97 Volvo 850 Turbo Wagon no spark
 

Hey all-

Just finished doing the head gasket on my 97 Volvo 850 Turbo Wagon, and now I am getting no spark.

So I traced the problem back as far as I could, the connector to the coil is getting power but the two terminals on the coil itself are getting nothing. Seemed obvious enough that the coil was the issue, so I grabbed one from the auto wreckers and the same problem persists.

Any ideas what it could be? Or did I just grab another bad coil at the wreckers?

The coils on these cars really don't go bad. There will ALWAYS be power to the coil connector (which goes to the power stage) but there will only be power to the coil when it's needed. You said you just changed the head gasket. If the engine was running beforehand, you messed something up. Nothing serious, though :) First, make absolutely sure the timing belt is installed correctly. Next, take the cam sensor off, unbolt the little sensor ring from the end of the cam, rotate it 180 degrees, then put everything back together. That ring is SUPPOSED to be indexed so that it only goes on one way, but they usually have enough play that they'll fit upside down. This will prevent the cam and crank sensor signals from syncing, and will keep you from getting spark. It's a common mistake. If that's not the issue, check the connectors and wiring to the cam sensor and crank sensor- they're both in that area- and make sure they are tight and not damaged. The wiring harness here sometimes gets brittle and will break after being disturbed for doing a head gasket.

Finally, why did you need to change the head gasket in the first place?

Thanks for the advice, I will start checking all of those now.

I changed the head gasket because after over 260,000 miles it developed a coolant burning habit, I replaced most of the other gaskets while there too.

Connect the positive end of the multi meter to the positive on the coil and then connect the negative to the grounding bolt located below the coil on the body of the car. If you get 12v then the car isn't grounding the coil. This can be caused by various things however you might want to test your power stage first.

To do this put the multimeter on the grounding pin for the power stage and see if you get 0 ohms between the grounding pin on the connector and the ground on the coil.

I was getting .9 ohms when my power stage broke.

Well going through the list of possibilities, I decided to test the crank and cam position sensors- crank position sensor checked out fine.

According to the manual I found, one wire should be a 12v power... which is good. The middle wire is a signal that should pulse between 0-5v when turning over... which I also have. The last one is an open ground that should read around 22 ohms in the run position.

I am not getting 22ohms resistance from that last wire. I'm guessing that may be where my problem lies? Does anyone have a more detailed wiring diagram of that area so I can trace it?

Is the wire shorted to ground or is it open? As I mentioned earlier, the wires in that area tend to get brittle due to heat, because they're all just kind of jammed in there against the head with a bunch of hoses, other wires, etc, all of which hold heat. Add in drops of oil from a leaking cam seal at the distributor, and it's a recipe for insulation failure. Start pulling wires out of the looms there, and checking. I'll bet you'll find the problem!

One other thing to make sure: did you have the engine out to do the head gasket, or did you leave the engine in place? If you had it out, make sure you didn't bend any of the crank sensor teeth on the flexplate/ flywheel. It's easy to do, and it will allow a signal from the crank sensor, but it'll be erratic and won't allow the sensor to sync with the cam sensor, which in turn will keep the engine from running.. If so, usually you can go through the starter mounting hole and straighten the teeth, usually. I had this happen to a customer's car- she had had a backyard guy do an engine swap, and afterward, the car wouldn't run right after it warmed up. It was caused by a couple bent teeth!

I did not remove engine from car to do head gasket.

I'm gonna start tracing wires this afternoon, hopefully something will turn up.

Before I do that though- a friend mentioned that some vehicles have fail safes that won't provide spark if the timing is off... is that a possibility before I waste my time tracing wires?

It's not a "fail safe" per se- the cam sensor signal and the crank sensor signal have to synchronize before the computer will start firing the plugs. Basically, the crank sensor tells the computer where the engine is in its rotation, and the cam sensor (on this car) tells the computer when cylinder #1 is at top dead center. You see, it takes 2 complete revolutions of the crankshaft for the camshaft to make one revolution, and for all cylinders to fire. If you were to look at your flywheel, you would notice that there are a bunch of square holes on a ring that the crank sensor "counts" as they go by. There's one spot where the hole is as big as 3 of the other holes. This spot is passed when cylinder 1 is at top dead center. On the cam sensor ring, there is a tooth that corresponds to cylinder 1 top dead center on the compression stroke ( as opposed to the exhaust stroke, the other time it's at TDC. These signals have to happen at the same time, otherwise, the computer doesn't know what cylinder is ready to fire, so it never starts firing. So, as I mentioned earlier, check your timing marks to make sure they're lined up. However, the fact that you got an incorrect reading from the one wire in the crank sensor circuit suggests to me that there's a problem with the wiring. I've seen it before many times- the tight quarters back there by the crank and cam sensors, surrounded by oil dripping from the head, hot air from the turbo- to- intercooler pipe, hot hoses from the coolant, the hot EGR pipe on cars so equipped, and no ventilation make for really inhospitable conditions for wiring, and Volvo kind of stuffed all that crap in that one spot. Everything can be perfectly fine, but then when you disturb that wiring in order to do a repair- such as a head gasket- something breaks or shorts.

Thanks so much for your help Carrots, in the end you were right about the wiring. There were several wires in that harness that were damaged or brittle. And after I got it running... I discovered that I put the plug wires in the wrong positions. :p

Ahaha congrats on getting it running! I messed up the plug order on a 84' Dodge Van one time, man did that thing backfire.

coil issues
 

after reading extensively about 'not firing', i finally figured it out. thought fuel pump, bought a guage, 44 lbs. changed cap, rotor, plugs, still no, whle trying cranking sunday, wife was watching and saw a puff of smoke by cylinder one, i shat, because all the dash lights went out, except for abs and check engine light, and the engine would not crank, found a broken ground strap, replaced that, all the lights back on and cranking, neighbor came by, change the coil, well i've checked and had 12v out of the lead wire to distributor, so i ignored that thought, well, he persisted, and to quiet him i plugged in the coil i got outta the junk yard, damn, it fired right up, don't know, but, trying it, they are easy to find in the junk yards and she did not even charge me for it, i replaced cam position sensor, changed the fuel filter https://www.autohausaz.com/images/0221601012.jpg
what is the 4 prong plug ???? i had something on a '87 that gave me an intermittent stalling problem

best practice is to start a new thread in the vehicle specific board and note your year and model.