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· Mmmm....Rover..
10,420 Posts
Recently I have had some time to "play" with the Pektron SCU as fitted to the facelift models.

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It suffers from problems that can be categorised roughly as follows...

Water ingress (25, ZR, and Streetwise only).

Water enters where the wiring loom passes through the bulkhead, it trickles along the loom straight into the SCU. If the SCU had not been mounted with the connectors facing upwards you would simply have ended up with a damp patch on the footwell carpet!

Corrosion will be visible on the pins of the SCU connector. The first item to fail in the SCU when water enters is the radio receiver circuit board. It is mounted at right angles to the main board and water will rest on it, symptoms include low fob operating range or no fob operation whatsoever.

If caught early the water ingress can simply be dried out and the SCU re-fitted.

If the SCU is damaged beyond repair then it is possible to copy the data from it and program this information into a new or used SCU, this process creates an identical clone of the original unit and retains all features along with the original fobs.

With a bit of dexterity, the SCU can be rotated so that the connectors face downwards and I would suggest that all owners of a ZR, 25, or Streetwise do this.

Fob and immobiliser faults...

A peculiarity of the SCU is that it will not allow diagnostics to communicate with it while it is in the armed state, therefore new fobs cannot be coded if it is armed and you have lost the originals or they don't work. I would assume that this was "designed" as a security feature, it's a pity that the dealer T4 diagnostics were not provided with a method of dealing with armed SCU's.

The MG-R solution when faced with this was to fit a new SCU and code new fobs, rather expensive.

Using a specialist diagnostic tool it is possible to program new fob to an existing SCU that is armed.

A design problem means that fobs that were programmed and working can become corrupted, this was a known problem when MG-Rover existed. The corruption is caused when the fobs are on the same keyring as those from the following manufacturers;

Any BMW vehicle.

BMW Mini.

Rover 75/MG ZT (Immobiliser is a BMW-based system).

These vehicles use a transponder-based system to communicate with their fobs, the transmissions corrupt the
Pektron fobs rendering them useless.
DO NOT keep MG-Rover Pektron fobs on the same keyring with any of the above.

In these instances, the only solution is to program new fob(s).

Please take note of the following!

If buying replacement fobs please be aware that they MUST be supplied with a barcode label, the data on this label is required for programming.

Used or secondhand fobs that do not have this data are USELESS and cannot be programmed to the SCU, even if they are supplied with barcode information they still may be unusable, see below.

When programmed the barcode data identifies the fob to the SCU, it also contains information regarding the rolling code that will be exchanged. As the fob buttons are pressed the code rolls forward but in a pseudo-random sequence that the SCU can follow. There is a "window" within which the barcode will still allow the fob to be programmed even after the fob has been used and the code has rolled forward. However, if the fob has been in use for several years the code will have rolled forward so far that the barcode will no longer represent the valid starting point, it will program fine but not operate.

The fob data contained in the SCU memory changes as the code rolls forward, if this data is extracted and used to program fobs they will work fine as it represents the current "window" for
that fob.

Memory Loss...

I have seen quite a few units which have suffered loss or corruption of the configuration data stored in the SCU memory, depending on the severity of the problem you will be faced with one or more of the following problems;

Fobs are transmitting but they do not operate the central locking and/or immobiliser.

The engine starts and only runs for a few seconds.

None of the SCU-based circuits operate correctly, for instance, the driver's window operates in reverse, etc.

Usually, these problems arise after one of the following has happened...

The battery has gone flat.

The car was jump-started.

The SCU was disconnected/re-connected while the battery was still connected to the vehicle.


If the fob data is corrupt in the SCU memory there is usually no option other than to code new fobs, unless you are lucky enough to have a memory dump taken before the SCU became corrupt.

The engine running for a few seconds is a definite sign that the immobiliser code shared between the SCU and engine ECU is no longer correct. The two units must be security matched again to rectify this problem.

Inoperative options or circuits controlled by the SCU can be re-enabled if it is a software fault. In severe cases, I rebuild the file using a known good base file and then add fob codes, immobiliser codes, and VIN details.

Circuit failures 1

Circuits that cannot be switched off ie. horn or foglamps will not switch off, are caused by an electronic fault that keeps the relevant relay switched on constantly.

Circuit failures 2
Horn doesn't operate.
The driver's window only travels in one direction or doesn't operate at all.
The rear fog lamps do not operate.
Central locking only locks or unlocks... or does neither.
The intermittent wiper function does not operate.
Doors are deadlocked (super locked).
Windscreen washers do not operate.

The last category is usually the fault of the Pektron branded relays, of which there are five.

The OEM (Original Equipment Manufacturer) relays fail because their coil winding goes open circuit which means that the relay will no longer operate when power is applied. I have dismantled many relays and each time I have found that the very fine wire in the coil winding has fractured, closer inspection shows that it has corroded.

