Discussion
Apologies for not searching but I just don't have the time at moment. My rev needle intermitently drops dead and is pushed back into life by a squirt on the accelerator. Could someone tell me where it is on a 97 4.2. Ta. I also don't know what it looks like so if anyone has a photo that would be great
theres a fat positive wire from the alternator running toward the back of the engine. follow this and you will find the big fuse. a black block about 2.5cm wide by 5cm long, should have a flip up lid. mine was cable tied to the coil pack plate. look down the back of the engine, central, where gear box joins to engine its around there.
Edited by plumAJP on Thursday 30th April 20:39
This seems to be a common fault report but I'm a bit confused. The tacho is driven by the ECU so I'm stumped as to why the big fuse causes the drop outs, not that I'm saying it's not so as many have clearly demonstrated that it is so. Mine was doing it both with the MBE and Emerald ECUs and then the alternator light came on and dumped me on the hard shoulder for an hour or two with a seriously pissed of wife.
I got it back to the workshop and changed the fuse (to a nail as a temporary fix!) and although the charging problem was fixed the tacho problem was not. Just every so often the needle would flick to zero and the engine stumble for a fraction of a second accompanied by a flash of the alternator light as the revs dropped too low to charge. I changed the crank pickup and that fixed it for me. My logic was that if the ECU stops telling the tacho what to read it is probably because it has lost sync with the timing pips on the flywheel.
I got it back to the workshop and changed the fuse (to a nail as a temporary fix!) and although the charging problem was fixed the tacho problem was not. Just every so often the needle would flick to zero and the engine stumble for a fraction of a second accompanied by a flash of the alternator light as the revs dropped too low to charge. I changed the crank pickup and that fixed it for me. My logic was that if the ECU stops telling the tacho what to read it is probably because it has lost sync with the timing pips on the flywheel.
I tend to agree with Jon on this one. I used to get this problem before I removed the crank pick up sensor and cleaned off all the accumulated metallic particles (its a magnet after all). I suspect that the layer of crap on the sensor was stopping it from picking up a signal when the engine was spining slowly. Faster spinning of the flywheel equates to a stronger signal, hence why it comes back in with a blip of the throttle.
I was retrofitting injection on a vauxhall magnum and had a lot of trouble with the crank signal dropping out at low revs and being pretty much absent when cranking (same Ford sensor). The gap between the end of the sensor and the toothed wheel was mega critical. I had an osciloscope (or is that an oscilloscope?) on it and found that at 1000 RPM the output was about 0.5v peak to peak and at 5000 rpm it was nearer 40 v. Any magnetic crud on the end reduced it to virtually zero as did a gap increase of 2 mm at idle. I think that if the cheaper big fuse approach fails the sensor is well worth a go, the new one was only about £30.00 and on a ramp it's actually less fiddly than the fuse.
They do build up steel crud on them as the starter grids against the ring gear and sprays filings around the bellhousing.
They do build up steel crud on them as the starter grids against the ring gear and sprays filings around the bellhousing.
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