'90 740 GL ECT Mystery

mightyjak · Post by **mightyjak** » 18 Sep 2011, 21:25

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.

lummert · Post by **lummert** » 18 Sep 2011, 23:53

What color is the sensor? And, is the new sensor the same color as the old one?

mightyjak · Post by **mightyjak** » 19 Sep 2011, 10:22

Old sensor was black, new sensor is blue. Same shape, size, plug fits (albeit loosely). Sourcing another one today from a different supplier to see. Wish I could find the old sensor to compare!

lummert · Post by **lummert** » 19 Sep 2011, 14:54

There lies the problem. The blue sensor has only one thermo sensor and the black one has a double thermo sensor. You will need to get a black sensor. I seem to remember reading that the black sensor can be substituted for the blue one but the blue sensor can't be substituted for the black one.

mightyjak · Post by **mightyjak** » 19 Sep 2011, 15:44

Aha! I was beginning to suspect something was up with the replacement sensor. I found and ordered a black one earlier today. IPD doesn't carry these at all, so I've been stuck parsing AutoZone, O'Reilly and Advance catalogs. While they've been helpful, their combined knowledge of 20 year old Volvos is (understandably) limited. Will let y'all know if this resolves the issue.

mightyjak · Post by **mightyjak** » 23 Sep 2011, 21:52

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.

mightyjak · Post by **mightyjak** » 05 Oct 2011, 19:27

I cooked the sensor, apparently. Replaced it 10 days ago and have had no issues since. I didn't even know that was possible.