The wire used seems to be of poor quality with inconsistent enamel insulation coating, where the enamel is missing the wire corrodes.

Some OEM batches are worse than others, with various production weeks in 2004 being extremely prone to failure. I have seen replacement SCU's that were manufactured in 2008 and 2010 which still exhibit
relay failure so the problem still exists!

These relays are unusual by the fact that every single unit contains two separate relays. In instances where one physical relay controls two circuits, ie. one side controls the horn and the other the rear fog lights, it is common where the horn side fails but the other side continues to work.

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This picture shows the SCU circuit board and each relay has its function shown...
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10/2011: I now have stock of the exact replacement and much better quality relay, the Fujitsu relay that I previously
had a stock of is no longer manufactured.

SCU Removal.

On all vehicles the SCU module was relocated or secured to withstand the Thatcham "5-minute" attack criteria, however, it is relatively easy to access on some vehicle but a complete pain on others.

The biggest challenge is when the SCU has to be removed from a 25 or ZR that has aircon fitted.

The SCU is fitted on the bulkhead behind the aircon evaporator housing. This is accessed by removing the glove box, the evaporator housing is towards the right with a metal retaining strap on either side. The housing cannot be fully removed as the evaporator contained within is connected by pipework to the AC system in the engine bay.

The original removal instructions were based on the fact that the early SCU bracket had a "hook" at its top which simply hung on a stud welded to the bulkhead.
All that was needed was for the lower housing nut to be removed, the housing pulled away from the bulkhead and the SCU lifted off the stud.

All change! The bracket was obviously modified at some point but the removal procedure was never updated.

The new design has a hole for the fixing rather than the previous hook and the bracket is now retained on the stud with a plastic cap. The aircon evaporator and housing have to be removed to allow access and SCU removal, this requires the system to be de-gassed and the pipework disconnected.

Here is the new bracket design...

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Some people have managed to loosen the various clips and brackets securing the pipework in the engine bay, this allows more movement of the evaporator housing and slightly better access to the SCU.

If the car does not have aircon fitted then it is easier to remove the plastic evaporator housing completely.

The procedure is as follows:

Remove the glovebox, two 8mm bolts secure the hinges to the metal framework.

Remove the metal strengthening strap that runs horizontally, with one bolt on either side.

The evaporator housing is on the right-hand side.

Remove the two vertical metal straps. One bolt is at the top of each strap and it is hooked in place at the bottom.

The housing is held in place with two 11mm nuts, one at the top and one at the bottom.

With these removed, the housing can be pulled forward and removed.

Here is the SCU with the later type bracket in its position on the bulkhead, the hole on the left is where the aircon pipes enter.

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On the 45/ZS the SCU is fitted behind the centre console, none of the above troubles here but there is a lot of dismantling required.

When the console has been removed you will need to unscrew the heater control assembly and the lower black plastic assembly from the metal framework, the SCU is located behind these.

The MGTF SCU location is also behind the centre console, roughly in line with the lower row of switches.
Remove the console side panel in the passenger footwell, don't pull it away too quickly as there is a wiring connection to the footwell light
that has to be disconnected.

The SCU retaining bracket only has to be removed on this side but the screw head is obscured by the bracket which is bent at 90 degrees.

Undo the screw, open the bracket and slide the SCU out.

The screws used are a variation on the TORX design, these are TORX-Plus Security which is a 5-lobe design with the pin in the centre.

The bit required is TORX-Plus 25.

Standard 6 lobe TORX security bits will not fit!

Here's a close-up of the screw head...

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The correct bit can be difficult to source, if you cannot find one then get in touch as I have stock of the correct size.

In true MG-Rover style, I have found the SCU brackets to be fitted with standard Pozidriv screws, 8mm hex bolts, and the Torx-Plus screw!

Relay testing and removal.

Despite being a multilayered circuit board with through-plated holes the relays are easily removed with a standard soldering iron and either a de-solder pump or de-solder braid.

The relay coil(s) going open circuit is the usual cause of failure. Each relay coil has two connection pins and a simple test is to measure
the coil resistance with a multimeter, a working relay coil has a reading of approx. 220 ohms.

I have yet to see damage or welding together of the relay contacts, despite their small size they seem to be able to cope with the current, especially the central locking and driver's window relay.

The picture below has the circuit board pads associated with the coil connections indicated with coloured dots, one connection (centre) of each
the coil is "commoned" with the other coil.
If you put the black lead on the centre pin and alternately place the other meter lead on the pin to the left and then the right you should get
the 220 ohm reading each time.
If you don't then that side of the relay is faulty.

This particular SCU had no drive to the driver's Window relay, the following picture identifies some of the drive electronics:
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A is the driver transistor for the key fob transponder coil, the device is a T2406.

B is the transistor responsible for controlling the courtesy light, it is a BSP77.

The IC marked C is a TPIC6C595, it controls all of the relays with the exception of the relay handling the fogs and horn.

