Hi, 1995 850 Turbo. I think I have a faulty ECU. How often do they go bad, and can I just plug in a different one without any "programming"?

 

Rarely if ever. Assuming your is an automatic you can use/test an ecu from another 95 Turbo. Your looking for a Motronics M4.3/072

 

Nothing like broken solder traces, cracked pins, vibration damage etc?

 

I stand by my 'Rarely if ever statement'.

 

Why do you think the ECMs failed ?? What testing have you done that's lead you to the ECM as the faulty component ??

Yes they can fail due to broken solder traces, cracked pins, vibration damage, corrosion in the contacts, component failure like a burned out resistor or transistor, improper installation like bent pins and a thousand other things.


BUT, It's much more likely something feeding information like a sensor to the computer or something controlled by the computer has failed or malfunctioned. Something simple like a bad ground or corroded connection shouldn't be overlooked either.

The ECM is the very last thing to check if you're having a problem. They almost never fail.

What problem are you having ??

 

One problem is the Radiator fan relay isn't getting the ground signals. Do they originate from the ECU? Plus the 850 is running rich.

 

The cooling fans ground goes directly to ground, it isn't switched by the ECM.




Why is it you think it's running rich Do you have a check engine light ??

 

Looking at that diagram, doesn't it say at the bottom that the yellow and yellow/white wires come from the ECM?

The guy I bought it from said the catalytic converter was running rich and it stutters on acceleration.

 

"One problem is the Radiator fan relay isn't getting the ground signals."

Not really sure what you've tested to get there since there is no "ground signal".

Yes those two wires will turn the cooling fan on (low or high) depending on the signal the ECM receives from the coolant temp sensor.

Some components do have a ground side that is switched. The cooling fan is not one of them. In their case if you went to test them by supplying battery voltage to them they can be perfectly good and yet not work because there is no complete ground circuit.
On most cars the horns are one example of ground circuit switching. Most cars the horns have power from the battery all the time and when you "honk" you are actually supplying the ground for the circuit and not supplying the power. It makes a big difference in how you test.

Your cooling fan should have a complete and working ground circuit at all times. The ECM switches the relay to supply battery voltage to the fan.
If you take a 12 volt test light or volt meter from the positive battery terminal to the ground lead wire running to the cooling fan motor the light should light or you should see battery voltage. If not you have a bad ground.

Do you have a check engine light ??

And if you think I'm being picky I am. I used to work on British and German cars way back when and those wiring diagrams run backwards to US cars. Their schematics run from "earth" to the positive battery post. If you're not paying attention it's easy to send something up in smoke or make a wrong call on if something is good or bad since you can hook it up wrong if you're thinking positive to negative.

 

Sorry if I've been spamming the board lately. I just don't want the car to overheat.

The radiator fan does not come on with the A/C. If I connect a jumper wire to the yellow wire connector, fan spins great, nothing if connected to yellow/white. When the car gets hot, the fan runs for two seconds,then stops. If I drive on the freeway, the temperature gauge drops to the four o'clock position.

Is this normal?