On-board Computer

Introduction

After a great deal of interest in members needing to know about retro fitting an OBC, I have decided to write an article to try and save having to repeat myself.

The 13 button OBC was optional on 6 cylinder cars in the UK and SE models were always fitted with it from the factory. When I was searching for a car, I really wanted one with the OBC factory fitted as I’d been told that it was incredibly difficult to retro-fit one, however, I ended up getting a car without. I then did some investigation into how these are actually fitted and found out that it’s not as difficult to do as I’d previously thought. For you guys who own 318’s where the OBC was not an option, a 325i OBC with an M10 318i instrument cluster has been tested and all the readings seemed accurate. If your car has an OBC relay and an ambient temperature sensor, then I believe this conversion will work for you.

Functions of the OBC

The 13 button OBC has the following functions:

• Time.
• Date
• Average Speed
• Consumption
• User selectable speed limit
• Remaining range on fuel
• Stopwatch timer (for measuring those 0-60 times)
• Outside temperature
• Code function

Parts you will need to fit the OBC

You will need the following parts in order to retro fit your OBC, I will start off with the essential parts, the bare minimum to get it working and then I will list the optional parts. I will also list their locations so that if you are removing the OBC from a scrap car, you will know where to look for the bits:

Essentials

• Main OBC unit (Mounted in the dash above the heater controls)
• Wiring loom (Runs from the rear of the instrument cluster to all ancillaries)
• Relay control module (Above ECU mounting plate)

Optional

• Outside temperature sensor (In nearside brake duct)
• OBC Horn (Just above the temp sensor)
• Temp sensor\horn wiring loom (Follow back from the temp sensor and joins to main loom shortly after entering the bulkhead)
• Audible gong (Mounted in trim panel above foot pedals)
• Remote indicator stalk (If you don’t know where this is, give up now)
• Tools required to do the job

Fitting

Before you begin to think about fitting the OBC, you will need to remove certain things from the car in order for you to access certain areas. First remove your stereo, you will need to use the relevant release keys depending on the make of the unit.

Next remove the lower trim panel above the foot pedals

Now you will need to undo the glove box, unclip the 2 pins that hold it up and allow it to rest in the foot well. Now undo the single cross-head screw that holds the upper trim in place, this is located next to the latch. There are also a couple of plastic screws which require a 1\4 turn to release. Now remove the panel.

Finally you will need to remove the instrument cluster. First put your hand up underneath the “bridge” shaped piece of trim directly beneath the cluster. You will find a round locking nut on each side that you undo by hand. Once you have undone these, remove the plastic trim.

You will see a couple of silver screws on the bottom and also a couple of black ones in the lower corners of the instrument surround panel, remove these and also another 2 screws holding the surround in at the top. Now remove the surround.

Finally you will see 2 gold screws holding the cluster in place these are approximately in the 10 and 2 o’clock positions, remove these screws and the instrument cluster will come free. Some people advise to remove the steering wheel to make it easier to remove but in my opinion, this is a waste of time as it’s not much harder to do with the wheel still in place.

Unclip the connections on the back of the cluster and remove. You do not need to worry about speedo cables as the readings are taken electronically.

Now you should have sufficient access to start fitting the OBC, start off by threading the wiring loom through the dash and guide each plug\connector to the approximate location (as shown previously) ready for fitting. You may wish to clip the loom in with the main wiring loom, this is entirely up to you and will make it look more factory should you or anyone else in the future, decide to remove parts of the dash again.

Now that your loom is in place, you can start connecting the parts to it. I would fit the actual OBC unit first, this can be quite fiddly and I must stress… do NOT drop the screws that hold it it, you will never find them again if you do!!!

You will need to release the heater panel in order to make life easier when fitting the unit. Pop out the rear heater switch and hazard light switch and whatever other switches or blanking plates you have there (should be 4 in total). Behind 2 of these you will see a screw, undo these 2 screws taking care not to drop them.

You now need to make use of the stubby cross-head screw driver, beneath the heater panel if you stick you fingers underneath you will feel another couple of screws, undo these and the heater panel should come loose.

There are 4 screws fixing the OBC in place, you will need to use the stubby screw driver again here to do up the 4 screws, I put my hand through the hole for the stereo and did them up this way. Ensure that you fix the green plug to the unit before securing it as it will be difficult to plug it in once the OBC is in place.

Refit the heater panel in reverse of removal.

I would now fit the relay control module. You will need to undo the 4 10mm bolts that hold the ECU in place and then remove the plate above the ECU. The relay control module is held in place on top of this plate by 2 large cross-head screws. Fix the relay control module and refit the plate and ECU, plug the loom into the module.

At this point I would test to make sure that the computer is working, plug the yellow connector into the back of the instrument panel and turn on the ignition to make sure that the computer lights up, it should light up orange and flash the letters UHR. If all is well at this point then continue.

Next I will explain how to fix the indicator stalk, this is fairly straight forward. Remove the lower steering column by removing the retaining screws and it should drop down. Next unplug the existing indicator stalk from the main loom and then undo the 2 retaining screws which hold the stalk in place.

You will now be able to swap the stalks over and re-assemble the trim.

Next you will need to fix the temperature sensor into the left hand brake cooling duct, there should already be a place that it plugs into, you will easily see this. You will also need to bolt the OBC horn and cover on, this goes up above where the fog light would be situated, it’s hard to explain exactly but you will know when you see it especially if you removed the computer from another car yourself.

Plug the temperature sensor\horn wiring loom into both the sensor and horn, feed it back along the side of the engine bay clipping it in with the rest of the wiring. You will see a rubber grommet beneath the brake servo, push the end of the loom through the grommet and into the car.

Now working from inside the car, attach the 2 looms together behind the glove box. You will see a main earthing point here which holds several brown wires onto the point with a nut, remove the nut and place the brown PBC earthing wire on the earthing point and secure the nut again.

You should now be left with 2 lose wires, a singe green one and a red with white tracer. These wires simply plug into the main control box, there is only one place that each will fit so you shouldn’t have any problems there.

Finally it’s just a case of reassembling the dash and glove box and re-fitting the stereo.

Article written by E30Adam and copyright to E30zone, must not be duplicated without written permission.