The IC marked D is also a TPIC6C595, it controls the Fogs and Horn relay.

All of these semiconductor devices are readily available components.

..... more to follow soon.


I can now offer full programming and repair services for the Pektron SCU/BCU.

Keyfob programming.

Enabling Central Locking, Electric Windows, and Foglights.

EKA Code retrieval.

Extracting Key fob data

Unlocking an armed SCU/BCU.

Creating a "memory dump", basically a backup file of the data stored in the SCU, a must-have if
you are faced with a corrupt memory at a later date.

Cloning units (ideal if you have purchased a secondhand unit or just want a second unit as a backup).

Forum supporters receive some of these services for free!


The self-test mode is useful if you are having difficulties entering the EKA code, the driver's door lock switches (sill switches) are also the
cause of some central locking faults.

Taken from the original SCU Technical Training course documentation:

SCU Self-Test Mode

Self-Test Mode can be entered manually as follows;

  • With the driver's door shut, depress the door sill button.
  • Turn the ignition on, off, and then back on again.
  • Lift the driver's door sill button.

This sequence of events must be completed within approximately 5 seconds. On successful entry, the horn will sound briefly as confirmation and the vehicle will become immobilised.
Once the self-test mode has been entered, normal functionality of the SCU is suspended and certain alarm-related inputs can be tested. A transition on any of the switch inputs will cause the security LED to flash briefly to acknowledge the input.
The following inputs can be tested by manual operation in this fashion.
  • Sill up
  • Sill down
  • Passenger / Rear doors open
  • Drivers door open
  • Bonnet open
  • Boot open
  • Key barrel switches
  • Inertia switch
Failure to access the self-test mode indicates a problem with the sill up, sill down, or ignition switch inputs.
A spare key will be required to test the key barrel switch inputs, as one will already be required to turn the ignition on.

The information in this thread is given freely by me to help others, however, I cannot abide by those who take this information and use it both commercially and claim it as their own - to this end I have, from the very outset, introduced subtle minor technical inaccuracies. They do not take away from the usefulness of the information given but do show up the idiots who claim to be but aren't technically knowledgeable to spot them!

If you want to use anything - just ask.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #5 ·
Is the problem just down to a blown relay as in your photo?

One last point, is it safe to just unplug the bcu to work on? will the central locking/alarm etc still work when plugged back in?


Your fault will be caused by the failure of one of the individual relays within the C/L relay on the far right in my photo.

Disconnecting the SCU should not cause any faults or problems, all of the settings are stored in non-volatile memory.

If you need a replacement relay the 5 Pektron relays fitted to this SCU are available.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #8 · (Edited)
Unlike the earlier 5AS alarm ECU, the SCU will not allow new fobs to be coded if it is armed.

So if you have lost fobs or they suddenly stop working and the car is locked the only option would be to replace the SCU and add new fobs.

Avon Diagnostics developed a tool which reads the SCU memory, mimics a fob and disarms the SCU, this then allows standard diagnostic equipment to code new fobs.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #10 ·
Unfortunately it isn't that simple. Replacing or wiping the eeprom does not result in an unlocked SCU, the microprocessor remains in the armed state.

Having watched the demo video for the Avon Unlock rig it would seem that the SCU is put in a learn or test mode, an inbuilt "fob" learns one of the vaild codes and this can then be used to disarm the SCU.

New fobs are then programmed in the normal way.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #25 ·
With regards to replacement scu boxes. When replacing them are the all the same.. As in will one off a V6 work on the diesel model or do they all have their own unique scu with different part number? I have to replace the one in my zs diesel and have noticed one on ebay complete with fobs off the V6 for £100 which would be bargain for me! Assuming it would work of course ;)

The SCU and the engine ECU share a digital security code for immobiliser purposes, if you replace only one unit then this code will no longer match and the result is an engine that won't run.

You either have to match the pair with diagnostic equipment or swap the original units for a complete matched pair taken from another vehicle, the engine types will have to be the same of course.

The SCU units are all the same.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #31 ·
I did a detailed pinout for the MFU some years ago.

As for dissecting it... no real point, they are so cheap to buy and the same unit is fitted to so many models, plus there is no programming involved when they are swapped.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #37 ·
I didn't think the 75/ZT used the same Pektron unit.

Generally if the control transistors fail they go short circuit which results in the circuit being "on" all of the time.

Have you checked the fuses?

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #44 ·
The SCU has to have the windows "enabled".

This in turn activates the SCU control of the external relay which supplies 12 volt power to the windows and it also enables the Drivers window relay within the SCU.

· Mmmm....Rover..
10,420 Posts
Discussion Starter · #57 ·
This IC can control up to 8 separate items, such as the relays fitted to the SCU.

A single channel in the IC can fail yet the other 7 channels continue to operate properly.

The horn fault is quite common with the SCU, I have never known it to be anything other than the driver IC.
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