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A temperature gauge reading at 100°C or more makes any owner nervous. But on an R107, the gauge and sender are 40 years old, and their readings can be off. Here is how to tell a real problem from a gauge problem.
Normal range
With the original 80°C thermostat, R107 V8s usually show roughly 80–90°C on the highway and may climb toward 100°C in traffic on a hot day before the fans pull it back down. A gauge that sits steadily in that range is normal. A needle that climbs steadily past about 110–120°C and does not come back is a warning.
How to tell gauge error from overheating
- Real overheating: coolant smell, steam, boiling in the expansion tank, the heater blowing cold, and the gauge climbing continuously.
- Gauge error: the car behaves normally, the fans are not roaring, the temperature reading jumps or reads impossibly fast after start, or reads very low on the highway.
IR thermometer check
An infrared thermometer is the quickest check. With the engine warm, aim it at the thermostat housing and the upper radiator hose near the engine. Compare with the gauge. Allow some difference because surfaces read lower than coolant, but a gauge reading 110°C while the housing reads 80°C points to the sender or gauge.
Sender test
The temperature sender screws into the cylinder head or thermostat housing. On many R107s there is a separate sender for the gauge and a sensor for the fuel injection or climate control. Test the gauge sender with a meter: resistance should fall as temperature rises, following the chart in the manual. You can also unplug it and ground the wire briefly (ignition on): the gauge should go to full hot, which proves the gauge and wiring.
Fix
- Clean the sender connector and the ground.
- Replace a sender that does not follow the chart.
- If the gauge is faulty, the instrument cluster can be repaired by a specialist.
If the reading is real, see overheating causes.
Quick answers
- Can a bad sender stop the car from starting hot? The gauge sender alone does not affect running. A faulty coolant sensor for the fuel injection can affect starting and mixture.
- Is 100°C dangerous? Not if it is steady and the system is pressurized and healthy; the boiling point is raised by the cap. Do not open the cap hot.
- Why does my gauge read low on the highway? A thermostat stuck open, or a sender reading low. See thermostat.
