1. Home
  2. Volvo automobiles
  3. 92 740...stuck open-loop?

92 740...stuck open-loop?

I replaced the O2 sensor as I was getting a code for bad O2 sensor. After disconnecting battery to reset computer, then all trouble codes were cleared. I am getting a "service engine" light for the first couple of minutes of operation...seems to be until the eng gets up to temp....but there is no CHK ENG lite, and no codes set. Before replacing the O2 sensor I was not getting this SERVICE ENG lite, I had ONLY the CHK ENG lite. Before changing sensor I was getting 16 mpg, and after it is still getting 16 mpg! So my theory is that the car is still running open-loop all the time. I vaguely remember checking dwell on a 81 Chevy years ago, you could tell when the car had gone closed-loop because the dwell would change from a fixed value to a varying value. So to confirm my idea, I would like to get a cheap dwell meter and check what is going on. But how do I connect it to this engine? For the CHevy, I think it connected to some signal wire on the distributor.

Reply to
geronimo
Loading thread data ...

You want a digital multimeter, or even better, an analog VTVM, but you don't see those much anymore. Connect right to the signal wire on the O2 sensor and it should start bouncing once it goes closed loop.

Reply to
James Sweet

Oh....OK. I am not sure about doing that.... I just spliced in the new O2 sensor, got the wires nicely covered with black heat-shrink, and don't want to mess with them...like to "y-in" a wire on the signal wire. Guess I could open up the harness back along the firewall and y-in to signal wire there instead. Well, I was at some Volvo part online vendor a couple of days ago, and they were offering a rich/lean gauge suitable for 740s. Don't recall the name of co. now. But it is a $60 .00 gadget. It works by reading the o2 sensor signal. Wish I had some idea of what the signal value is when it is at optimum ratio! Maybe radio shack still sells a cheapie micronta that is analog? The other thing to considder is that if the circuit is high impedance, you couldn't put a low-impedance analog meter on on it, as it would load the signal way down. I think it is high-impedance. Geronimo

Reply to
geronimo

...noticed you did say "VTVM", not "VOM". Haven't seen one of those hi-Z critters in ages. Geronimo

Reply to
geronimo

Yes, a VOM will put WAY too much load on the O2 sensor. I use a Fluke DMM with an analog bargraph.

A standard O2 sensor doesn't have a specific voltage output for stoich, but rather a "knee" if you graph the output. Lean of stoich you'll get close to

0v and rich of stoich you'll get around 0.8v. In correct operation it will oscillate with the average being something around 0.5v.

Even with the O2 sensor disconnected you should get better than 16 mpg. Is the coolant temp sensor ok? Bad sensor or wiring will make it run pig rich.

Reply to
James Sweet

Reply to
geronimo

One that I picked up from a junkyard was bad, it read open circuit. The car had a straight body in pretty good shape so I suspect that's what landed it in the scrapper. Good one should read a few kilo ohms at room temp as I recall and should respond significantly to temperature change. When they fail, they usually go open.

Reply to
James Sweet

Hey you guys he said it was the service light, it's a 740 the service light comes on at pre set intervals and has to be reset. Theres a little plug on the dash that comes out and you will need to press the switch with a pen or pencil and that will turn off the light till the pre-determined interval turns it on again, reminding you to service the car.

B. Franklin

Reply to
berns

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.