 should be to +12V and 85 the switched earth. Without a diode it makes no difference. The relays inside the MFRU have resistors for protection.

I did this on my own car for a while and actually wrote an article for Low Flying covering it, although since then I've discovered that some cars use three of the relays and some use two (the Main Relay is optional) and my solution wanls only applicable to the two relay looms.

A question though; why do you want to do this? After a lot of testing I've come to the conclusion that the "Hugh resistance in the relay contacts" is mostly a myth, they are robust well designed relays and most of the resistance which causes starting problems is in the wiring. With the currents drawn by the starter solenoid you can drop a measurable voltage along even 50cm of 2mm2 cable. The main benefit of "relay mods" is actually because people supply the new relay directly from the battery through a shorter wire which is what really fixes the issue! If you just replace the MFRU with h relays you won't get this benefit. I've actually reverted back to the standard MFRU on my car.

If you want I have the internal wiring diagrams for both the EU2 and EU3 MFRUs so you can see how things are wired (they are slightly different)? Andrew

[Hanns Per]

thks have read and partly understood :-), here are 2 connections to the MFRU in my installation. The reason for this rebuild is to move all the electrics under the scuttle and to make it as simple and straight as possible. ECU MBE9A4 fuses and relays (without the MFRU)  will be there on one spot. behind the middle part of the swoopy dash. I also would like to control the 2 fans of the radiator individually which is not possible with the MFRU. With the 2 pics. below does anything additional come to Your mind, thks Andrew

PS I sent You the connections with separat mail I could not manage to get a good picture in this post

[anthonym]

having taken my MFRU to bits and having had how it works explained to me (by Andrew), I changed my mind away from replacing it with relays. It's a tough bit of kit, waterproof, has circuit protection (those resistors) and perhaps paradoxically I think it simplifies the wiring. 

I'd be surprised if you can't add an over-ride switch for each fan. Something to do with 12v-12v deactivating a relay.. still struggling with that concept.

Anthony

[Hanns Per]

So I did replace the MFRU with individual micro relais, all with diodes und put everything under the scuttle. Thanks to Andrew, your information was very helpful. Here are some pictures of my winter modificationitis:

%5Bimg%5Dhttps%3A//i.imgur.com/SiwTF9H.jpg%5B/img%5D  %5Bimg%5Dhttps%3A//i.imgur.com/EbthJMS.jpg%5B/img%5D

%5Bimg%5Dhttps%3A//i.imgur.com/DtoT6Fz.jpg%5B/img%5D %5Bimg%5Dhttps%3A//i.imgur.com/xQGBEYh.jpg%5B/img%5D

%5Bimg%5Dhttps%3A//i.imgur.com/i9rZpUp.jpg%5B/img%5D

[revilla]

Nice work!

[anthonym]

wonderfully simple, it makes my underdash area look like a snake pit