Ignition pick-up coil / Crank position sensor (nerdy AND boring) - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 167 (permalink) Old 11-20-2010, 07:32 AM Thread Starter
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Ignition pick-up coil / Crank position sensor (nerdy AND boring)

A cursory trawl through the forums shows this component to be responsible for no end of problems on Bonnies. I can't understand why this should be so. Similar devices have been used on all manner of bikes for donkey's years and they mostly last the life of the engine. It's a relatively simple, passive device with no moving parts. The working principle was worked out by Michael Faraday as long ago as 1821. Another mystery of British electrics...


This simple component is just an assembly consisting of a metal bracket holding a plastic housing that encloses the two active elements in it: A coil of very fine enamelled wire wound around a magnetised iron core. It is in effect a tiny generator stator.

I have got hold of a spare one and close examination shows a well-built, robust and well epoxy-sealed assembly that should be reliable. I can't see the windings inside though, so this is where the troubles could stem from, cheap enamelled wire or faulty soldering or strain-relief of the many turns of very fine wire necessary to obtain the sort of resistance and inductance values required in such a small component.

On carbed models Triumph call it an "Ignition pick-up sensor" and on EFI models it's called a "Crank Position Sensor". Other manufacturers use names such as "Pulse coil" or "Pulse generator".

It's actually the same component on both: Part number T1290131 and costs £49.61 ($79).

On both models it's positioned inside the triangular cover on the right of the engine and fitted very close to the alternator rotor.

On carbed models the rotor has some metallic strips bonded to it called reluctors that pass very close to the magnetic core as the rotor turns. Everytime they do this a tiny electrical pulse is generated and sent to the igniter. This signal is processed by the program or map loaded into the igniter to work out the moment of ignition and the degree of advance and retard to be applied, depending on engine revs. The advance and retard function is entirely electronic, unlike the old centrifugal weights system. No moving parts, just time-delay circuitry that applies more or less delay to the received signal.

On EFI models it also sits close to the rotor but this is fitted with a toothed wheel. Everytime a tooth passes the sensor it also sends a signal to the Electronic Control Module (ECM) and it's used not only to work out the ignition timing but the moment of fuel injection as well. Between the signals sent by this sensor and the MAP sensor (manifold absolute pressure) the ECM knows exactly in what position the crank, and herefore the pistons, are at any given moment.


If the pick-up sensor is on the bench like the one in the photo the resistance can be measured using the multimeters probes. If however it's still fitted to the engine the connector can be difficult to access, being stuck between the right hand seat rail and the top of the airbox. Brilliant bit of engineering this...

Unplugging is often impossible without further dismantling. You can see the two wires that go into the connector though, so you could just use the following method:

Using a couple of pins prick the insulation of red and black wires under seat tube above the airbox. Drive the pins deep enough to pierce the insulation and contact the conductors inside the cable. Connect the multimeters probes to the pins and set the meter to the appropiate Ohms range.

The quoted resistance to be expected is 560 ohms +/-10% at 20ºC .This means you should see somewhere between 504 and 616 Ohms. If you measure it with the engine hot the resistance would be higher, as much as 640 Ohms or so.

You could even measure the voltage output with this method. Just ensure the two pins and meter probes are not short-circuited, set the meter to a low DC volts scale and start the engine. You should see at least 0.6 volts although at idle this would be pulsating and the digital readings difficult to pin down.

This nice drawing I've found shows how the pick up generates two pulses: one positive and another negative, the first one as the reluctor first meets the pick-up coil and the second as it leaves it:

A simple tester can be made using a couple of green LEDs that will detect any pulses above around 2.2 volts, both the positive and the negative pulses will show up as the LEDs light up alternatively as the engine is cranked:

The gap for the pick-up coil is quoted in service manuals as 1.0 mm +/-0.20 mm but was changed during production due to warranty issues. The following was sent out as a statement by the Factory in a technical release to dealers:

Affected Models: Bonneville/T100, America, Speedmaster, Thruxton and Scrambler.

Should a bike (see above) demonstrate faulty ignition coil type symptoms (most commonly an engine misfire), please initially check and adjust the ignition pick-up (IPU) air gap (we recommend an air gap of 0.8mm).

Since changing the IPU air gap in production to 0.8mm, we have had no ignition coil warranty claims.

As far as I know this only applied to carbed models with chronic ignition faults, often blamed on the ignition coil.

The 2008 service manual of which I have a copy, only covers bikes up to 2008, including EFI models, and the gap is still quoted there as 1.0 mm so maybe that technical note came out later than 2008.

The pick-up coil to rotor gap is set by positioning the alternator rotor with a reluctor strip facing the magnetised core and measuring the gap with a suitable feeler gauge. If found to be incorrect the fixing screws are loosened slightly, not too much, but enough to enable the component to move a bit, ensuring the feeler gauge is a firm sliding fit between the two points and re-tightening the screws to 10 Nm (7.3 Lbs/Ft). Re-check the gap after tightening.

Last edited by Forchetto; 11-21-2010 at 04:35 AM.
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post #2 of 167 (permalink) Old 11-20-2010, 08:02 AM
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have to check that on Davids bike.. he has allready changed the coil ones!

I never seen so many sissy men on a bike forum. "Good judgment comes from experience. Unfortunately, the experience usually comes from bad judgment."
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post #3 of 167 (permalink) Old 11-20-2010, 08:07 AM
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For someone like me who has no concept of electrical wizardry that was a good read, thanks Forchetto.


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post #4 of 167 (permalink) Old 11-20-2010, 04:25 PM
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Always wondered how that worked. However, after a frustrating bout with intermittent spark, I dismantled, checked resistence and gap, found satisfactory, but the problem persisted. A dealer ultimately fit a new one and problem gone. Yup, those buggers cause some headaches!
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post #5 of 167 (permalink) Old 11-20-2010, 05:04 PM
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Linking this in the electrickery part of the info sticky.

This thread is a lot better than the crush washer one.

Actually, I would like a pickle.
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post #6 of 167 (permalink) Old 11-21-2010, 07:29 AM
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Just happened across this post. Great write up, explanation and contribution Forchetto. The Bonneville library of technical fixes etc. in the stickies is really filling out much of it thanks to your well presented write ups. Thank you bro.
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post #7 of 167 (permalink) Old 11-21-2010, 08:34 AM
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I agree, another winner from Forchetto. Nice job!
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post #8 of 167 (permalink) Old 11-21-2010, 05:29 PM
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well done Forchetto, brilliant write up

have saved it for future reference. thankyou

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post #9 of 167 (permalink) Old 11-21-2010, 09:21 PM
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Thanks a bunch... the power of this forum is incredible... leraning a ton from guys like you Forchetto...

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post #10 of 167 (permalink) Old 11-23-2010, 06:21 PM
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Have you got a pic of the efi alt rotor?

Why do I feel young on my bike
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