88 TC fan trouble...not the usual

June 10, 2009, 07:22:08 AM

Heres the deal.

We just completed some modifications on this car that involved removing the lower intake and putting a knife edged one on, and we re-used all the sensors. The car ran perfect and the cooling fans turned on prior to this.

Symptoms:

Chugging at idle, more profound when the car warms up.

Rev the engine to about 2K, and it smooths out. if you close the throttle suddenly, it chuggs again. if you let it down real slow, it is smooth but idles high.

Temps get to around 230 (high as I'll let it go) with no cooling fan operation.

Unplug the VAM, and it smooths out perfect, but slightly high and PIG rich, you can see the smoke. Reconnect it and its rough again

We took this vam and put it into another 88 TC, and that car ran perfect. No issues, so the VAM is good.

KOER test came up with a code 21 ECT problem.

Here is what we have done so far, with results:

1. Ohmed the ECT at a certain temp (warm) it checked out okay.

2. Ohmed accross the sig return and ref voltage wire at the injector harness with the ECT plugged in, same result as above.

3. Ohmed accross the pin 7 and 46 at the eec connector with it disconnected... NOTHING. Ohmmeter says 0.L . Unpinned the two ect wires at the injector harness and put just them in the engine harness side, and nothing. We wiggled the wires and got an ohm reading real close to what we got above. let them go and it reads 0.L again.

4. Ran 12V to both fans direct, they operate.

5. Jumpered the ECT connector and ran the engine, Fans both ran. This is what was wierd.

6. Replaced the Injector harness with a tested and rebuilt harness, no fan operation. Temps were taken with an IR heat gun, at the T-stat housing.

So since the ect plug jumper trick worked, we thought we were dealing with a faulty ECT. We replaced it with another, and still no fans. This test sghould have told us that the wiring, ECM, and the IRCM and its associated wiring are okay. But if we couldnt get an ohm reading at #3 above, but then we got ohms when we wiggled.....this would indicate the wireing is in fact NOT good.....right? So how did the fans turn on when jumpered with the engine running....but not when the ect showed that the engine was hot? Would this mean that even though there is voltage going to the fans correct, the eec is not seeing the resistance and therefor not turning the fans on? Can this circut run the fans when jumped, but not when the ect shows the resistance change?

Help, I'm about to take a bat to this thing.....i get body parts back this weekend, and the Bronco 2 is on its last leg as we speak...I need the TC to run!!!!!

Another random thought (keep in mind Ive been working all night and am tired LOL

When jumpered, the fans worked with teh engine on. Could this happen even if the sig return wire was bad?? Mabey it was using the other sensors on the sig return ( ACT, KS, so on) as a ground, tricking the comp into running the fans???

My brain hurts...I hope to have some answers by 530 tonite so I can try to figure it out again. :punchballs::punchballs::punchballs:

FYI we tested for vac/boost leaks and everything sealed correctly. We have none to speak of. TPS and Idle speed were both set correctly too. Everything but the ECT circuit seems to be working as intended.

88 TC fan trouble...not the usual

Reply #1 – June 10, 2009, 09:27:15 AM

Your head hurts??? My eyes are spinnin'

from reading all that... LOL

OK I'll make a stab here... Since signal return is common to all sensors and only the ECT has a issue, that really only leaves the ECT, it's associated wiring, or maybe the EEC as the culprit... Now since you've covered your bases and repl the harness and ECT that leaves the wiring at the EEC or maybe the EEC...

Have you measured the voltage across the ECT at various temps??? As it's resistance changes, the voltage will vary as well... For instance at 86*, voltage across the ECT(it's two terminals) should be around 2.62v and the resistance of the sensor is 24,270 ohms... On up at 230*, voltage is down to .36v and ECT resistance is down to 1,550 ohms... Now if your voltages are off more than 10% you have a wiring issue(possibly a high resistance) or the voltage divider resistor inside the EEC is open(OK it isn't open the fans ran when the ECT was jumpered) or changed value...

One thing you can do is compare the ECT sensor voltage/resistance to the ACT sensor(at this stage of testing, voltage readings are more important than resistance), they both operate exactly the same... Also If you can come up with a 10K to 25K ohm pot(maybe Radio Shack), you can sub it for the ECT... Adjust it till the first fan comes on, then measure the voltage and the pots resistance to get a idea of just where the fan will activate... I believe the first is supposed to activate at around 215-220* & second around 230-235*... Voltage at 212* is .47(1/2 volt for practical purposes), and sensor at 2.07K... So the fan should come on somewhere between voltage/resistance listed for 212 & 230*...

To sum up, I'm saying it would be good to know the approx temp the fan will activate... If it's say 250*(.28v) and wiring and sensor are OK the EEC is the culprit... But in reality, if you're sure the ECT is good and the wiring back to the EEC is OK(remember the voltage readings are important and should be performed), that only leaves one thing...

I'll look on the Rat Shack site to see if I can locate a suitable pot...

EDIT R/S lists this 10K pot that would work for testing... You'd connect the center lug and one of the outers in place of the sensor...

http://www.radioshack.com/product/index.jsp?productId=2062354

88 TC fan trouble...not the usual

Reply #2 – June 10, 2009, 04:27:00 PM

Do you see the resistance of the ECT sensor drop as the engine warms up?
Do you see any coolent in the ECT sensor hole when you take it out?
Is it possible the coolent is not flowing in this part of the manifold because of an air pocket?

Sorry for nothing but questions when you are looking for answers.