Looking for crank sensors install tips & tricks

[johnmc111]

86 951. So I have already toasted a sensor because since I didn't remove the bracket to remove and install the first set of sensors, I didn't check the gap and paid the price. Now the bracket is out because of the smashed sensor and I am looking to reinstall it and the sensors.

Hoping someone might be able to provide some advice on the process to make it as least painful as possible. I know about the 8mm washer to set gap, I'm just looking for things like the order to do stuff, should I grease/lub the sensors to install them and the procedure for checking the clearance by turning over the engine by hand.

Links to articles or videos would help too.

Thanks! - John

[wwdwgs]

Refer to Clarks garage or just google "clarks garage" for more instructions and tips.

[Cochrane 951]

What I did was glue a washer to the bottom of an old sensor that was 8 mm set it and tighten the bracket then replace the sensor with a new sensor.

[grendiers]

There should be plenty of dirt outlining where the bracket was. Did this on a 924s back in the day. May need a small mirror to see it.

[johnmc111]

Thanks for the help. I've read Clark's guide but I was just wondering if anyone had anything to add. Here is a specific question regarding the spacer for the speed sensor, the double holed washer. Is it true that some engines use this and some don't? If so how do you know? Is it by engine number or model year? Although the car is an 86 the engine was out with previous owner and has been heavily modified and I'm not sure it is original to the car. The bracket is probably not original because the end has been cut off. When I removed the sensor, I didn't see or hear the spacer fall off.

[curtisr]

If you were able to remove the bracket then you're ahead of the game. I couldn't get mine out. &*&^&%!!
Sure, a little lube is OK but you shouldn't need to. It sometimes takes a bit of penetrating fluid to get 'em out. Anyways, bolt in the sensors, attach bracket, push down until it stops, tighten bracket, install washer (used on 944 Turbos) and you're done.

Alternatively, you can use a gapping tool [available here (no affiliation)] to set the height (sensor shaft plus 0.8mm).

Some folks will glue an equivalently sized washer to the bottom of an old sensor (they're both the same size) and use it set the correct height.

[Monkey Wrench]

what Ive done on Volvo's just measure the length of the sensor to the base, then measure the distance down to the pin or gear that it is sensing, then subtract and you know the clearance. I just use a vernier calliper.
In a volvo it senses a metal plate ( flex plate) with holes in it, on a Porsche there is one that senses a pin and the other looks at the flywheel gear teeth, I think.

I know the air gap on a volvo is 2mm but on a porsche not sure, I would think it would be something close to that.

anyway my point wasn't to compare vehicles but just say you can use the pointy thing on a vernier calliper, to reach down into hole, and subtract the difference. referencing where the sensor seats.. then you know the exact dimension of the air gap. you'd have to turn the crank to the place where the pin is to take that distance measurement.

i paid a coule hundred for my good starrett one years ago but there are 10 dollar ones and they work ok. I even find the dollar store plastic ones useful when accuracy isn't an issue, like measuring a bolt in length.

if you take one measurement and zero it there , then take the another measurement it shows the comparison without needing to do any math.

if you hook up a scope you can look at the actual pulse and that may help as you'd see changes in the pulse amplitude and wavelength with distance.

the sensors are a coil of wire so they have a certain resistance can if you wag a screwdriver past them with it hooked to a meter you'll see a volt or so of pulse. if it has no continuity the coil has a wire break and it's bad,, if it gets whacked that may happen. if that coil were shorted part way through the bundle it may short some coils and have lower resistance but that's probably rare.

if the porsche has two and they are the same part number you should be able to compare and get similar resistance.

I think it may be more likely to have a wire issue as the wires shake about and get heated and oil contaminated. after 35 years its probably a little brittle. If they die, engine stops running as it needs it to track the ignition pulses. that would cause a loss of the pulse on the primary coil..

If I suspect an ignition fault , first thing I'd look fo is does it or does it not have a pulse across the primary coil when cranking. no pulse at the coil primary, means no spark. a small bulb put across the primary coil is an easy check , it should blink when its cranking. I'm personally starting to just put an LED there so I can see at a glance if it's pulsing.

if I do have a pulse at the coil , or not , I think that's great to know first in the event of a no start. a bad sensor or bad ground or bad hot to the ECU or bad wiring could make the car loose it's primary pulse at the coil , so I just jump straight to that. It dumbs down the troubleshooting somewhat. that pulse is pretty well proving all the other parts before it, are at least intact and functioning.

on the coil there are a couple of spade connectors, a low watt bulb or LED can be put across it, the lamp is then sharing the load with the primary coil but if the bulb is just little it's not robbing enough to make any noticeable difference.. i have a couple of cars where I've just left it in place.

if you wanted to remove it having a small bulb and two alligator clips is a handy little device that you can throw in the trunk. you can do pretty much any electrical check with just that test bulb and a bit of wire if you had to do some side of the road troubleshooting.

there is a little amplifier thing it may have other names..
it is bolted somewhere near the inner fender on the drivers side with some heat transfer putty to conduct heat away from it. the wires than lead to the coil. the thing is fed from the MCE, on that side it is delicate. in other words not high amperage. basically a signal. its basically a transistor and it makes heat, that's why the heat putty.


