Fixing a “Limp Home” Mode Issue

The E32 750iL uses an electronic connection between the driver’s right foot and the throttle plate opening. No more mechanical cable.

This system is named “EML” which is an acronym for Elektronische Motorleistungsregelung which is … electronic motor output regulation. I think. German is my first language but my Oma didn’t teach me BMW tech stuff.

An EML light in the instrument panel should come on briefly when the car is started. This shows all is well. When there is a problem, the system would rather not go crazy and maybe accelerate when you didn’t want it to. So, if it self-detects a problem, it shuts down. That means the car goes into a mode where whatever your right foot does has a very minimal effect as to engine speed, bringing it from idle to barely above that, and also the transmission in “drive” mode is in third gear. This tends to enable you to get home slowly but surely, sort of like a wounded animal limping home.

As to troubleshooting: it’s either a false alarm or not. The former means that a bad EML light can actually trigger the problem. Troubleshooting this aspect is probably the easiest. The ideal is to have another good, running similar car and to swap instrument panels, and to draw conclusions from what you observe as a result. If that doesn’t make for a quick fix, you can diagnose the instrument panel. The later ones have a self-diagnoses built-in. The earlier ones require a lot more work, and the risk of breaking parts, and the need for an ohmmeter.

The instrument panel for the E32 BMW came in three versions. The first one had a gray back plate. This was used until 2/89. The second one had a white back plate.  This was used from 2/89 to 9/90. The third one had a blue back plate and that was used from 9/90 until the end of the production run.  The colors imply different internals and non-interchangeability specific to the gray back-plate, though the white back-plate and blue back-plate versions are very close. The latter two save the car data on a chip that can be moved from instrument panel to instrument panel.

Basically, our BMW 1989 750iL car is in “limp home” mode. This mode is announced by the EML light in the instrument panel not coming on. This could be due to a fault in the instrument panel, or elsewhere.

To check the instrument panel, I got another gray-back-plate instrument panel and installed it.  The problem did not go away.

Perhaps both instrument panels are bad, perhaps it’s time for me to stop suspecting the instrument panels. 

I have started dismantling the original instrument panel. There is a back half and a front half, and they hinge together. The hinge comes loose, and so I’ve split the two halves. The rearmost half, into which the EML lights fit, has a light gray plastic housing and a circuit board. I’m assuming that the component-measuring I need to do is from behind, so I need to remove the circuit board from the gray plastic housing.

I found it hard to do this. I pried loose the 4 little feet that affix each plug base to the circuit board. it loose but it doesn’t seem like that’s the right way. I’m trying to figure out how to get these apart safely.

I’m enthused about this because one of the two EML lights seems blown (black glass) and the other might be unhappy too. I’m trying to rule out electronic causes before I start unbolting expensive parts … two blown light bulbs would explain the issue away, 100%.

Next hurdle: how to remove the printed circuit board from the back plate.

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