My car has two problems. A few years ago the motor started to hunt at idle. I checked everything and swapped out almost all of the fuel injection electronics and checked for vacuum leaks and even got the local Volvo spesialist involved. I came to the conclusion that it's probably the engine wiring harness particualrly to the IAC. But then I left the counrty for a few years. The car was run (by my parents) at least every other day untill it started randomly cutting out, few weeks before I returned. Then it totally cut out and won't start. A week later I came home. I have the Volvo green shop manuals and have been troubleshooting the system as well as I can. I found this forum yesterday and checked a few related posts for a "no start" problem. I was referred to a really good home written manual on that from this site. I had actually checked almost all of it. But tonight I'm finding myself scratching my head again. I haven't really checked for spark (I need an assistant) I a few minor things that I am 99% sure is good and I plan on finishing that tomorrow with a friend. Right after I was checking the Hall Effect sensor the engine almost started (I'm really confused as to why). Then back to nothing. There is just one thing that I think is wierd. If I remember correctly, when you turn the key to the run position the fuel pump should run for a few seconds. I think that this is reguardless of the hall effect sensor. I can override the relay and they run but the motor doesn't fire up. I think it is a bad computer. What do you all think? If you need more info just ask.

 

Welcome.

Did you check if you have power to the coil? Assuming you have the wires set up to the proper firing order? It is 1-3-4-2 with #1 at the front of the car.

Wait for the experts to get on.

 

This is another good nugget of info. I decided to just check everything off on the flow chart on the "no start" troubleshooting article. It checked the Hall sensor and checked out good. So I went back and checked for sparked. The coil sparked two or three times then never did anything again. I checked the coil and it all checks out fine according to that troubleshooter. Firing order is also good. So according to the flow chart I need to check out the computer. Then only thing is I'm surprised that I saw it spark a few times. Wierd. I wouldn't think that a computer would work intermittently. So tomorrow I'm going to find one to swap in. Tell me what you think.

 

The fuel system won't fire up if the ignition system signal to the control unit has failed. Check the wiring to the hall effect switch, also check the power stage unit on the left front fender by the injector ballast resistor pack.

 

after you check what Redblock has metioined....

You can then check power to the ICU. It is under the drivers side dash.
The fuel ECU is located in the passenager lower kick panel.

Here is a helpfull link: http://www.vcoa.org/700-900-faq/FAQSummary1.html

 

I went back and double checked my Hall Sensor. Apparently I hadn't fully understood what the test procedure checks for. My local Volvo mechanic said that he checked them at one point by using an old needle ohm meter and it should fluctuate between 0 and infinity when you spin it. Mine fluctuated between infinity and almost infinity at evey now and then. SO it looks like that may be the problem. I just rebuilt my old distributor three years ago. Now the bearing is pretty sloppy. Any suggestions on what I should buy? I found a good looking one on Ebay for $140. I think I also read somewhere that you can send a false pulse signal to the computer and get it to fire up that way.

 

Go to www.stepbystepvolvo.com for free guide to 740 no-starts.

 

Actually I already got that guide. It's helped me a lot. I've got the Volvo Green shop manuals and sometimes they are hard to use. With the "step-by-step" guide it looks as though the hall sensor is bad. I wasn't expecting that since I just rebuilt the thing a few years back. but before I drop some cash on getting a new unit I want to know how I can do it cheaper, what the sensor does and how can I directly test the sensor? I went back into the Volvo green manual and found a test for it and adapted it for my situation.

The Volvo test for the Hall sensor, which you do after testing the wiring to the hall sensor (it's outlined very well in the "step-by-step" guide -I think it's in the section "sidebar 2,") you test the sensor in the car by checking point 14 of the EZ-K computer (while it's all plugged in, to do that you have to have ignition off, disconnect the plug take it apart, there are two comb looking things that pull out to expose the testing points on the plug and reconnect it). You then touch a multimeter's red tip to the #24 piont in the EZ-K's plug. Connect the black end to ground and now you can read voltages coming from the sensor when the ignition is on, but before you do that, you need to disconnect the coil (I think the red and white lead #1 on the coil so the motor doesn't start). Then some one needs to crank the crank shaft by wrench with the ignition on to in effect turn the distributor and give you reading changes that you can read with the multimeter. You should be getting two reading from you multimeter: 1.8vlots (or a little less, but no more (says the Volvo manual)) and an almost nothing reading (0-.7v).

That is a lot of work just to have the hall sensor powered up to test it. My distributor was already out so I tested it on the bench using this system I devised:


I spent some time finding out what the "- 0 +" all mean exactly (they are written on the bottom of the connector on the distributor) and here it is:
"-" ground
"0" signal (it goes to point #24 on the EZ-K computer)
"+" hot end (it powers the sensor)

This test sould tell you if the sensor is working or not. It should switch between 1.8v to almost nothing. My sensor (which I'm now confident is at fault) Reads the "almost nothing" at .2v (that's good) when the aperture it over the sensor (you can see it when the cap, rotor* and shield are off. *Be careful not to break the rotor when pulling it off. Mine is on really tight). But when the metal is between the sensor pick-ups I get 0v. I think that's when I should be getting 1.8v. So that seems to be my problem.

Anyway, here are a few pics of the test I did:

*******the pics are gone now and I'm not really sure if this test is conclusive*******


So I'm off to find a new hall sensor. I'll tell you what happens next. At this point this test is still unconfirmed. I haven't tested a "known good" sensor, so test at your own risk!

 

Great job on the follow-up and for devising a test on the Hall sensor itself! Here's to all us amateur mechanics!

 

So earlier today to car started for the first time in a long while. I got a new hall effect sensor for $20 and put it in (I found them here cheap http://www.bbautomacao.com/home_hall...e_sensors.html, I saw other places that wanted them for $90). Just make sure to wire it right into the connector at the base of the distributor. Black is negative (-). Red is positive (+). Green is the signal (0). The above test that I devised is inconclusive. It's seems like it should work, but something with how the rest of the system affects the readings make it impossible to test just the hall effect sensor alone separate from the car. So, with a working sensor you still get no reading. However I did get some differing impedance readings and when wired up wrong it gave me some other faily predictable readings. So a similar test might be conclusive. I'd have to spend more time comparing. Up until my started running, I wasn't sure it was going to work. The good news is I'm back in business!

One quick question: I got another less-serious problem, should I start a new thread so others can look up the problem easier or just add on here?