850 Turbo No Spark Frustrations

bgkast · Post by **bgkast** » 23 Sep 2012, 17:51

I have been battling an intermittent no spark condition on my 1995 850 Turbo Wagon since I bought it last November. This issue has left me stranded at least 5 times (it seems to only fail to start when I am away from my house

). So far I have tried replacing the Cam Sensor with a new BOSCH unit and a used unit, replaced the ECM with a known good used ECM, checked the crank sensor resistance whenever the issue occurs (always tests at 300 ohms), and replaced the coil/driver twice with used units. I have yet to have any codes triggered by this issue. The car always seems to start working again fine every time I get under the hood and start testing things, which makes diagnosis tough!

I have been following the troubleshooting procedure here: http://www.justanswer.com/volvo/3bzfo-1 ... place.html but have not back-probed the Cam Sensor plug as described in step 5 because I can't figure out a good way to do it without damaging the wire insulation.

I have also probed the CMP plug with the CMP disconnected as described here: https://www.matthewsvolvosite.com/forums ... 01#p143318 (I realized later that this was for a V70 and may not apply to my 850)

So far the only strange results I have found is when probing the Cam sensor plug with the sensor disconnected and the ignition on I get 10.5 volts or sometimes 3.2 volts at terminal 3, and 5 volts at terminal 2. According to the second link I should be getting battery voltage. I have checked the resistance of the sensor wires between the plug and the ECM and they seem to be fine.

Any help would be greatly appreciated.

Thank you!

bgkast · Post by **bgkast** » 24 Sep 2012, 23:32

Anybody?

Post by **phils94850** » 25 Sep 2012, 02:36

I just went thru a NO start problem with my wifes 93 850, i was stumped for days and for some reason long story short it started up and has been running fine. Electrical thats intermittent is the hardest thing to figure out. Ive researched as you have done. All i can say is check and recheck all your connections, there known for getting brittle and causing problems. Have you replaced your coil ? there known for going bad.

bgkast · Post by **bgkast** » 25 Sep 2012, 09:23

Thanks for the reply. Any particular connections more notorious than others? I've replaced the coil assembly twice with used units.

What could cause the intermittent low voltage from the Computer to the Cam sensor? That seems to be the best and only lead I have found.

byeboy · Post by **byeboy** » 25 Sep 2012, 09:29

Have you considered the Ignition Switch?

My Cam Sensor connection was very bad once I closely examined the female part...the male prongs were barely contacting. I ended up destroying that plastic housing (it crumbled away), and just cleaned the connections, put some electrical grease on them, a piece of shrink wrap tube over each, and it was good to go,,,all for free!!(My favorite type of repair!)

Post by **phils94850** » 25 Sep 2012, 09:59

I would say check the wiring from the cam sensor to the ECU, over the years there notorious for breaking down.

Post by **cn90** » 25 Sep 2012, 10:49

Intermittent no start, 2 main culprits:
- bad fuel relay
- MAF

Search forum for detail.

bgkast · Post by **bgkast** » 26 Sep 2012, 19:36

Thanks for the help. I'm going to double check the wiring for any breaks with a multi-meter again before I bolt the computer back in its box. I will also take a close look at the female plugs to the Cam sensor and crank sensor. I have considered the ignition switch, but the computer seemed to be getting good voltage from the blue and red wire, and the starter seems to work fine when the issue is occurring. Are these ignition switches known for failing in this way?

Can the fuel relay or MAF cause a no spark condition? I have been focusing on the ignition system because when the car won't start I get no spark between the coil and ground when cranking the engine.

Thanks again for the help.

Post by **Ozark Lee** » 26 Sep 2012, 20:01

A bad MAF or a bad fuel pump relay will not cause a no spark condition. They can keep the car from running but it won't be because of spark.

Have you pulled the ECU and cleaned the edge connector? The cars are getting to the age where the edge connectors on the ECU and TCU can start to tarnish and get intermittent. Other sources of intermittent cam and crank position sensor signals are from the ignition wires. In their natural state the wires from the sensors want to loop up next to the distributor. Volvo has a TSB out on it but it is simply to add a zip tye to both wires and fold them neatly away from the ignition wires. That particular problem resulted in random stalling and it doesn't sound like your problem at the moment. Once you figure out the main problem that would be a nice finishing touch.

...Lee

bgkast · Post by **bgkast** » 26 Sep 2012, 22:50

I poked around on it a bit more tonight and all of the wires between the computer and sensors seem solid. The only thing I noticed was that the main ground to the computer (large brown wire) had about .3 ohms of resistance to the negative terminal of the battery. I wanted to clean up the connection to ground but can not find the end of it. This wiring diagram http://www.volvoclub.org.uk/tech/servic ... agrams.pdf (Page 12) says it should ground to the engine at terminal G113 on the Left front of the engine...anybody know where exactly?

I have pulled the computer and the contacts did not look corroded, but I will clean them with some contact cleaner. I'm out of ideas so I guess I will put it all back together and cross my fingers.