This one is puzzling me, what’s your thoughts?

[Woodyyellow]

Electrical puzzle?

Engine starts and runs fine, no problems there at all! Water cooling light extinguishes as it should after 4 seconds or so, BUT every now and then it will start flashing at random times. 
 OK I thought, as I need to replenish the coolant, I will drain and thoroughly flush the system, refill with correct type and ratio of antifreeze and water. 
 Random flashing again! 
 OK, bad contact with wires onto coolant level indicator sensor, so cleaned and sprayed with electrical contact cleaner. Have a new coolant sensor on order.
 Thought it had fixed it but then went on 230 mile round trip yesterday  to collect eBay purchase, more about that later - it is van related!
 During the journey was noticing the light flashing again in what I thought was a random sequence. Little by little I began to understand when and why it was doing this. Should say at this point I have a dash cam wired in permanently, don’t know if this is relevant?
 Put indicators on, no flashing light.
 Apply brakes / lights, no flashing light. 
 Put hazard lights on, flashing coolant light when on and for several minutes after switching them off! 
 Put side lights on, flashing coolant light on and for several minutes after switching them off! 
 Flash headlights to call someone forward, flashing coolant light when on and for several minutes! 
 Put headlights on, flashing coolant light for several minutes when on and after switching them off!
 Noticed when big power drain, ie headlights, tachograph would stop working!
 Managed to get home before it got dark as not sure what was happening on the outside of the van!
 So my diagnosis; either the alternator is not able to keep up with the electrical demands at times so pulls power from the battery to assist with needs, then takes a time to replenish battery and whilst it’s doing this the coolant sensor is starved of power, or battery is failing so alternator is struggling to meet demands.
 Initial tests done:
 Battery voltage at rest ie. engine off:12.75
 Battery voltage after starting / stopping engine and coolant light stopped flashing: 13.3
 Voltage at alternator with engine running coolant light stopped flashing: 14.4
 My thoughts are that the alternator is not producing enough power output for demands of the systems so could do with replacing / upgrading. Internal voltage regulator faulty?
 Or, battery is on the way out and needs replacing, no date label on battery
 Or, even both need replacing!!!!
 So what thoughts are out there? Am I getting close to a possible solution or am I miles away? Please?

[MarkHughes]

I would check all the earths first. And then check the voltage on the system when running before thinking about an alternator swap. Does it have a voltage regulator on the dash clocks that might be on the way out ?

[Aidan]

the earth for low coolant sensor is commoned in the engine bay where it meets the 7 pin plugs, 4 wires into 1 coolant temp, coolant level, number plate light and heater motor and commoned to engine bay earth too, pins 4 and 7 so lot of connections there as well as the coolant low reservoir control unit in the relay plate,

I'd clean the connections on the control unit 1st, (relay pos #3,) maybe change it for another one and look over all the connections tracks 70, 98, 150 only about 20 in all, diagrams 97.209, 97.211, 97.216 in Bentley for 1990

weirdly I just checked my 1990 WBX SA van, and I don't have a coolant level control unit currently fitted, should be a 191 919 376 A, a #43 I believe - might be why I'm not getting any low level warnings

[colinthefox]

My temperature warning light has a habit of flashing when my phone is charging and nearly full, and has done for years. I now think of it as a "phone nearly charged" light.

[Woodyyellow]

Thank you both, Mark and Aidan, that gives me some good leads to try and trace the fault.
Will report back in a couple of days with any progress.
Cheers, John

[clift_d]

Our coolant light also comes on at random times. Unfortunately it’s not consistent … it’s sufficiently infrequent to make you question whether the level might need checking … a legacy of head gasket problems we had many years ago, but so far it’s always been a ghost in the machine. I’ve tried cleaning the sensor contacts, but I’d never thought about the engine bay earth. I’ll try that next time.

[silverbullet]

Failing capacitor in the temp gauge gives intermittent warning lamp symptoms iirc?