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Tech Article

BMW E30 3-Series Temperature Gauge Malfunction

Last Updated: 03/08/2017
   
Parts Group: Temperature gauge
We sell good, original BMW, used E30 temperature gauge units. If you like, you can skip directly to where you can buy this part:This article discusses the part with the main focus being on troubleshooting.

Sometimes people think they need the gauge because their wiring and sending unit are okay so they figure it must be the gauge, and then they find out that -- oddly -- it's not. I've seen an old-school pro auto electrical guru (a very savvy guy) become very puzzled as such. He replaced every traditional part and still the temp gauge on my E30 didn't behave.

He was overlooking this subtlety: starting with the BMW E30 as in 1984-1991-or-so, the 3-series has a service interval indicator so that you don't just always have to get your oil changed every 3,000 miles or so. The optimal number varies based on many variables, such as temperature and engine speed, all of which are measured by the electronic control unit anyway. Using this data, it calculates precisely when the next oil change is due, and it displays the developing news in advancing green, yellow and red lights on the instrument cluster.

This works great until someone disconnects the main battery in which case that tracking system would normally lose all its data. That's why each such BMW has its own internal battery backup. Those batteries work well for 15 to 30 years, and then they start dying and then the cluster (typically the tachometer and temperature gauge) go haywire too. There are at least four different types of battery boards so how they misbehave probably varies too.

If the batteries soldered onto the instrument cluster get old, they can cause the temperature gauge to appear to be the problem. If you haven't replaced them, then (much as I'd love to sell used good temp gauges) I would start with that. I took the one for my E30 to Batteries and Bulbs and they replaced the batteries AND soldered them in for less than $10.

Ironically the temp gauge on my personal 1989 325i is behaving lazily (and so I need to mess with my batteries too, again) but after a few miles of driving the temp gauge suddenly wakes up and behaves fine due to the batteries having recharged.

The malfunction gets worse faster when the car sits for long periods of time without being started. That makes the internal backup batteries run down much faster.
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