Hello all, am new to the Volvo scene, so pardon my initial ignorance.

Have a 91 740 with unknown mileage (apparently all 740's have no odo). Had the emissions done, and it did not pass. The car did not have a CEL, though it does light up when you turn on the key. And, the little black box on the strut tower--diagnostic box, I understand--gives no codes either. It seems like that little black box is dead, it doesn't even give a 1-1-1. Does this little boxcontrol the CEL, and if so, how can I fix this warning system?

Thanks in advance.

 

If you push the button does the LED light up?

If not open up the Button area and clean teh contacts. That may be why it isn't working.

 

How can the car not have a CEL if it turns on when you turn on the key?
BTW 740 odos are notworse than any other car's odo.

 

Tech--No lights from the diagnostic unit at all. I'll try the contacts. It should be getting 12v from the ECU, right? I'm thinking that the unit's dead and not communicating to the ECU.....

TedV--I don't know, that's why I'm trying to track it down.

 

you might be doing this, but I cant tell from the post.

the ignition needs to be in the on position for the diagnostic diode to work.

 

The buttons get dirty all the time and don't work when pressed.

 

Thanks for all your help.

Got it back from the shop yesterday, after they replaced the cat and passed emissions. Tested the diagnostic box, and now it works (dunno why, maybe the shop got it working....)

Thanks again.

 

Glad it is all fixed.