Compiled by Chip Baines for an '86 528e, posted by John in VA, Thu Sep 03, 2009

Original thread: E28 Late On-Board Computer (OBCII) Information

This is general information for the late On-Board Computer (OBC II) in an '86 528e. It also applies to other mid-to-late 80s cars. Most of this is in the owner's manual, except:

• How to calibrate the range display
• How to display the OBC version
• How to replace the bulb / light bar
• What to do if the OBC display reads PPPP

Set the time

If the battery is disconnected or becomes drained, the OBC defaults to factory settings: metric units and 24-hour time. Once power is back, reset the unit.

1. Press the HR-DATE button until the OBC is displaying the time (not the date).
2. Press the recessed change-over contact button near the lower right corner of the display with something pointed, like a ball-point pen. The display should change to four digits.
3. Set the time in HH:MM format with the numeric buttons along the bottom (1000, 100, 10, and 1), each corresponding to one digit. Example: 11:32 is 1000 once, 100 once, 10 three times, 1 twice.
4. In metric (24-hour) mode the first digit is 0, 1, or 2, and the display is labeled UHR. In 12-hour (US units) mode, repeatedly pressing 1000 cycles the first digit between 0 and 1 AM, then 0 and 1 PM.
5. Press the recessed button again to start the clock.

Set the date

Press HR-DATE to display the date. Format is MMDD in US units, or DDMM in metric. Set it the same way as the time, then press the recessed button to store it.

Metric versus US units

Press AVG MPG, then press the change-over contact button. Repeated presses cycle between:

• ----L:00 — liters per 100 km (metric: km, liters, °C, 24-hour clock)
• ----MPG — miles per US gallon (English units, 12-hour clock)
• ----M/G — miles per British imperial gallon (same English units, but an imperial gallon is 20% larger, so the calculated mileage is 20% higher)

Be sure you have MPG vs M/G set correctly.

The CODE function

The CODE button controls an alarm and ignition cut-off.

1. Turn the engine off but leave the key in accessory (radio on, engine off).
2. Press CODE. The display should change to ----CODE.
3. Enter a secret 4-digit code with the 1000 / 100 / 10 / 1 buttons.
4. Turn the key off and remove it. The alarm is now set.

The next time you try to start, the OBC flashes CODE and sounds the bong. The car will not start without the code. To disable the alarm, turn the key to accessory, enter the code, and press SET-RES. Then start the car.

If you try to start without the code, the engine will crank but there will be no spark. Do not keep cranking — you may flood the engine. If you enter the wrong code three times in a row, the alarm goes off (horn and lights) for about a minute and cannot be shut off early.

If you forget the code, disconnecting the battery for a moment or two resets the OBC and it "forgets" that the alarm had been set. This is not a very secure system.

Average MPH and average MPG

Press AVG MPH or AVG MPG to display that value. SET-RES resets the calculation. Reset average MPG with every full tank if you want a per-tank number. In metric mode these are average km/h and L/100 km.

Maximum MPH

This function bongs if you exceed a stored speed. Press M MPH, enter the limit with the numeric buttons, then press M MPH again. The LED in the button lights when the function is on; it flashes and the OBC bongs when you exceed the limit. In metric mode the limit is in km/h.

Timer

Press TIMER, then SET-RES to start. Press SET-RES again to stop. The button LED stays on while the timer is running.

Accuracy:

• Tenths of a second for the first minute
• Whole seconds from one minute to one hour
• Minutes from one hour to 99 hours 59 minutes (then it rolls over)

If you leave the timer running when you turn off the ignition, it keeps tracking elapsed time.

Temperature

Press TEMP for outside temperature. The OBC bongs if temperature approaches freezing (around 36 °F). Switch °C / °F by changing metric vs US units. A reading like −28 °F usually means a broken sensor, unless you actually live somewhere that cold.

Range function and calibration

Press RANGE for an estimate of remaining distance. Do not rely on it. Below about 10 miles you are likely to be pushing soon. The display can show "virtual" leftover fuel.

This calibration is not in the owner's manual:

1. Park on a level surface.
2. Press 1000 and 1 at the same time. The display shows liters of fuel left in the tank. (A later reply said 100 and 1 — that combination is the firmware version, below.)
3. When the low-fuel light first comes on, the tank should have about 4.5 liters left (some people say closer to 6.5 — treat this as approximate).
4. There is a small adjustment screw behind a plastic cover in the lower left corner of the OBC. Turn it until the display reads 4.5 L.

Display the version number

Press 100 and 1 at the same time. The display shows the OBC firmware version (example: 5 .8 TYP).

OBC bulb / light bar

The display is lit by a light bar that slides into the upper right corner of the OBC. Needle-nose pliers make it a snap to replace. If the readout is dim or faded, replace the light bar rather than looking for internal batteries.

OBC light bar. It slides into the upper right corner of the unit.

Changing the bulb means pulling the OBC from the dash. The bulb sits under a cover on the side or rear of the unit.

It is also possible to convert the display from incandescent to LED. Red and red/orange LEDs are used to match stock dash lighting, or you can convert the rest of the dash to LED and match the OBC. See the thread (fastpat, 2010).

What to do if the OBC displays only PPPP

This can happen after an electrical surge. Reset it:

1. Press CODE.
2. Press the small recessed button on the lower right.
3. After a second or two, the LED in the M MPH button flashes once and the OBC is reset.

All memory is wiped, so you will have to set the date and time again.

From the discussion

Light-bar replacement and the LED conversion note are from later posts in the same thread (John in VA, fastpat).