Coolant Temperature Sending Unit - location

Specific conversations and info for the BMW E28 M5 and M535i.
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Posts: 83
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Coolant Temperature Sending Unit - location

Post by Syndrome »

Hello. My coolant temperature gauge in my 1988 M5 is always pegged at maximum. The car has not run in over a decade, so I am assuming the engine is dead cold. I’m trying to understand where the temperature sending unit that feeds the gauge is installed on this engine. There are two sensors on the driver side of the radiator, and two that are on the coolant pipe just above the exhaust manifolds. I think there is a pressure sensor on the coolant expansion tank too. I’d like to know what each sensor is for, that way I know what to keep an eye out for when driving the car.

But my main and important question for now is, is one of the four sensors I have found is the one I have to check for the coolant temperature gauge in the instrument cluster, or is there another temperature sending unit somewhere else that’s the culprit?

Thank you!
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Re: Coolant Temperature Sending Unit - location

Post by Galahad »

I don't have an M5 so someone correct me if I'm wrong. The two in the radiator are temp switches for the electric aux fan, the one in the expansion tank is the level sensor that trips the check control for low coolant. The remaining two (on the coolant pipe) would be a temp sensor and either a cold or hot switch (can't remember off the top of my head).

Gauge pegged hot is an odd failure mode, normally it fails to only reading cold. Possible issues I can think of:
your temp sensor failed shorted (seems unlikely, they usually fail open)
the gauge cluster went bad (possible but seems unlikely since you have a late cluster). I have an early cluster with an intermittent connection to the temp gauge but it reads either stone cold or whatever the temperature is.
the other sensor is a cold switch (closed when cold) and someone swapped the two connectors

I'd start by measuring the resistance of all your coolant sensors and seeing what you get
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Re: Coolant Temperature Sending Unit - location

Post by tylerrose »

The sender unit for the cluster is located in the coolant pipe above the manifold. It has two spade connectors attached to it.

The blue (if it hasn't faded) JPT-connected sensor on the pipe is the DME temp sensor that dictates fuel trim.

The sensor in the expansion tank triggers a light on the check panel if the coolant level drops.

The two on the radiator are low and high speed aux fan switches. Low speed closes at 91 deg C and high speed closes at 99 deg C.

To diagnose the gauge, I would start at the spade-connected sensor and work back to the cluster. As stated above; the default position is cold if the connection is open.
Intermittent temp gauge issues may occur with bad SI board batteries, but in the cases I've seen, this also pins the gauge cold.

Best of luck!

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Re: Coolant Temperature Sending Unit - location

Post by RossDinan6 »

I have not needed to troubleshoot this issue on my E24, but I do so in the marine industry. VDO in my world pegs the gauge high when open and low when open. Mine car is apart at the moment so can't confirm. Pull a connector from the sending unit, check the gauge, then short it to ground and check the gauge again. The direction the gauge goes isn't real important. Confirm it pegs in both directions. This will prove the gauge and all connections. If It does operate this test operates the gauge you need a sender, I'd use a meter to confirm the resistance. If not the gauge and wiring need to be checked.
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Re: Coolant Temperature Sending Unit - location

Post by ///M »

The two plugs on the coolant pipe above the manifold are for coolant temperature. The Black two pin plug is to the gauge and from the ETM should be Brown/white wire. It goes to C101 engine plug. It then changes to Brown/violet and goes to the instrument cluster blue plug pin 12. The temperature gauge is connected to the service interval (SI) processor.
To show hot the wire is grounded so if you remove the black plug you should be removing the ground signal. If the gauge still shows hot then it is either stuck or is being grounded by another source, possibly the SI processor.
The Blue two pin on the manifold is to the DME and should have a brown/red to the DME and brown to ground.
Search for e28 wiring diagrams online there is one for 1988 that has M5
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