wstrohm

I had removed the "fuel recall" (circuitry that was added to make fans run after engine is turned off) from our '94 FD some years ago. So the fans should not run if engine is cold. But:

Turned on ignition but did not start engine after 4 days not running. Cooling fans came on; not good. Tried again today; fans did NOT run. Measured the resistance between TFA and GND terminals of the Diagnostic Connector ("Data Link Connector"). Result was 294.1 ohms. Changed meter to read voltage and turned on ignition. Voltage was 11.7 volts. Turned ignition off and re-measured resistance between TFA and GND terminals. This time it was 181.3 ohms. Had thought that maybe the fan switch was closed, which would account for the fans running with ignition on, but switch was open (today anyway). But why did the resistance change after seeing voltage? And I still don't know how to reproduce the cooling fans ON when engine is cold.
Any advice?

DaleClark

iTrader: (55)

The fan recall just runs the fans after you shut off the car if it gets too hot. So the key is off, engine is off, fans keep running for 10 minutes, that's all the recall stuff did.

If you are putting the key in first thing in the morning and the fans are running some input is kicking the fans in. You don't have the AC on do you? Or, it could be a short in the wire going to the fan switch on the back of the thermostat housing (broken wire shorting to ground) or one of the fan relays is getting stuck.

Dale

wstrohm

Thanks, Dale.

I was checking for a potential short of the fan switch to ground by measuring the resistance at the "TFA" terminal of the Diagnostic connector to ground with ignition off. I read 294.1 ohms, so there is no short there or from the fan switch on the back of the thermostat housing to ground (it's a common point). The weirdness at the moment Is the 181.3 ohms I read later, after temporarily checking the voltage at TFA with ignition on, then turning it off. Why the change? Is there "self-heating" of the switch thermosensor by applying a voltage? There shouldn't be, right? The resistance should only be a function of coolant temperature. (Maybe it was some other component that changed resistance... there are a few other things in parallel with that switch seen by the "TFA" terminal.)

None of the above answers why the fans were running upon ignition turn-on. I had removed the recall wiring a long time ago, so I don't think I screwed that up. And now the fans do NOT come on when engine is cold, so maybe the problem "fixed itself." I'll report later if it happens again.

DaleClark

iTrader: (55)

I'm not sure exactly where the TFA terminal goes and how that works. The fan switch on the back of the water pump housing is super basic, once it hits the temp it's set for (stock 107 deg. C FC switch 97 deg. C) it shorts to ground so it grounds that wire out. Shouldn't be any real resistance, just 0 resistance to ground when it's triggered and open circuit when it's not.

We'll go with the "it fixed itself" . Could also have been a fan relay that got stuck closed or something.

Dale

wstrohm

Dale, on a stock (pre-recall") '94 FD, the "TFA" terminal in the Diagnostic Connector is connected to the cooling fans #2 and #4 relay coils' "cold side." (See page Z-40, Section Z, chart B-2 of the 1994 workshop manual.) Its purpose is to test turning on those fans by jumpering "TFA" to the "GND" point in the connector with ignition ON. The fuel recall, which added the fan timer to run fans after ignition turnoff, also connected the fan switch in parallel with the "TFA" terminal.

I used the schematic for the recall addition, when I thought to check the switch for accidental grounding. But I forgot that the circuitry is back to its stock condition!
The resistances I was reading were to ground through the ECU's input port that normally controls the fans. So the readings were reasonable for that condition. But I still don't know why they changed from 294.1 ohms to 181.3 ohms.

scotty305

iTrader: (2)

What you're seeing is most likely because of the ECU's transistor circuit that activates the fans. That transistor should act like zero resistance when it is activating the fan, and 'high enough' resistance that the fan will not be triggered when the ECU is not trying to activate the fan. It's not too surprising the resistance changed after the ignition key had been powered up. Try repeating the check with the ECU or the X-05 connector unplugged, the resistance should be much higher (above 200k to ground).

Also, are you sure that's 181 and 294 ohms measurement, not 181k or 294k ohms? Sometimes multimeters can be tricky to read if they are auto-scaling and you don't use them often.

wstrohm

"... are you sure that's 181 and 294 ohms measurement, not 181k or 294k ohms?" Yes; I verified that by changing scales several times. It was ohms, not kilohms.
Thanks for your reply, Scotty 305.

scotty305

iTrader: (2)

180-290 ohms seems low to me. If you're able to repeat the measurement with the ECU unplugged and then again with the the X-05 connector that's a good way to check that resistance is inside the ECU or in the harness. I checked on my car, which has a different ECU than stock so it's probably an apples-and-oranges comparison. On my multimeter, the resistance between TFA and GND was dithering between about 60k ohms to 200k+ ohms.

wstrohm

In fairness to the seemingly low values, an in-circuit resistance measurement from the Data Link connector "TFA" to "GND" will also see in parallel through the coolant fan relay coils to other loads, such as the "Heater Control Unit," so it may not be as odd as it looks.