First U to be sure - check the wiering. Disconnect ECU and with ignition on check for voltages at ECT terminals. Gremlins must be found.

 

Well, its not the grounds and its not the EEC (ECU) module. I put a spare EEC on and the same CEL occurs.

Voltage at the ECT connector is 5.6v KOEO and will drop to 2.7 as the engine warms up. Voltage is the same when the harness is probed above the connector, so its not a high resistance at the connector.

Maybe an exorcism is next!

 

Hi Dave:

Yeah, I just got back from a camping trip to Yellowstone. Anyway, I still think that you may have a ground problem. How about running a big ground wire between the negative battery terminal to the engine block?

Another possibility is that you may have damaged some wires or pulled a connector loose while jacking up the engine. Can you pull the connectors apart and lube them with some dielectric grease before snapping them back on?

I don't have the schematic for the '91 4.0L, but I suspect it's pretty much the same as a '92. So here goes. The ECT, IAT, TPS, O2 sensor, Octane Plug and the Data Link Connector all share the same ground and the wire is colored GY/R (Grey/Red??). The ~5V on the ECT is LG/R (Light Green/Red??) and came from the PCM.

Since you don't have a problem with any of the other components, I suspect that the ECT return wire has been broken somewhere. Try to warm up the engine and measure the voltage between the ECT input and the GY/R wire on the ECT itself instead of the engine/battery ground. If the measurement is out of whack, then you know that's your problem. In that case, it'll be an easy fix.

Worse comes to worst, you can connect the GY/R on the ECT with any of the GY/R on the other components to bypass the return wire on the ECT completely.

Good luck

Tung

 

The easiest way to connect ECT sig.return wire to TPS sign.ret. wire, becouse they are close.

 

My electrical ignorance is about to show.

There is a power side to the ECT and the Sig Return wire. With the key on, I measure 4.8v on the power side and have continuity to ground on the Sig Rtrn side.

With the key off, I have continuity between both the power and Sig Return. Both sides also have continuity to ground. I can measure 28.2 ohms of resistance between the two connector pins. Does this seem right or is it indicative of a low grade short? If there were a short, wouldn't the reference voltage be too low instead of too high?

 

Hi Dave:

I'll make some measurements after I got home today to see what's the normal range for these. I'll post them for you tonight.

At any rate, you might want to take the connectors off the ECT and the TPS and measure the resistance between the two return signals. They should be connected regardless of whether the key is on or off.

 

copper, et. al.

I'm both pleased to tell you the problem has been solved and embarrassed as all get out that the entire problem was from the greenest, most rookie mistake you can make.....I tagged the wrong connector. In the immortal words of Bugs Bunny.."what an embezzle. What an ultra-maroon!"

What an idiot!!! I even tried at one point using the other connector but got the same CEL light. Well, DUH, until the code is cleared it will still light up. In my frustration, I forgot all the basics. I want to thank those of you who burned up the brain cells on this "problem" and want to say I'm truly sorry for the wild goose chase. This has to be one of the all time embarrassing blunders. All is well now in Aerostar land. Thanks again for all the assistance.

 

been there, did that
hardest part is getting beyond, "I could not have made a mistake"

which 2 so all of us can learn?

glad you found it
how's the 4L running now with the new head gaskets?

 

Old girl is running great! She's a tad bit livlier - or so it seems to me. Worth all the effort.

I even knew beforehand there was a spare connector and still managed to mislabel it. Both connectors are identical. My wiring loom is covered in OEM black plastic protector down almost to the connector and then has tape from the plastic loom protector to the connector itself. I didn't want to rip off the tape and the wire colors are impossible to see otherwise. I only discovered the mistake by probing the EEC connector looking for continuity between it and the connector. When the pin numbers didn't match.......

I replaced the radiator cap and now that I have full pressure, there are few new leaks to take care of. That aftermarket radiator bypass valve was the first to go - leaking like a sieve.

 

The pink one is the one used for the temperature sender on the digital dashboard. Normally it's not used. The correct connector for the ECT is the black one.

Dave:

It's OK. We all made mistakes like that. I went out and probed the resistances on the connector for you this afternoon when I got home. All for nothing I guess

If it made you feel any better, I bought a weight lifting bench for the boys today and we spent hours putting it together, making one mistake after another. It got pretty boring removing the left hand base and swapping it to the other side, two or three times before we got it right

 

Copper,
Thanks for making the effort. Can always count on the good people on the Aerostar forum to go over and above in their efforts to help out. Get those boys started early and soon they will bench pressing you! My boys can easily put me to shame.

 

I noticed that unused plug on my 91 model with analog gauges (my 90 has digital gauges). It actually has a plastic cap over the pins to prevent someone from accidentally plugging it into something. So I wonder what it uses to feed the temp gauge?