ECT and voltage divider help?

[Daniel Houlton]

I'm working on a schmatic for a small controller for an electric 
radiator fan and I have some questions on reading the ECT sensor.

Doing some testing on my truck I found that the sensor is fed 5V from
the ECM (measured with the wire dis-connected from the ECT), but 
connected it only reads 3V when cold.  The voltage then drops to 
around .65 V at normal operating temp.

It's not really important, but I was somewhat confused how hooking
the wire to the ECT sensor caused the voltage to drop from 5V to 3V
(or .7V or so when hot).  After some research on the net, I found that 
it's set up as a voltage divider.

The 5V (OK, actually like 4.9x or so) goes through a resistor in the
ECM before coming out to the ECT.  The readings I were taking were
between the two series resistors so it's working as a voltage 
divider.  After some math, I found the internal ECM resistor is 
around 1765 ohms, confirmed by both hot and cold readings.

So, my question is, how do I switch something based on this voltage?
Basically, I want to monitor the voltage to the ECT sensor and when it
drops to around .4 or .5 V I want to trigger an output (that eventually
drives a relay).  Then, while the output is triggered and the voltage 
goes back up to around .7 V to turn the output back off.  And I want
to be able to fine-tune the on and off voltages with a couple pots.

Any ideas how I can do this?  I don't know a lot of the solid state
devices.  Would I be using a transitor, op-amp, etc?  

Will tapping into the ECT wire to read voltage somehow screw up the 
reading to the ECM by acting as a sink or source?


Also, I'm including several inputs that can trigger the fan relay like
the A/C clutch, air compressor clutch, and a couple extras as well as
a manual on, manual off (which over-ride the other inputs) and an 
automatic setting.

The inputs are diode protected to prevent backflow from one signal to
another and they run into a low amp relay.  That relay is controlled 
by the manual off/on/auto over-ride and triggers the 30 amp fan relay.

My question is, that he low-amp signal relay (needs to handle about 200
mA max to trigger the fan relay coil) seems kinda big and bulky.  Is
there a solid state device that can replace this?  Typically, a relay
is used to drive a big load with a small signal, but I want to drive a
small load with a small signal.  Just wondering if there was some kind
of small IC that would do that instead of a big relay.

Oh yeah, one more thing.  I've found sockets for different types of
relays in Jameco and a couple other catalogs, but they aren't very
high amperage and I'd rather use a common automotive 30A relay.  I
can't find board mounted sockets for these automotive relays though,
just pigtail sockets with wire leads.  Anybody know where I could find
a board mount socket for these?


thanks a bunch

--Dan
houlster at inficad.com

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