Constant flashing coolant light

I have 1989 GTI Cabriolet with the DX engine.

The coolant level sensor light on the instrument panel has started flashing continuously.

Before that, the gauge was a little temperamental. Every so often it'd give no reading for a while, then would inexplicably come back to life.

I have checked that the coolant level is OK and that the sensor is clean - they are. I also pulled the plug off and bridged the two terminals to see if it stopped the flashing - it didn't.

I've just driven 4:45mins from Yorkshire to London and the car ran fine with no trouble at all, bar the flashing light, which kept flashing.

I assume it's an electrical fault. Any suggestions on where next to check or what I should replace?

I do have a spare gauge so could give that a try. Just want to be sure I haven't missed anything obvious before I do.

Thanks!

I get the same problem every now and then, I believe there is a relay on top of the fuse box can remember if it's 43 or 57 in my picture, if you unplug it and clean up the connections, refit and see if that helps?

Constant flashing coolant light can be a bad gauge, a bad wire, a bad sensor, and a bad control unit.

The most common is that the sender has 2 metal tips, while they look clean they can calcify.  Since you really can't short the sender out to test it, there is IIRC about 180 ohms of resistance in the tips when they are in coolant.  Remove the sender and use sand paper on the tips.  When replacing it be sure the jug is to the full mark prior to replacing the sender, it is a 27 or 28mm socket that you can use to remove, Lubricate the o-ring.

The sender sits in a plastic cup, and on older style Expansion jugs, they didn't have a "Air Bleed" hole at the top so your sender can be sitting in a air pocket and not in the fluid.

If your Fluid is more than 50/50 in a dilution that can interfere with the proper operation.

The Gauge itself can get "Cold Soldered" joints internally.  There are 3 studs for the gauge, one in the center is for the flashy light.  

These gauges are basically a toaster.  They use a nica-chrome wire wrapped around a Bi-metallic spring.
As the sending unit sends resistance to the gauge from zero to infinity (ok not infinite) the wire heats or cools bending the metal spring that the needle is attached to.

The 2 larger studs on the back can get cold soldered and half work.  So the bias that is needed for the flashy light isn't correct and can trip that.  

You half to take the face off the gauge and re-fresh the solder.

The connections at the bottom of the cluster can be tightened to make better connectivity.  You remove the fingered plastic shroud off the connector.  Carefully remove the mylar off the tongue.  On the plastic tongue you will place one strip of electrical tape, (pin side of the tongue.), replace the mylar, replace the shroud.

lastly there is the 10v stabilizer that controls the voltage to the water temp and fuel gauge, they go bad.
so follow the Bentley and test it a wee bit.

In the archive section below I have how-tos that link to threads I have done on the repairing of the flaky water/fuel gauges.  

to rule out level sensor and wiring, just remove the 42 or 43 relay from the holder above the fusebox.

if it stops, then go over level sensor and wiring. if not, its temp sensor, wiring or dash clocks at fault.

make sure sensor connection is clean, wire is well terminated and no breaks in it that could short to ground. check the brown/white wires to the side of the head, one of them is the earth for the clocks

Thank you for all the advice - as ever, it's very much appreciated!

So as a first step, I pulled the 43 relay from the fuse box. There was what looked like the remains of a chrysalis on one of the prongs. I cleaned it up, put it back in, and that seems to have stopped the flashing warning light. So easy!

I'm not sure how best to trace the fault that causes the gauge itself to randomly turn on and off. I presume there must be a loose connection somewhere in the circuit.

TeeTee said

I'm not sure how best to trace the fault that causes the gauge itself to randomly turn on and off. I presume there must be a loose connection somewhere in the circuit.

That is the Cold Solder joints on the mounting studs… repaired about 4 of them (Golf, and 3 cabbies).

Heres the link.
Repairing your Flaky Water Temp Gauge | Volkswagen Owners Club Forum

ity may also be loose connection at the main clock connector on the bottom, mine did the same.

unplug and check all the pins are nice and tight, they get squished down with age. look at all the contact pads on the cluster as well, plus you can use chortles tip of removing the connector socket then packing out the blue foil with some card