[Sean's Jaguar XJS Tech Pages]
Useful Tools and Gadgets: the Timing Light

Last modified 2005 APR 07 04:40:30 GMT

Timing Light

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My Sunpro inductive timing light CP7804 (Made in the USA!)

A timing light is a necessary tool for not only checking and setting your timing, but also for performing simple ignition diagnostics (for instance, you can clip the inductive clamp to ANY of the spark leads to check that the distributor is sending spark for that cylinder - of course, it may be grounding at the plug, but OPEN plug circuits are trivial to detect with this simple test). DO NOT get a cheap "inline" timing light - the type that you connect by removing a spark lead from the distributor, and inserting some probe (usually something like a spring), and tap the line that way. They're cheap, and with the voltages present in even a typical ignition system (25Kvolt), you can get in trouble, but with a high energy system with voltages from 40 to 50Kvolt (or more!), it can be doubly dangerous. Inductive sensors avoid the danger, and sense passing current using a clamp that you just clip onto the spark lead. They operate on the same principle which clamp-on AC ammetres work.

Now, as anyone who owns an XJ-S knows, the battery isn't in the engine compartment. Further - at least with the V-12, the timing is checked from a mark on the BOTTOM of the crank pulley, and the spark leads are in the valley above. With an stock timing light, the only way you're checking the timing is to snake the timing light leads up through the engine compartment to get them where they need to be, because they never make the test leads long enough to contend with this sort of engine design. This exposes them to heat (ever touch a wire insulator - or anything for that matter - to a hot exhaust manifold?), and/or rapidly moving parts (belts, and the cooling fan). As has been said, contact with parts moving at 3000 rpm will prove to be a memorable experience. You'll also no doubt have restricted range of motion with the timing light because all the slack in the cable has been taken up by winding it down to the viewing location.

One solution is to make and install an alternate timing plate, so you can check the timing from above (foolish me, I didn't do this while I was in dealing with my water pump a few months ago). So what's a DIY'er to do?

Modify your timing light! This not only makes it more Jaguar friendly, but at the same time, gives you another tool.

First, take the inductive trigger line and cut it, perhaps 3 feet (an arms length) from the inductive clamp. Take a suitable piece of 2-conductor wire (16 AWG "zip cord" or "lamp cord" - same stuff as is used for household extension cords or for rewiring lamps works well), approx 12 feet in length (you could use shorter if you wanted, but a length like this ensures you won't be short on reach). Strip all wire ends (both sides of the cut wires on the timing light, and both ends of the piece you're about to patch in). You may want to check out my tutorial on splice soldering. On the timing light side, sleeve both individual conductors with shrink tubing, and a larger shrink tube over the pair. Trim the prepped leads to about 1/4 inch, and then solder them to the light, pull the individual shrink tube over the solder contacts and briefly heat them up (a pocket lighter works well, but don't heat in one place, or it'll burn). Then pull the larger piece over the pair, and shrink that. Properly done, your completed splice will look like so:

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At the other end of this cable, solder an appropriate 2-conductor connector. I used "CINCH" connectors, pictured here.

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These are polarized (one pin is larger than the other), and so, when making the patch cable, I was also sure to maintain matching polarity on the cable (the zip cord has ribbed, or a stripe on one conductor, and at least on my timing light, so did the original cable). This shouldn't matter, but it certainly alays any concern that the timing light circuitry might care.

You could instead use BNC connectors -- the same sort of connector you'd plug directly into an oscilloscope (use the MALE connector - the one with the pin in it - on the pickup for reasons which will be obvious shortly).

Of course, you also solder up the mating connector on the clamp side of the wire as well.

To use, you just plug the connectors together, and you have a timing light with 12 feet more of reach.

The fun doesn't stop here -- if you just wanted the extra reach, then you could do without the connectors, and just splice the extension in (and in fact, you may choose to do this). The reason I went with a plug, is because I picked up a second male cinch plug as well (I put the female on the clamp side), which I can solder to a test lead to use with an oscilloscope. With this (which is nothing more than a non-calibrated ammetre clamp), it is possible to use it as a trigger for other events: a two channel o-scope can measure one event on one channel - say ignition noise, injector pulses, or a signal from the ignition amplifier, and show that in the time domain with the pulse of any individual (depending on which spark lead you clamp it over -- or ALL, if you use the HT coil lead) spark firing. No, I don't have any specific tests to conduct with this just yet, but since the tool could be adapted for this for a minimum of cost (the cost of the connectors - in my case, about US$2.60 apiece, and they're only that much because the electronics supply shop local to me likes to overcharge), why not add a tool to the toolkit? There are places that sell pickups for using with a scope, which are not very different from what I'm suggesting here. Note that if you use a male BNC connector on the clamp, you don't really need to make a to-scope cable, since the pickup wire can connect directly to a scope. If you wanted a better pickup, you could determine the polarity of the pulse, then replace pickup clamp wire with a shielded cable (a cable in which the positive lead runs down the middle, and the ground runs through a a mesh shield around that), which would clean up radio interference from the signal. For simple ignition triggering, this is unnecessary.

The clamp, plugged in, looks like so:

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Subsequent to taking this photo, I cut down the clamping screws, then used shrink tubing over electrical connectors so that I don't have to worry about one of these screws dragging down a fender.

With the increased reach of the inductive lead, I can clamp 6A (which fires at the same point in the crank revolution as 1A, but is much easier to reach), then route the lead up over one of the bonnet catches, and down over the side.

What about power you ask? Refer to the battery terminal block and charging extension -- I hook it up as described, but instead of connecting the charger, I connect the timing light! No fidgeting with getting to the power buss on the firewall, or connecting to the alternator mains, or any of that. Plain and simple. Before using the terminal block, I used to just clip the timing light to a spare battery...


Sean B. Straw
Post Box 751224
Petaluma, CA 94975-1224 USA

EMail to: Sean.Straw+Jaguar@mail.professional.org

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