The E28 temperature gauge is not buffered like later BMWs (E39 and on). The needle actually moves. That is normal.

Original threads: FAQ — temp gauge troubleshooting (rob6118, 2007) and Where should my temp gauge be / temp range of temp gauge.

How to read it

Approximate E28 temperature gauge readings (Shawn D. and Rod Paine). Sensor and gauge working properly.

• Normal running: needle straight up or a little to the left of 12 o'clock (about 180–195 °F on the chart). Many healthy cars sit nearer 180; 195 is where a lot of people want a 535i.
• After a short stop and restart it may read a bit high — heat soak in the coolant next to the sender. That should settle once you are moving.
• Hot at idle or in traffic, normal on the road: look at the fan clutch before you tear into the gauge.
• Needle bouncing, or hot while driving but normal after you shut off and turn the key back on: bad engine ground (see G103 below).
• Thermostat opening temperature moves the needle a little. What you want is a steady needle, not a particular tenth of a clock face.
• An IR thermometer on the radiator will read cooler than the sender (the sender sees water leaving the head). The top radiator hose is a better IR spot.

Unlike an E39, this gauge is not damped. If it moves with load, traffic, or A/C, that does not by itself mean the car is overheating.

First: is it actually hot?

Before you pull the cluster, put a meat thermometer in the expansion tank on a fully warm engine. That is the number that matters. A flaky gauge can make you crazy while the coolant is sitting at 180 °F.

If the radiator is much cooler than the reservoir while the gauge is high, suspect a thermostat that is not opening. The radiator is out of the loop until the t-stat opens; some heat soaks through the upper hose, but no flow through the bottom of the radiator.

Sender test (Bentley)

Ground the sender wire to the engine block (or to the sender housing).

• Gauge redlines → the wire and gauge path are good. If the gauge was dead or low with the sender connected, the sender (or its ground) is the problem.
• Gauge does not redline → the fault is the wire, cluster, or gauge — not the sender.
• On a working cold car, grounding the wire redlines; plugging it back onto the sender puts the needle down on the blue mark. That is a pass.

Do not unscrew a hot sender. The system stays pressurized for hours. Forgetting that produced a 15-foot geyser of scalding coolant.

Sender resistances

• Sender housing to thermostat housing: low, on the order of 7 Ω.
• Sender terminal to housing, coolant hot: about 30–50 Ω (one measured 40 Ω).
• Sender out of the car, cold: open circuit until it reaches roughly 65 °C.

Also check sender-housing / block ground all the way to the battery negative. A rotten engine ground (G103, above the starter — rodpaine) has thrown several E28 temp gauges off.

While you are at the sender, look at the wires. Corroded or torn leads at the connector are common; replace them while you are there.

Cluster, SI board, and the gauge itself

If the sender test says the car side is fine, the fault is wiring into the cluster, the SI board, or the gauge.

1. Check the sender connector for broken wires and corrosion. One working car showed about 8 V there, engine warm.
2. Intermittent voltage or an intermittent needle: pull the cluster (see Service Indicator Reset). On the back of the SI board, the connector that shares the temp and speed (tempo) wires can wear through the plating into the board. Fresh solder on that pad has fixed intermittent readings.
3. The SI-board ground pigtail must be on the pin on the main board near the night-time illumination bulbs. An unplugged pigtail has killed the gauge.
4. The nuts that hold the gauge board to the main board need to be tight — same nuts as the fuel gauge.
5. Cracked solder joints are common. Use a magnifying glass and reflow anything that looks cracked.
6. To test the gauge movement itself: two rivet-looking grounds and a nut (power) on the back of the temp gauge (fuel is the pair below it). A AA battery across the terminals should redline the needle. If it only moves when you help it past halfway, the movement is sticky or the needle is bent.

A needle sitting in the wrong place can simply have been knocked off its stop. Rotate it back until it sits on cold.

SI batteries

The Ni-Cad cells behind the service-indicator LEDs should read about 3 V. They keep SI memory. They are not required for the temp or fuel gauges. Early clusters have been run for years with the batteries removed (and the SI lights disconnected); gauges still work. If fuel or temp die when you pull the batteries, look for cracked solder, not the cells.

A remote battery box is covered on Service Indicator Reset.

Four usual causes

1. Bad sending unit
2. Bad or corroded wires at the sender
3. Bad connection or cracked solder on the SI / cluster board
4. Engine / G103 ground

From the discussion

• Ground the sender wire as a pass/fail of the wiring, not as "sender is bad if it redlines." A good system redlines when you ground that wire (EvanJacob, garageboy).
• Measure real coolant temperature first. Everything else is secondary (garageboy).
• SI batteries do not feed the temp gauge (turbodan, doug).
• Reflow cracked joints; check the SI ground pigtail and the two gauge-board nuts (karl, wkohler).
• A sticky or bent needle can look like a dead gauge (karl, mckend).
• Air left in the system after cooling work will make the gauge read high until it burps out — sometimes after a long drive and cycling the heater valve (rob6118 on t=34159).