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Tech Article

BMW E28 5-Series On-Board Computer

Last Updated: 07/09/2018
Parts Group: On-board computer
We sell good, original BMW, used E28 on-board computers. If you like, you can skip directly to where you can buy this part:
This article discusses the on-board computer (a.k.a. trip computer, info screen, or info display screen), for the BMW E28 5-Series with the main focus being a technical analysis.

BMW sourced the on-board computer from a high-quality vendor: VDO. This part is basically a trip computer with a display and several function buttons, embedded in the dashboard. It helps with calculating gas mileage, plus it has functions that come in handy when rallying. I used to navigate at road rallies and I once lost our team a victory because I didn’t do the math correctly while the driver was hurtling along at high speed. That day, I would have appreciated having an on-board computer.

This part is totally different than the computer that enables the car to drive, by controlling the fuel injection, ignition and so on.

Aside from version-related updates, the very same on-board computer part is used on multiple dissimilar models, so regardless of which 1980s BMW you drive, as far as the driver is concerned, the interaction is the same. I love that. That’s a similar approach to the instrument clusters on the various 1980s BMWs all being nearly identical. To me, this sort of consistency is the mark of good design.

To make the electronics coordinate well with the various different models and their very different types of engines, VDO added a coding plug in the back of the on-board computer. This coding plug calibrates the on-board computer so that it works precisely and correctly for the relevant model. That way, VDO needed to make only one type of on-board computer yet its internals behave differently depending on which plug is shoved into it.

These coding plugs are sold separately. For example, the eta-engined BMW 528e made before 1988 uses a different coding plug than the super-eta BMW 528e made in 1988 ... different characteristics, hence different coding plugs, yet the trip computer they use is identical. Not surprisingly, the 533i uses a different coding plug yet.

By implication, if your on-board computer failed, don’t throw away the whole thing. First, remove the coding plug since that is the right one for your car, and then insert it into the replacement trip computer you buy. This will correctly calibrate the replacement computer to your specific model of BMW. Inserting or removing the coding plug is trivially easy and takes only a few seconds ... pulling or pushing.

I haven't verified this personally, but I've been told that using the wrong coding plug can make the on-board computer misbehave completely.

These on-board computers use a backlit liquid crystal display, and the lighting comes from a light housing, whose incandescent light bulbs eventually fail. Replacing the light housing is as simple as pulling it out and shoving in the replacement light housing. On that subject, if the light housing on your on-board computer has failed, don’t throw away the entire computer. Just replace the light housing. Conversely, if the on-board computer has failed but the light housing works, remove it before discarding the computer.

We happily sell used, original-quality examples of the coding plugs, light housings and on-board computers.
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