'90 740 GL ECT Mystery
 

1990 740 GL B230F Auto

I was on a two hour road trip last week. Halfway through the check engine light came on. I stopped, fueled up, checked the code and got 1-2-3 for ECT. Thinking it was meaning the sending unit rather than ECT, I continued on. I lost power about ten minutes later, due to my A/C compressor seizing and the belt consequently breaking. I pulled to the side of the rode and called a buddy with a trailer. I was able to drive on and off the trailer with no trouble.

Once back in the garage, I started and idled to temp without any trouble. No leaks, no gaskets blown, etc. I pulled the intake manifold to make it easy and have done the following: replaced flame trap and all hoses related to flame trap; cleaned the oil box the flame trap connects too; replaced the thermostat, spark plugs and spark plug wires and replaced the intake manifold and EGR pipe to manifold gaskets. At start up, it barely cranked, idled very rough and stuttered constantly. I managed to get the RPMs up and it smoothed out, but something's up and it sounds like the replacement ECT is bad.

However, I checked the old and new ECTs with a voltmeter and got the following (ambient temp is about 75 degrees): old @ 3400 ohms and new @ 1600 ohms. So, the car ran fine but threw a code with the old ECT, but the new ECT specs out but runs rough. I'm at a loss. I tried to check voltage at the computer as suggested elsewhere, but couldn't get anything. Key was on, sensor connected to harness, checking at the harness side of the ECT in the cab.

I realize that there could be something else at play, so tore the manifold off again to put the old sensor back in to just see. Unfortunately, I managed to lose it somewhere. Frustrated, I put everything back together with the new sensor, swapped the old spark plugs and wires back in and it started. Rough, but it started easier. And it idled rough (but not as bad as before) for about a minute and then gave up the ghost.

I checked the codes after replacing the sensor and still got 1-2-3. I cleared the codes and checked again and got the same.

So, what next? Does this sound a like a defective sensor even though it specs out? Does it sound I missed something else it is causing the trouble? Any help is appreciated. I realize I'm more of a parts changer than a mechanic.

Did it seem like a tight fit to get the ect plug back onto the sensor? Many redblocks have an issue with the wiring being a bit short. What happens is the wiring in the plug pulls out just enough to not make good contact w/ the sensor...the end result is the same as a failed sensor. More than once I have had to splice in an extension to make the connection w/o having the harness under stress.

Also, and I mean no offense, but you did swap the sensor by cylinder #3, correct? The sensor at #2 is for the temp gauge. Occasionally that is an issue. The one you swapped is like this one? http://www.fcpgroton.com/product-exe...ategory_id/194

No issue on the length of the wiring harness, although it did seem a tad loose once connected. I was looking for the old one to compare the fit, but have lost it.

And no offense taken! Sometimes we overlook simple stuff. I did switch out the sensor at the #3, but was Duralast unit from AutoZone. Am ordering another tomorrow to be sure I didn't get a bad sensor.

Take a good look at the harness connector and make certain the micro-connectors aren't corroded, pushed back or bent too.

I had that thought after reading someone else discovering a bent blade being responsible for a bad connection. It's a weather-tight connect and metal appears to be shiny. I sprayed some electronics cleaner on both sides of the connection to be sure. No corrosion or bent connections and wires are pliable and in good shape.

I had installed the sensor with a small amount of liquid thread sealer on the threads only. When I reinstalled the sensor, I cleaned the threads and block to be sure of good contact.

MYSTERY SOLVED: The blue sensor is not the same as the black sensor. I got my replacement ECT from AutoZone and it was blue. I thought nothing of it, until I discovered the color actually meant something! Blue sensor at Azone @ $15, black sensor from Advanced was $45. There is a difference.

Mystery unsolved! Same code thrown again today! This is wearing me out.

Recap: Replaced sensor with correct unit on Monday, everything was ops normal. I did neglect to refill the coolant after replacing ECT, thermostat and lower heater hose. Drove it lightly around town for the last two days and everything was perfect, but the needle started rising today. Stopped and put in a gallon of antifreeze, back to normal with the exception that the 1-2-3 code won't clear. Have I 'cooked' the sensor?

Also, during the last 1.5 yrs of owning this car, every time I hit the brights, the temp needle would peg to the left. Since the previous owner had installed cruise control, I chalked it up t something crossed in the steering column. Now, however, the temp gauge operates normally.

So, bad sensor or wiring elsewhere. I read about checking voltage at the ECU in the cab, but couldn't get a reading. Instructions I was following stated getting a reading on the #5 and #16 pins on the harness side of the connection, however there is no #5 pin in my harness. Any suggestions greatly appreciated.

Cooked the sensor and that was that. Replaced the ECT 10 days ago and have had no trouble since.