Cluster or Temp Gauge Problem?

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Syndrome
Posts: 128
Joined: Nov 23, 2021 3:46 PM
Location: DC

Cluster or Temp Gauge Problem?

Post by Syndrome »

My E28 M5 is making amazing progress. I’m now looking at a problem that is and isn’t a big deal, but I want to fix it. Basically, the water temperature gauge in the instrument cluster jumps to beyond maximum as soon as the key is put into the second (run) position.

I have new sensors in the coolant pipe, and new sensors in the radiator. When I disconnect them all, individually and all together, the gauge is always at the top reading. I’ve checked the continuity of the wires from the sensors to the diagnostics port, and it all seems to be OK. I don’t know where the wires go from the continuity port, so I have not checked them further.

Could it be that there is a problem in my cluster? I will say that I was adventurous and I opened my cluster to replace the speedometer gears, which were getting buttery. Did I do something to mess up the team gauge while I was in there? No idea, and no idea how to check. It would be nice if I had another cluster to swap in. I can’t even find one to buy.

I would love to have some advice re. I have a beautiful original copy of the “1987/1988 BMW 535i, M5 Electrical Troubleshooting Manual”, but I haven’t figured it out on my own nonetheless…

Thank you!
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Cluster or Temp Gauge Problem?

Post by Mike W. »

IIRC the gauge goes to max when it's an open circuit, so I'm guessing one of the plugs on the coolant pipe got switched as they're all different. I think it's unlikely you did something to the cluster. The radiator uses a switch or switches, not a sensor and it just serves to turn the aux fan on and off. The location of the sensors matters less than having the right plug on them. The thermo time is easy, it's brown, should have a brown plug to go on it. I thought M's used the same sending unit switch as regular E24s, which is 2 push on wires, not a plug, but that's not what I'm seeing right now, but I don't know if it's an updated parts diagram or not.
Syndrome
Posts: 128
Joined: Nov 23, 2021 3:46 PM
Location: DC

Re: Cluster or Temp Gauge Problem?

Post by Syndrome »

Hi. Thanks. Right. The sender on the coolant pipe over the exhaust manifolds has a brown and brown/white wire. These have a smaller connector and a regular sized one, so there’s only one way to plug them in. The other sensor next to it is the thermo-time sensor with a two pole blue plastic connector. At the radiator, there is a three wire switch that turns on the fan, if I’m not wrong. There *may* be another temperature sensor/sender beneath the switch on the radiator, which I think may be how the switch actuates. I can disconnect them all and nothing changes at the gauge. I can’t imagine what the problem is. When I lived in Italy, there were specialized mechanics for electrical problems (They’re called “elettrauto”). I wonder if I could find an American version near me who could figure this thing out. I wonder if I would be able to find a whole other cluster to buy.
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Cluster or Temp Gauge Problem?

Post by Mike W. »

I doubt there is a problem in your cluster, though it's possible. There are auto electric specialists, there's at least one and maybe two in the mid sized central town near me, population ~200K. But...

OK, sounds like your sending unit should look like this,
Image

If so that's good, that's the gauge sending unit. That is what the temp sensor is looking at. The large terminal is for the gauge, the small goes to the warning light on the cluster and is not a sensor, it's a high temp switch for a warning light. Ohm it out, I forget what the spec is, but if it's infinite, it's bad. The spec I bet is the same as non M cars and I'm sure is listed here many times. My guess is you have either a bad sending unit, unlikely, but possible. Or a break in the wiring between it and the gauge.
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