850 Turbo No Spark Frustrations

[Ozark Lee]

There are a couple of points on the block. I have not gone through the schematics but my guess is that the ground point for the ECU is the one just below the oil trap. The other one I remember is, more or less, below the rear knock sensor and it has several small wires bundled into it. The main ground from the battery is on the face of the rear of the block, below the starter.

...Lee

[danger2manifold]

I recently had a similar no-spark condition and wanted to add a data point for myself as well.
Over the past 8 months my 350k km 1996 850 Turbo developed an intermittent no-start condition (ie extended cranking without sounding like it was able to catch at all), 1 in 100 times or less. It would always start on the second try, or once I opened the hood to begin checking for spark etc, it would behave. Then the no-starts ramped up, to the point of several per week, luckily always at home or work. Fuel system relays were good, cam and crank sensors were replaced in the past 16 months, so I was baffled when I checked and found I had a no-spark condition. Despite the recent sensors, I checked them anyways, and they had the correct resistances and voltages, and cam sensor would vary appropriately when cranking. Diving into the forums and wiring diagrams, it seemed like I had low voltage to the DI power stage at the coil (multimeter showed 0.27 volts when the troubleshooting guide says to expect 0.7 to 1.3 volts, but looking back maybe my multimeter was filtering out the spikes?). Based on that, I cleaned up the grounds as best I could (without removing the intake manifold), when that didn't improve anything, I bought another ECU to see if swapping would change anything - it didn't. Swapping a junkyard DI stage / coil also didn't do anything.

I wanted to do one last-ditch check before digging in to replace the starter based on the previous posters' interference comments, so I ran a new ground wire from the ECU ground (labeled 31/33 at the front of the block, just above the starter) to the chassis ground just in front of the battery. Keeping in mind I had previously "checked", ie removed and noted only minimal corrosion, ground 31/33 with no effect... with the new ground jumper wire, it fired up immediately and more eagerly than it ever has. So if anyone else out there needs to hear it... checking and cleaning grounds makes allllll the difference on these cars. In my case, I didn't actually clean ground 31/33 effectively enough the first time around, but a jumper ground to the battery cured my laziness.

[Williamad33]

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: viewtopic.php?f=1&t=31301#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!

Intermittent no-spark issues on these 850s can be a real pain . Since you’ve already gone through cam sensor, ECM, crank sensor checks, and even swapped coils, I’d start looking at the wiring and grounds more closely. Corroded or loose grounds on these cars (especially the ones under the battery tray and near the ECM box) have caused exactly the kind of “randomly won’t start, then fine again” symptoms you’re describing.

Also worth checking the ignition switch itself – worn contacts can cut power intermittently and mimic a bad sensor/ECM. If you’re seeing odd voltages at the cam sensor plug, it could be a sign the switch isn’t consistently delivering full battery voltage.

[bronco]

Have you ever tried flexing parts of the wiring harness to check for faults?

Have the car running and try moving parts of the harness bundle to make them lay different or put tension on them and try to recreate an issue. put a different tension on a plug maybe , undo it , twist or reroute it and plug it back together
Sometimes there is a fault which works 99% of the time but you hit a bump and a harness section physically moves stresses or flexes one wire or connection and breaks contact.

Maybe at some point while looking for a problem someone stuck a wire with a sharp probe or jammed a tester into the back of a plug to probe a wire and created damage to the wire or left a hole and that area is now open to weather allowing it to corrode.

[bronco]

Another thing worth checking for is wire turtling