'95 850 fan stuck on: code 123 driving me crazy

My 850 GLT 190K miles has had this annoying problem every 2-3 weeks.
- Usually occurs after rainstorm/damp day, doesn't matter if car has been driven.
- Car is dead, no battery power, no lights/start/clock... dead battery
- ECU A2 code is 1-2-3 (coolant sensor signal faulty).
- I hook up charger, engine cooling electric fan immediately is on full blast,
almost too fast.
- I've caught the fan on full blast beyond normal post-driving fan on.
- Fan shuts off after I charge battery and reset ECU

What I've already done:
- replaced thermostat, coolant temp sensor, battery, fan relay.
- checked all wiring with ohm meter

I and my mechanic are out of ideas... and I don't like the idea
that my wife would get stranded somewhere w/kid. Mechanic says
next step is replace ECU, which seems extreme and expensive.

Any and all help would be very appreciated.
-Pete

 

What I've already done:

Can give you a few hints you might try.

The error code means there is a fault in the signal from the engine coolant
temperature sensor.
Since you have replaced the sensor, the fault is probably in the wiring or
the connectors. The observation that often happens in wet weather, also
makes me think its wiring or connectors that are bad.

You can only locate the fault in the wiring, when the error is present on
the car, that is when the fan is running half speed.
It continues to run for three minutes after you shut down the engine.

The resistance in the circuit is faulty when the error is set, that means
the resistance is higher than 50.000 ohm or lower than 50 ohm.
You should read about 2200 ohm on the sensor at 77F with the sensor
disconnected.

You should read 5 volt between the wiring to the ECT sensor when the sensor
is disconnected and the ignition is on.

Do you have other error codes stored?
If you respond to the hints above, I will be able to help some more.

Niels

 

Look down below the thermostat area along the engine block.
Follow the ect wire as far as you can see it.
Mice like to chew on volvo wire harnesses.
I have seen a few where the mice chewed on the ect wire
and caused the fan to run all of the time.

Can also try removing the fuel brain and plugging it back in.
This will sometimes reset the fuel brain.

BD

 

Where is the fuel brain? do you mean the ECU?
(theres the ECU box with 2 units in it, and then the tranny
brain near the master cylinder)

thanks for all the advice, mice have been an issue before
--p

 

It is in the black box look for the ECM marked BOSCH the other should be
marked AW this is for the transmission

--
"*-344-*Never Forgotten"
Is for the New York City fireman who lost their lives on September 11,2001.
The official count is 343, but there was also a volunteer who lost his life
aiding in the initial rescue efforts. And I will never forget them as long
as I live,
nor should any American.
"Mow Green"