I think the coil actually gets grounded intermittently that way. the ECU sends a pulse and I believe it basically is making and breaking the ground side contact causing the pulse. I think the coil is actually seeing a ground pulse and it will be hot on its other terminal with key on. that's the primary coil side. the secondary coil is sending juice to the rotor and on to the plugs, that is high voltage. the primary is 12 V.

the coil is just a transformer, it is changing that 12 V pulse to a high voltage pulse, the secondary has many turns so that ups the voltage to create a long spark.

each time the coil is interrupted by opening there is a high voltage induced in the secondary.

the factory engineers already figured out how many turns is optimal to generate the right voltage, but you can run out and buy aftermarket coils that make the claim of producing " "stronger spark" I think if that was really a gain it would have been easy to change the number of turns at initial engineering stages.

this I believe is just mostly fancy marketing and a lot of peole buy in and replace coils to find no difference..

the amperage is not governed by the coil. you could double the number turns and that would create higher voltage but at at less amps.

a transformer doesn't; create power, it transforms power, it has some losses. an aftermarket coil could have a different number of turns, it could put out a different voltage , sure ..
is that better? the coil doesn't create energy.

a higher voltage jumps a longer distance. so if you'd want to increase the chances of having carbon tracking in the cap or a problem like a spark jumping from a plug wire to ground then that's what you'd want, higher voltage.

the stock voltage is plenty to jump the plug gap and I believe the engineers made no mistake. I think what happens is people run out and buy a aftermarket high output coil with some catchy name and think it has improved things, In reality did was change the number of turns from what the engineers previously decided was optimal.

a lot of times people have a no start situation so they go buy a coil, hoping that fixes something.. yes they can fail but it's rarely the case.

I have seen the studs on the coil get rusty and had an instance where my volvo would not start.

testing for a pulse across the primary proved it had a pulse but no spark was being generated..

this was because although the spade terminals on the coil were seeing a pulse, the rusty threads that attach that spade terminal to the windings of the coil , had an issue. i merely had to clean the threads that protrude from the coil and I was operational again. I'd suggest if you are looking at a coil and the threads or connectors seem rusty or dirty, to clean them.

I think it actually could be possible for an aftermarket coil to put through more amps, to do that you'd put more stress upon the transistor thing on the inner fender as it would need to handle more power.. it would also increase power draw at it's feed.

I believe the MCU is a bit isolated, its basically triggering the transistor which is in turn handling more amps ..

"transistor "may not be the exact correct term. I think Porshe and volvo shared the exact same one in the mid 80's and I've never had one fail. I believe its operation is somewhat akin to a relay in that the trigger is low amps and it is triggering a higher amperage connection without loading the trigger side. The ECU or MCE provides a signal but not many amps. You might think of it amplifying this signal to one of higher amperage, to in turn feed the coil it's higher demand.

as you go from car to car there may be differences but I;d say most cars use the same essential principles. so that check of the primary pulse probably applies to most other cars.

in my ford van I've seen it loose its ignition pulse due to a failed ford ignition module. in my volvo 740 Ive seen no pulse caused by a failed hall sensor
in some cars use the distributor hall sensor as the input for timing and TDC the Porsche 944 and later volvos used a TDC sensor.
The porsche 944 used two sensors.. in the volvo it used a bunch of holes and one hole missing for the index pulse or TDC. and only one sensor.
even if its an old car with a points distributor there is still a pulse at the primary when the ignition is functional.
some cars now have a coil for each cylinder, why I don't know, just different. my 1982 force 85HP outboard
motor has an ignition issue, the coil is made of plastic, it generates the spark by a magneto I believe and a trigger, anywhy what happened there i s it is a coil of wire , bound by plastic, the plastic shrinks breaking the wires.

as a result my boat will run a mile and then die because the coil that generates the current for the spark is damaged. when cold it runs fine, once warm it dies. I believe this is being caused by a coil of wire where the leads are broken but they are touching , sometimes, causing an intermittent issue. heat expansion causes them to open and close up.

sorry this is off topic or off track I just find it interesting and somewhat beneficial to try to understand how these problems are common in other vehicles and to make some comparisons.

I'm not trying to deter anyone from following the steps of clarks garage i just feel a general understanding helps in addition to a logical sequence.