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Old 02-12-2018, 02:39 AM   #1
Join Date: Feb 2018
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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
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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
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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
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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
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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
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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
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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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Old 02-25-2018, 04:10 AM   #8
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@AndrewBird - I'll take apart the cluster sometime next week hopefully and crawl under the car to check diff sensor/wiring.

@cheffy30 - God knows when those 2 were replaced by previous owner... seems to have been neglected in the mechanical aspect so they were due to be replaced by me soon.

@coldweatherblue - Bought the cluster from a trust worthy guy who supposedly checked them before selling them to me for 90$.. which seemed like a fair deal. I've seen online prices of clusters and I don't wanna gamble a few hundreds on another set that could act the same way.
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Old 02-25-2018, 10:42 AM   #9
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Greg just went through the gauge cluster on my M3. The coolant gauge was stuck on hot and it ended up being the SI board. Thankfully, I had a good spare and we got it all fixed up. If you can find a good Si board and fix your odometer gears (and speed sensor) you could just grab a good fuel gauge and replace that.

You can fix it cheap by buying used clusters and/or parts at the cost of time, or trade time for money and send it out for restoration.
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Old 02-28-2018, 05:12 AM   #10
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From what I've seen, Gregs///M does awesome work. If electronics scares you....SENT IT TO HIM!

But....with that said...There's 4 main gauges. Fuel, speed, tach, and water temp.

The first one, and last often are the issue of the brass nut on the back of the cluster is loose. You pull the steering wheel, pull the cluster, tighten those suckers back up and all is good. It get's a bit trickier with the tach and speedo.
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hey does anyone know anyone who gets upset and makes electronics?
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