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Old 02-12-2018, 02:39 AM   #1
mike24
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Should I trust 30 years old gauges?

Hello good people.
I have a '90 coupe with M40B16 in it.
Bought it last year in November, after wishing for the past 10 years to own one. I dove in head first...which is the worst thing to do in any situation, let alone buying a 27 years old car.

Long story short, the car had some nasty plasma-type gauges which I threw out as soon as I got it. Installed a stock set that I bought second-hand from god knows how old-of-a-car (has 300.000+ km on the clock). Speedo doesn't work (neither did the one I threw out so I'm guessing it's the sensor leading from the diff). Gas gauge is not accurate at all, I have to rely on the warning light. Economy gauge is either max no matter what, or 0 when not touching the throttle. Rev counter works good from what I can tell.

My biggest concern is the temp gauge. When I put on the ignition, it snaps instantly to red without the engine running. I took the chance of driving it to work and back (1.5 hours of quite heavy traffic) and there's no sign of overheating. No weird smells, no excessive heat in the engine bay, no coolant leaks in the parking spot, heater works perfectly.

Important thing to mention: temp gauge worked fine after I've installed the new cluster. Drove it daily in Dec and Jan, staring out cold and slowly rising until reaching the center..and staying there as far as the engine ran.

What should I look for next? Sounds more like an electrical/gauge issue then anything else...
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Old 02-12-2018, 02:12 PM   #2
Gregs///M
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Unless you buy the cluster brand new from BMW direct or have us refurbish it, do not trust it. Your cluster has the typical failures, SI board dead, solder joints on speedometer and main board failed, odo gears broken.
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Old 02-13-2018, 07:12 PM   #3
g.ruff561
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Hey bro. Don't trust nothing 30 years old. Gauges, wires, hoses...women. IJS.
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Old 02-13-2018, 08:54 PM   #4
mike24
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Thanks for the input guys!

I'll try the old sandpaper-rubbing-trick on the connectors, maybe get the temp gauge working at least. From what I noticed, most of the times, cleaning is all these cars need.
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Old 02-14-2018, 08:24 AM   #5
AndrewBird
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There is a grounding nut on the back of the temp gauge that likes to come loose. Try tightening that. Also look closely at all the solder joints on everything and see if you see any cracked joints. These can be re-flowed with a soldering iron and a little solder.

The wires at the speed sensor in the diff get brittle and break easily. I'd start there for fixing the speedo. The fact the MPG gauge doesn't work points to this as it needs a speed signal in order to function correctly. A bad speedo wouldn't cause the MPG gauge to not work, so there is something else wrong (not that the speedo AND something else could be wrong though).
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Old 02-14-2018, 08:32 AM   #6
cheffy30
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timing belt and water pump should be next on your hit parade...
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Old 02-14-2018, 07:13 PM   #7
coldweatherblue
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Hi, I have a cluster with the same issue.

I tried tightening the grounding nut and sanding connections and nothing worked.

Suspected it to be a short in the cluster board/solder joints and swapped it out with a known good cluster.
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