Yesterday,driving my '87 SDL at @70MPH,thje low coolant light came on for 15..20 mins.and then went out for 5 mins.,then back on for 15,etc....over and over. The coolant level is fine,the upper rad hose is soft in the A.M.,and no leaks to be seen.I recently replaced the monovalve and heat is fine.When key is in ignition,but not turned on to start,the low coolant light does not come on.The others do come on,battery,seat belt etc..Where do I go from here?Does the grey sensor switch have anything to do with this?? Any help would really be appreciated..I'm just learning my way around this great car.Thanks

It is somewhat of a problem if the lamp does not illuminate with all the remaining lamps when the key is in the #2 position.

The lamp is fed power whenever the key is in #2 or #3.

It is grounded one of two ways:

1) Through the cluster. From the lamp, through a diode, and to the charging circuit via the charge indicator lamp in the cluster. If there is no alternator output, then the bulb will illuminate via a ground in the alternator.

2) Through the coolant level switch. The switch closes and sends current to ground. It shares a ground with the washer fluid level switch. The coolant switch has a brown wire with a yellow stripe coming from the cluster, with power. The switch has a brown wire which is ground.

You can do some tests at the coolant switch to see if you have a proper ground and whether the switch works correctly.

However, the lack of illumination when the engine is not running is still a question?? Bad diode that won't allow current flow??