Lately I've been noticing random slight twitches in the tach when waiting at the lights. Just small, about a needle's width. These are accompanied by a quick but definite jerk or jolt felt through the seat (both mine and the car's), and a small twitch to the left (lower) of the ammeter (or is it technically a voltmeter?), again barely half a needle's width.

It seems to do it more when the engine is warm, and lately I've noticed (as the weather warms??) that the revs tend more towards 600 rpm than 700 rpm at idle. So the twitch might be from 600 to 650, just for an instant.

Looking up tach twitches, I seem to get lots of stuff about major swings, but these are very minor in terms of how much the engine surges. Suggestions I've read cover everything from a failing coil to ECU issues to a dirty throttle body. (As an aside, I haven't found much in the forum about cleaning XJS throttle bodies. I took a look at mine, and there does appear to be a minor blackening discolouration around where the throttle rests at idle. So perhaps that's the first priority....)

As usual, any hints or suggestions will be most gratefully received.

 

I'd say you have a fault in your 'primary' ignition system. That is, the 12v side of the ignition. Not the high voltage output side to the spark plugs...known as the 'secondary' side.

Loose connection at the coil(s), failing ignition switch, ignition amp.module problem/poor connection....that sort of thing. The clue is the tach twitch coinciding with (what sounds like) a misfire. The tach takes its reading from the primary side of the ignition system

Cheers
DD

 

Thanks, Doug. Always helpful.
So the "primary" side is from the coils to the distributor, with the "secondary" side from there to the plugs?
Loose coil connections - I'll see if anything stands out. By "ignition switch," you don't mean the one I turn with the key, right? Where would I look for the ignition amp. module? If it's a connections issue, I might be able to fix that, so long as I knew where to look.

 

Okay, I took a look at the coils. Being a facelift V12, there are two small coils nestled in the vee. The rear coil, with three wires coming out of the connector, appears normal. The front coil, with two wires coming out of the connector, is not normal.


I'm guessing the insulation shouldn't be peeling away like that....

I don't know if that could cause the issue, but a prompt fix seems in order. High-temperature insulation tape wrapped around the wire, or is that just a cowboy fix?

 

Hi Someday

If you can take the wire out of the Plug without cutting the wire, which 'may' be held in by a grub screw?

Then it would be better to 'get off your horse' and get some Shrink Wrap

These come in a kit of various sizes and just slide over the offending wire, where a bit of heat from a heat gun, will see the Shrink Wrap er...... Shrink! and pull itself tight on the wire

Not expensive to buy for just a few Yen

Maybe that bare wire was causing the problem (on occasions)

 

Thanks, pardner. I've read about shrink wrap stuff, and I think I've seen it in the shops. The heat gun will be harder. It'll require (a) a heat gun, and more crucially, (b) a power socket I can use. That will be harder to track down. Mind you, the heat in the vee may be all that's needed....

I think I might also try running the engine at night with the bonnet opening and seeing if anything looks sparkly down by that bare wire.

 

Pro tem use some ordinary pvc electrical insulating tape. But this may be symptomatic of other loom wires cooking hard too.
I would follow Doug's advice and check all the non HT wires in the system. (ie the 12 volt primary side wires.
Also, there is stuff called self amalgamating tape which you could use, the silicone variety of which is heat resistant.

 

Hi Someday

Once you've got the Shrink Wrap on maybe you could find someone who has got a heat gun to give it a quick blast or heat

Or dare I suggest that you borrow your Wife's Hair dryer! (although I haven't tried it that might work)

 

I do investigate the visible wiring on a semi-regular basis, but hadn't spotted this fraying. But I only noticed the symptoms this month, so maybe it's new and I will definitely check the other wiring carefully. I'm not entirely sure which wires I should check, so will check all I can see....

 

It probably would, from what I've heard, but that still runs into the problem of how to power it....

 

Hi Someday

Don't try this at Home! but I have even done the Shrink Wrap using a Cigarette Lighter

Fire Extinguisher handy just in case!

Also if you borrow her Hair Dryer without asking her permission, keep all sharp objects hidden away and Sleep with one eye open!

Ask me how I know!

 

Well, if the owner and car were not in Japan, but, stateside, I'd suggest Harbor Freight for the heat gun. I have one. It is great.


There are a couple of hair blow dry guns around. will work, nut just a tad short on heat.


When I did a ton of splice and older in converting my I6 to V8, I used book matches. OK.


I have a backup. A piezo ignition butane lighter for the fire place or BBQ.


And, there are mini torches. My son fancies those and says my stuff is antique and outdated...


Carl

 

That tach is a great tattle tale. Ignition is off a bit.


And, oh yeah, that bare wire is "the smoking gun" A wiggle and the wire goes to ground and a momentary misfire takes place.


As Doug says, start the examination the primary power source. The ignition switch, yes the one that accepts the key to turn and fire the engine. That wire should go to the + post on the coil.


But, surely checking the wire from the + post to the distributor is also a good idea.


Carl

 

Hi Someday

You need some of these Shrink Wrap Tubes



Assorted Shrink Wrap Tubing



Lots of different sizes to fit almost any Wire



Slide the Tube over the Wire



Carefully Heat the Shrink Wrap Tube



The Shrink Wrap Tube holds really tight and almost any source of Heat will do but keep it away from anything inflammable

That Tube could have done with a bit more heat but I was also holding the Camera and taking the Photo which made it a bit 'Awks'

 

I think I'll try Greg's pro tem suggestion of PVC insulating tape to start with, and see if that fixes the issue. If so, I can look into more long-term solutions. If not, then the issue is presumably elsewhere. I haven't noticed any + or - markings on the coils, but will take a closer look, and check the other wires as much as I can.

 

Well, I wrapped a but of insulating tape around the bare wire, and checked all the other wiring I could see, finding nothing obviously out of the norm, and this evening we took a drive down to Costco. Revs stayed steady at 700, or a hair under, and while there were the usual citations [vibrations! Bloody tablet...] which I still want to eliminate, the twitches and jolts were gone. So thanks to Doug and Greg for where to look and how to fix it, and everyone else for their advice.

 

SDSD
Well done. FWIW I would not be surprised if there is not a fracture/partial fracture in the copper part of the wire, and wrapping it up held the broken ends together better. It could have been shorting to the coil outer also, as that is very close and with bonnet shut it might have been pressing against it. Either way, good stuff.

 

Good work.

When working on the car for any reason always keep an eye peeled for similar wiring faults...as well as dirty, loose connectors and the like. I can all but promise they're lurking. Fix 'em as you find 'em!

Eradicate them, stomp them out, drive them back from the ramparts

And *whenever* you come across a ground point, clean it. Don't mull it over. Just do it

Cheers
DD

 

Well, pretty much all the connectors are dirty, but as far as I can tell, that's the external plastic housing being mucky, and the connections themselves are well protected. Or I could be deluding myself. I don't want to start pulling on connectors without know exactly how to open them, however. Some of these are very firmly connected. I jiggled lots of wires and connectors, and didn't find any loose ones.

I did spot a couple of ground points, like on top of the radiator housing (at least I'm pretty sure that's what they are: several wires terminating in a nut). While pretty clean, it certainly wouldn't hurt to undo the nuts and give the metal a good scrub. I've got plenty of anhydrous ethanol, which should be okay. Also might get a toothbrush and use it on those mucky connector housings. I have a car show coming up, and it would be nice to have the engine bay looking somewhat clean....

 

If the wires are black, they are grounds. I usually use a bit of fine steel wool to clean any corrosion off joints like that. A bit of dielectric grease on the now clean surfaces upon reassembly will keep any moisture out.