940 LPT cutting out and RPM tac bouncing.

[TonyS9]

The rpm needle should not move until the ignition system is firing the coil (its triggered from the coil back EMF). And the computer won't do this until it finds TDC (which it doesn't remember after a few seconds of not running, otherwise it does use the multiple triggers around the flywheel). So normally it should fire when the needle moves, but it takes a few pulses of the starter for this to happen on average as there is only 1 reference point per engine rotation.

If the rpms are showing ok when running badly then it might an ignition problem after the coil, eg in the disti, cables or plugs. Or it might be a fuel problem, although the the computer usually knows about fuel mixture problems.
The ignition amp is known to fail at higher temperatures, but this drives the coil so you should be able to see it on the rpm needle, however I'm not 100% sure.

To check the fuel pump check for priming when turning the ignition on, listen for the whine, if its gratey/grindy or not happening the pump could be on the way out. Pump replays are also a common failure (always carry a spare). The pumps do wear out and need replacing after 150K or s, but it does vary. In my case the pump failed while at speed on the motorway, with rpms still showing as expected. I deliberately kept it in gear, and luckily it came back after a few seconds. I had 1 or 2 other similar incidents of not starting then starting before determining it was the pump. When listening it was intermittent, and grindy when it did go. You need a good quiet location to hear it, lift the back seat up helps a bit.

I'd probably be looking at the ignition system first. Get a set of spark lights, or a single spark monitor, this will help check actual spark rather than just using the rpm needle

[volvo always]

Thanks for the info,

I have ordered a Bosch crank Sensor, so may arrive tomorrow.

I replaced the in tank fuel pump a few years ago as it was on it's way out at 191,000 miles. First breakdown. Fitted a Bosch fuel pump from Ebay, cost £80. Fiddly to fit.

Ignition module was also changed around a year ago too. Have also bought a cheap £20 one off Ebay as does seem electrical related.

When it's running it runs well, no misfire, smooth and picks up well. Idles around 800rpm. A bit less when AC compressor kicks in momentarily.

James.

[volvo always]

UPDATE

Installed the new crank sensor yesterday and drove it around 15 miles and so far no issues.

The ignition module was not the right one, so ordered another.

Replaced the passenger side suspension yesterday after doing the drivers side the day before. Quite enjoyed it.
All parts replaced, Arms, track control arm (Kit), drop links. Ball joints, track rod ends all TRW brand.

Worst issue was getting the bolts through the car mountings to align up. Cheap £40 Impact gun was brilliant. Ball joint on drivers side well loose and noisy!


Had the tracking/ alignment done as bad vibrations in the steering wheel over rough lanes after changing the suspension. Smooth roads no vibration. Steering feels quite tight and no noise coming from the suspension. After alignment went down same lane and a massive improvement, but still a slight vibration in wheel on very bad surfaces. Will have it re-tracked when I do the inserts as not convinced Kwik fit did the best of jobs. Love to find a place near Burton On Trent that only do alignment.

Awaiting my new RIDEX damper inserts to come into stock and be delivered. Need to replace as passenger side insert leaking. MOT 2nd of November. Believe process is to remove brake caliper and ABS Sensor or disconnect in engine bay and undo from ball joint bottom nut. Remove the small bolts on top of strut housing and remove strut. Use spring compressors on spring and then undo top nut. Stilsons on nut retaining the damper insert and re-assemble.


On the whole, enjoyed spending 6 hours doing the suspension and at least all the bolts came out!

Many thanks. James