My girlfriend started my 1990 240 gave it gas but without releasing the key.

The gear on the starter broke into bits and bent up the flex plate that the TDC sensor reads. It also damaged the sensor as bits got carried up around with the ring gear and flex plate motion.

i straightened it out from under the car even drilled an access hole near the removable transmission screen plugs to see the flex plate better.
i might have .010” of runout I have it pretty flat now it had some damage but I spent a few hours getting it best I can.

I’d like to know what the distance from the sensor tip to the flex plate should be. I think I have about 3 mm or maybe 1/8 “ gap now but I can measure that by comparing depth of hole down where sensor fits , to flex plate , to the length of the sensor.

it ran after straightening that but when I put it on the road it stalled intermittently and wouldn’t start had it towed home I assumed I had just not straightened it well enough so I spent a couple more days trying to iron tgat out and get it as close as I can. It doesn’t seem bad now .

I’ve been going absolutely crazy trying to get it started again. Replaced sensor, several more attempts at trying to get that flex plate just as straight as I can. Don’t want to pull the transmission because I have it pretty well straightened now and I’m not sure it is the plate that’s the cause.

i have an oscilloscope for general electronics can I connect that to one of the 3 sensor wires to observe the wave pattern? I couod check the sensors I have for how many ohms if resistance they have.

if I turn the key it fires and runs for a revolution but doesn’t seem to spark past then. Occasionally it will idle a bit and then stall

it backfires a bit I think because it keeps turning off the spark so some raw fuel builds up in the engine.

I’ve had the plugs in and out between tests refreshed the plugs

I tried swapping the fuel pump relay assuming it is the white relay near the ignition computer above passenger foot area.

checked fuel flow, two short cranks and I got 1/2 litre out, it’s pumping fuel ok.

tried swapping ignition computer , main computer. Tried connecting a different O2 sensor , mass meter. Ignition coil, I put the old part back as each did nothing.

I believe it is giving an initial spark and then the ECU of ICU do not latch and continue with providing spark. I can hear the pump relay unlatch

I tried swapping the transistor behind the air filter box maybe I can try another. Maybe it’s intermittent or maybe I have a new but intermittent TDC sensor it was not a OEM one last time.

I’m ok reading the meter and doing checks. And I need to analyze this more closely but I’m stuck without a complete understanding of a logical sequence to troubleshoot this

I haven’t been able to retrieve any codes it does have the box with led sensor. I don’t know if I can use that under a no start condition. Should it store a code to report if if it’s not getting a valid TDC pulse ?

i have 2 740s that have the older sensor in the distributor and run , plus one 740 that I disassembled. Also similar, so I can use any of those parts that are compatible to swap for troubleshooting they are all 88/89 era the ignition computers have a different plug and are not compatible but I think the ECU is compatible

the 740 has a relay near the radiator water bottle if I pull that they won’t start but I’m not sure if the 240 has that relay or where it is or what that does.

if anyone has a bit better understanding of how the ignition system latches or what to check with the ignition amp or how to verify if the pulse from the TDC sensor is ok or not , these seem to be good directions for me to troubleshoot in


it always fires in first crank. It has fuel, I don’t think it has a spark past the initial first revolution.

removing trans to install a new flex plate is a big job and I do have mine fairly straight after it’s collision with the starter gear fragments. I don’t want to do all that just to learn it’s not the cause of my issue

I wish there was some way to convert it to the older distributor type system but the ignition computer is different. Wiring changes are a bit daunting I did pull the whole harness from the 740 I disassembled.

my brother converted a couple of similar cars back to carbs and points and was able to pull the computers right out but that again is a big job and I’d need a different manifold and old style distributor .

this engine is in decent shape I rebuilt it some time ago with new pistons wrist pins rebuilt head new cam oil pump etc so it’s a fairly tight engine. Idled perfectly and had lots of power before this.

I’ve been stuck on this issue a long time. The car isn’t even that perfect but it’s sort of personal I just want to win this ! I’ve kept up the car mechanically so even though it looks a bit like something from a mad max movie with missing clearcoat etc if I can fix it it’s a perfect winter car I did the heater fan job exhaust brakes struts all the front end parts rebuilt the seats u joints fuel pumps etc

it might end up to be a bad connection. I can send a picture of the clumps of hair I’m pulling out trying to solve it this one has got me.


the fuse near the coil seems ok.

the other Volvos I have have had similar issues with bad sensors within the distributor and they cause similar non start issues


thanks for listening

Phil

 

I found some good procedures for troubleshooting this on another site. . I think it's resolved now, the car runs. what caused it is now my only concern. It may have been a loose connector at the coil because the las thing I did was hooked up a test lamp to the coil. then it ran! I had even swapped the computer, the ignition computer the fuel pump relay, checked it was pumping gas from both the main and pre pump, I even swapped the coil for another coil.. and still had no start just firing when the key turned than no more spark AFTER FIRST REVOLUTION.
maybe something else is flakey like the power stage transistor? anyway if it comes back the first things ill check are the coil and power stage transistors.
I did previously have issue with that TDC sensor but managed to straighten out the flex plate, replaced the sensor and the starter motor.. when a no start issue returned I thought maybe the flex plate is still too bent up, or new sensor was flakey or something relted.. so I was under there straightening it 'BETTER'

the symptom is similar, no spark , no firing no start. cause was damaged sensor, bent sensor plate and faulty ignition coil connection .
I'll spend some time cleaning connectors then out it on the road , see what happens. maybe I should order a powertage transistor and a TDC sensor for good luck.