amplifying signal from crank position sensor

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Now that I think of it, the 3.37 volts is ground for the op amp. So maybe it would work to run it off the vehicle's battery... Should I do this?

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Conduct another test first. While cranking are you getting a spark?

Pull the _ignition_module_ and take it to a shop that can test it.

...Jim Thompson

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Assuming the ignition module is that thing with the coil clipped into it. I pulled that out. Then I took the coil out of the ignition module, clipped a cap across the primary terminals and touched 12 volts to it, got a phat spark. So the coil is ok. But there are apparently some electronics in the module, because it has more than just two wires going in and there's a cavity under it filled with some potting material. So now I have to go and find somebody who can test this module? Sheesh, why can't they make cars a guy can FIX.

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Forgot to mention, no spark during crank.

Hmm, by looking at the specs i'm guess it's a reluctant type of pick up? since the ohm reading is low on yours, it can be assumed that a couple of turns are shorted there by reducing output voltage.. how about using a step up transformer. something in the audio class should do it. you can use an pot on one side to shunt out over voltage.

Using something like a BuckBoost configuration.. Radio Shaft has an audio output transformer you could use by putting the primary leads across the sensor, then, connect one of the leads of the secondary to one of the primary leads. the other secondary lead will be a lead going to the computer. and remaining lead of sensor which is also connected to the primary of the transformer will go to the computer.. in the old days, Picture tube boosters were like this. you will have to play with the phase polarity to get it to boost. The ratio of this transformer should be close enough to give you a slight raise in voltage. the primary has a CT so you can play a bit.. You could also use the 1:1 audio xformer they have with the same config, that would double it. use a shunt R on one side to drain out over voltage. so in your case. 0.4 volts would be 0.8 in theory how ever, roughly thinking about loss from the sensor due to extra load and all it most likely will end up to be around .6 volts/

Question: I have in my box of magic junk a pre-wound pot core of a power ferrite material with a 1:1 ratio, almost an inch and a half in diameter. I think each winding has 860 uH inductance. Would it work?

Nope you need something with much higher L in it. You could try it but I don't think it'll work. something in the H/ high mh range..

I typed in "audio transformer" at the Radio Shack site and found one

one

Okay, I found a 1:1 transformer at RS, item 273-1374

It is a Variable Reluctance Sensor.

The amplifier will be a differential amplifier with about a few k of input resistance. There is probably an added dc bias current at the inputs that allows detection of open-circuit sensor, (and sometimes short-circuit).

It is therefore not a case of simply providing an extra stage of amplification. Any extra amplifier will probably have to fake the floating dc resistance of the VRS. This will be a transformer output, padded up to about the same 240 ohms output resistance.

In the event of a 'failed-sensor' detection, (real or false), the electronics might well cut the engine.

Poor connections can often cause a false failed-sensor.

So the first thing to try is to get rid of that plug and socket, and do a hardwire, (as suggested in the URL). Use a three-way screw terminal block.

A VRS has an output impedance that looks like an inductor and resistor in series. In an industrial app I have seen the output increased with a capacitor across the output leads.... of the order of 0.1uF or less.

Perhaps you might try your cranking experiment again, stabbing various cap values in parallel to see what the effect is.

I could be wrong about this, but it seems to me the meter should see a bit of voltage across the pins in that case, which it doesn't. This vehicle is from 1989 and may be a bit less modern in that respect.

Well, yesterday I pushed it to get it across the street to a legal spot. Damn jeep.

I tried pinching the connectors to make them tighter, but I didn't want to cut the connector off the wiring harness.

That's a good tip.

I had another idea of sticking a dime-store magnet on top of the cps to make the magnetic field stronger. I figure it probably failed because heat and vibration weakened the magnet. There's certainly plenty of both at the top of the bell housing, and that vehicle has

Thanks for the info Tony.

If you use a $5 Nd-Fe-Bo magnet, you can probably remagnetize the thing, but be careful you do it in the right direction, or you'll screw up the timing something awful.

Cheers,

Phil Hobbs

Uuuuh? Is there really a magnet there, or just a steel "tooth" going by a VR pickup that has current flowing thru it.

I know for a fact that ignition (distributor) VR pickups have no magnets.

...Jim Thompson

Hmm, could be. The ones I've seen (not automotive, admittedly) used permanent magnets. That way they don't require power.

Cheers,

Phil Hobbs

I misspoke, there are no _rotating_ magnets, but there is one underneath the rotor assembly.

But flywheel pickups don't use magnets, at least those I know of... there's no convenient "loop".

...Jim Thompson

Any magnetic sensor is going to have exactly the same problem...if you use a bar magnet with a field of 1 kG, and lots of turns, it doesn't take much to give you a nice big signal. Remagnetizing the magnet with the wrong polarity is likely to make the ignition fire half a tooth early or late.

Cheers,

Phil Hobbs

Might be worth measuring and see if there's current flow in the pickup coil.

...Jim Thompson

Okay Tony,

The capacitor trick worked. I tried 0.1 uF first; car started ran bad. Bunged in a 0.01 uF and ALL'S WELL.

Jim & Phil: The crankshaft position sensor definitely has a permanent mag. It attracts a piece of steel. A passive device like that ought to last the life of the car, by rights. However, there's a lot of heat and vibration at the sensor mount on top of the transmission bellhousing. Weakens the magnet as the engine hours pile up. This thing is a routine maintenance item, but if it fails out in the middle of nowhere, PITA because your car won't run without it. This has given me an idea to sell a little device containing a capacitor that can be hitched onto the wiring harness with those quick- snap connectors that pierce the insulation of the wires. Cheap insurance at a few bucks. I think the off-road crowd would buy them. You're on top of a mountain somewhere and your jeep won't start because your cps has 130k on it. Snap the magic doodad on your wires and away you go.

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Buy a new sensor !