UK Car Repair: Vauxhall Astra 1.7TDS Overheating, vauxhall astra, jumper cable

Question
QUESTION: Hi, I have a 96 Astra Estate 1.7TDS with the isuzu diesl engine. It has started to overheat going up to between 100 and red on gauge. Checked thermostatic rad switch which is operating fine. Have checked rad fan and this is working fine. Have checked relay and this is fine. have also checked earths and continuity between components via loomm and this is fine. rad is hot top and bottom as are hoses. Any ideas please why the fan is simply not cutting in?

cheers
ANSWER: Are you sure it is actually getting hot as opposed to a gauge malfunction?  And, do you have AC? if so, there is a separate pressure switch in the circuit! The relay has 4 wires, the battery feed for the motor goes to terminal 30 and when connected, out at 87 to the fan.  The relay feed goes to term 85 then grounds via the switches through 86, you can test the fan system first by using a jumper cable between 30 and 87 on the relay socket, or, you can leave the relay in and earth the the 86 terminal with a temporary wire connected under the relay to a good earth.  Let me know.

---------- FOLLOW-UP ----------

QUESTION: Am sure that it is actually overheating as it only occurs under heavy load situations or when stationary and held at 2500rpm. Cools immediatly when load situation is passed or driven slowly ( 30mph ) to create airflow. I dont have a/c, and have checked that fan works by jumper across rad fan switch terminals. Feel sure that fault must be somewhere in the loom, but simply cannot trace it. Many thanks for you advice and thoughts on this.
Cheers

Answer
Well you have two options, trace the circut and find out which sensor is faulty or, fit a separate switch that you can control from inside the car. You can do the latter as a temporary measure or/and override then find a wiring diagram and trace the fault!