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The Problem:
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A volvo 740 B230K with no spark.
Have tried so far:
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All sorts of things, but will narrow it to what I believe
might be the salient points (my logic may well be flawed -
if it is then please don't be shy).
Distributor cap rotor and HT leads were changed at the
beginning. I now look for a spark by having the coil HT lead
jammed in close to the exhaust manifold with a small gap,
which allows for good spark visibility. Hence can ignore
the distributor altogether. Coil impedances seem to be
within spec (Bosch coil), coil HT replacement lead has resistance
of about 2k ohms, original lead was about 3k ohms. Switch on
ignition and get 12v at +ve coil connection.
Turn engine over and get no spark, no tachometer reading and
can't hear much in the way of fuel pump activity (although
in a noisy environment I doubt I can be certain about hearing
fuel pump).
Swapped out coil for another, still nothing. Swap coil back.
Next deducted hall sensor. Removed hall sensor connector and
checked voltages - red at +12V, blue at +5v, black at 0V with
essentially zero resistance to ground, so that looks good.
Make jumper connection between blue and black wire and
toggle it repeatedly with ignition on. Still no spark.
So that seems to indicate trouble in the ECU, power stage
amplifier, associated wiring or perhaps high voltage coil
breakdown(?).
Removed power stage amp connector and checked voltages with ignition
on. Pin 1 (red and white wire) +12v, pin 2 (brown) 0v connected to
ground, pin 3 (black) 0v but not connected to ground is merely single connection to shielding of signal wire and hence is free floating,
pin 4 (blue) +12v, pin 5 (grey) 0v. The signal wire is connected to pin 5, toggling the blue and black wires on the Hall sensor connector gives
a nice clean +5v pulse on the signal wire, indicating that the ECU
is doing its job.
Switch off ignition, reconnect the power stage amp, switch ignition back
on, and then toggle the blue and black wires on the Hall sensor connector again. No spark.
Looks like the power stage amp may be at fault. So I swapped
in a replacement power stage and repeated the toggling procedure.
Nothing.
I swapped in yet another power stage amp and I start seeing
lots of sparks from the coil HT lead when the ignition is on, sadly
though the sparking occurs whether I toggle the hall sensor connector
wires or not. Still, at this stage I'm glad to see any kind of spark,
and it takes some suspicion away from the coil(s) - maybe wrongly.
Anyway I reconnect the hall sensor, get in and switch on the ignition
witout cranking. The tachometer needle is twitching all over the place
and (I'm told) the coil HT lead is producing a nice set of blue sparks.
I can't say I heard anything from the fuel pumps but as I said it was
a pretty noisy environment.
Being pretty desperate at this stage, I tried starting the
beast (hey! a spark's a spark and we all do dumb things). So I
connect the coil lead to the distributor and give it a go for
a few minutes. Unsuprisingly this didn't work. The tacho
was still twitching throughout so the power stage amp was
still triggering during this process. After this I smelt the
exhaust pipe. Hardly any smell. Suggests that the fuel pump
probably wasn't going, as otherwise I reckon there'd have been
a stong smell of fuel.
OK this is clearly not a healthy power stage amp and lord
knows what it's done to the ECU. I still see a nice 5V pulse
when I toggle the Hall sensor connector leads with the
ignition on, but by this stage I'm harbouring suspicions
against everything. So I swap in an ECU from a B230E volvo 740
(it's an EZ118K system just like with a B230K so it should
be fine) and yet another power stage amp. I switched on the
ignition, toggled the Hall sensor connector and checked the
signal wire. A nice clean 5V pulse. Connect in the power
stage amp, toggle the Hall sensor connector - No spark.
Disconnected the power stage amp and the negative lead from
the coil and checked the resistance to ground of the lead
with the ignition off (hence the resistance to ground of the
fuel pump relay and the tachometer and whatever else might
be attached to that wire. It was pretty much infinity, hence
no shorts to ground there. Reconnected the power stage amp
and checked the resistance to ground through the power module
with the ignition on. It was about 2k ohms. My meter is an
analogue so it's not going to catch sharp spikes. However,
when I reconnected the Hall sensor and measured the resistance
to ground through the power stage while cranking it drops to
about 1.5k ohms, consistent with the power stage switching
on for a series of short spikes.
Any Ideas?
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I'm stumped. I have numerous suspicions though. As you can probably
guess from above, I reckon the fuel pump relay is likely knackered,
however, I can't really see how that couldstop the system sparking,
especially when an oddly behavingpower stage can kick it into life
- albeit in an uncontrolablean unusable way. I'm not sure about the
coil(s), but would be very unlucky to have two duds, and again I
have seen sparks, so clearly they work sometimes. Some kind of
intermittent capacitance to ground causing smoothing of the power stage
spikes?? Who knows.
I hope someone can help here, otherwise a perfectly reasonable
volvo 740 is heading for the scrap yard for the sake of a
missing spark.
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