My 740 Turbo wont start

The crankshaft sensors have been known to fail. Early ones were in the distributor, later ones were at the top of the bell housing. My '85 is in the distributor, dunno about your 88... I think that was right about the time of the changeover. They have been known to fail, sometimes permanently (like mine did earlier this year), sometimes for moments at a time (very frustrating). If the tach doesn't kick every second or so when you crank the engine it could be the sensor. If the tach does kick, look elsewhere.

Mike

Have you replaced the engine wiring harness yet? If it's original, it's bad.

300 series had electronic ingition with bell housing crank sensor since about 1984 (B200K/E etc). Injection models have a relay module to confirm the pulses to keep the fuel injection pump and sparks going.

I always thought the 700 series were ahead of the 3 series.

-- Tony

James you say this alot, how many exampes have you seen? I find it hard to believe every 88 or older 740 (or whatever the year is) that they are all damaged or unreliable. Yes connectors and corrosion can be a problem, but usually there is a route cause.. missing splash guard, or other waterproofing, large amount of driving in heavy rain/ mud / salt, or not enough driving to dry off properly, sitting out in rain/wind, leaking battery, oil, heat guard etc.

Is this really the common problem you say it is. Surely 'the whole wiring loom' can't be faulty.

Volvo have used the same wiring and connector technology since the early days of 240s (and probably before on 140 etc), there are plenty of them around and wiring looms are not mentioned much here.

-- Tony

In 1988 Volvo went to conventional PVC wiring, so the OP's harness should be okay. Before that for some years they used some concoction that crumbled with age. Apparently the idea was that the insulation would degrade rapidly when the car was scrapped, but in practice it fell apart much earlier. I replaced my engine harness about ten years ago because of that.

Mike

Well let's see, both of my cars, my mom's car, my dad's car, my friend's car, 3 cars belonging to another friend, essentially I have yet to see an '81-'88 200 or 700 series Volvo that didn't suffer from harness rot. Some climates may be worse than others for it, but it's a fact of life, the wiring insulation rots and crumbles under the hood.

It's the heat under the hood that causes it, if you doubt it, you haven't cut open an original harness from that era. Do a search for Volvo wiring harness rot, you'll see what I mean.

The technology was the same, but the insulation used on the wires themselves was clearly not. There has been some debate as to whether this was changed in '88 or '89, I believe some early '88's had the earlier type of wire.

Fair enough, I'll have to check my 87 360.

Could be limited to hot climates or cars with air con?

-- Tony

I have two 740's an "86 Turbo and '91 NA.

Both have original wiring, and neither requires replacement.

To troubleshoot: hook up a timing light, see if spark flashes when at TDC.

If it does, rule out crank sensor; if not, check the sensor.

I'm not sure this applies to the 360, weren't they made in an entirely different factory? AC was standard on all Volvos sold in the US market during most of the crumbly wiring era. 200 and 700 series are all we had during that period.

The climate here in northwestern Washington is not terribly severe.

Feel the wire going to the oil pressure switch or peel back the sheath on the large bundle under the intake manifold, if the insulation is not crumbling I'd love to know what you did to prevent it. Washing the engine often enough to remove any traces of spilled or leaked oil may do it but very few people aside from me ever do that.

The cars are Pac NW cars, both were well maintained, and the engine compartments were clean when I got them.

On each car I removed the intake manifold as part of a thorough inspection.

Each had MINOR wiring issues which I quickly addressed with electrical tape, typically short leads to a sensor were worn, once or twice worn away, leaving bare wire (an inch).

On each car I cut into the covered harness in several places: everything was fine.

This was definitely NOT the scenario with the 240s I have owned, which all required more extensive repair or replacement to sort out.