After-run relay diagnostics

[Luis Marques <marques@ti.com>]

All this talk about sensors and senders prompted me to do some snooping
around the wiring in my car ('87 4kcsq) and I discovered that, in fact,
it had the after-run feature for the radiator fan.  I had never seen it
in action, so i decided to investigate.

This relay is located on the auxiliary relay panel under the fuse panel
(at least on the '87 4kq's).  It has five pins labeled "30", "31", "87",
"15", and "T".  It also has a sixth pin, but is not connected to
anything.  It should receive battery power all the time trough pin 30. 
Pin 31 should be grounded all the time.  Pin 15 receives battery power
from the wiper relay when the ignition is on.  Pin 87 sends power to the
radiator fan.  Finally, pin "T" goes to ground through the after-run
switch on the valve cover.

The relay has a timer so that the fan only runs for a fixed amount of
time after the engine is shut off and its warm.  It works using a
discharging capacitor.  When the engine is on, this capacitor is charged
from the power supplied into pin 15 from the wiper relay.  Once the
engine is turned off, if the engine is warm enough to close the
after-run switch, this relay will turn on the radiator fan and begin
discharging the capacitor.  Once the capacitor is discharged, the relay
opens and the rad. fan is turned off and will remain off until the car
is used again.  Note that if the wiper relay is bad or missing, the
after-run feature is disabled!

It turns out that there was nothing wrong with my relay!  It just wasn't
getting battery power in pin 30.  This pin feeds to a 4mm red wire that
is connected to a bus under the fuse panel that is connected directly to
the battery.  I connected this big wire back into the bus and presto!
the relay works again.  Now the only thing left to do is find out if the
after run switch is good or bad, since I have yet to hear the fan after
shutting off the engine.  Either the engine hasn't gotten warm enough to
trigger this switch, or it has failed open.  Does anybody know how often
these switches fail? 

I hope this info is usefull to others on the list.

  Luis Marques
  marques@ti.com
  '874kcsq