I have a 1994 1.8 440LI volvo. Abou 8 months ago i replaced radiator, water pump, thermostat and fan switch. All was fine. Recently my tempreture gauge has been going into red and the warning light comes on. However engine does not appear to be overheating. Replaced the tempreture sender unit on the front of the cylinder head and the gauge sensor on the back of the engine, Still going to red and warning light coming on. Car drives fine.
Can someone please help

my first guess would be thermostat, check, is the radiator getting warm, how warm are the hoses from thermostat to radiator, how hot are the hoses to the heater matrix - if you turn the heater on when it says its running hot, how hot is the air coming out, if its very hot then the temperature is high, leading to the thermostat as the main cause, if the radiator stays relatively cool, that would also point to the thermostat.
Grendel

I've heard that the temp gauge itself is subject to malfunction, you should take it out and ohm it. Should give 50 ohm on both fuel gauge and temp gauge. You should clean the brass nuts that acts as conections from the PCB to the gauge, they're a bit of a weak spot.
Clean them good and use some contact spray, refit carefully and see what happens! (IMG:style_emoticons/default/smile.gif)

I think it must be the gauge,Car happily drives, but gauge and red temp warning light saying car is over heating. Done 50 mile trip in the car all fine. for a few days gauge has been normal. now back to saying getting hot. Any ideas on how i get gauge out

It should be some instructions around here. It's pretty much like in the haynes manual, but with some modifications.

BEFORE YOU DO THIS, DISCONNECT SPEEDO CABLE. It's at the rear of the block, all the way down there, and it has (atleast on mine) a protective silver hose, will make it easier to identify. Clean around the edges with some kind of thinner. Push it down while you pull the pin gently out, and cover the hole immediately, so that you don't get any dirt down there.

Back in the car:
1. Remove the plastic frame around the instrument panel. Should be pryed gently off somehow.. A friend of mine did that part, can't recall.
2. If you have left hand drive (like me), there will be two screws at the LEFT side of the instrument panel. Unscrew them!
3. Grab underneath the instrument panel module, pull it towards you while you push it to the LEFT. It's just tracked in place with some pins at the right side, will just slide out.
Disconnect the wires, disconnect the speedometer cable.
4. Take the whole thing out, and find the brass nuts and the bolts behind the temp and fuel gauge. Clean them, clean the bolts, use contact spray. Ohm the bolts, they should give you 50 ohm resistance.
Unfortuneately i don't remember the details in my head, but if you look around here or at volvoforums.org.uk's 400-section, there will be details!

Hope this helps!