Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
81
You forgot the K’s. The resistances are all in the thousands.
Yeah, that’s because I never managed to nail down which was which. It’s wholly unimportant anyway, the only reason the resistances are there is to limit current in case something gets shorted out somewhere. Both the tach and the EFI are looking for a signal, not current. You could swap the 6K and 10K resistors and it wouldn’t make any difference. Or you could install two 6K’s or two 10K’s. I would have loved to sat in on the meeting at Lucas where they decided on two different resistors, probably involving a heated argument.
There are no such ‘protective’ resistors on the xk - and you said previously that the resistors were there to convert the xk AB14 to V12 configuration…??
Like you; I suppose that the ECUs and tachos looks for a signal - ie frequency rather than current. However, the V12 frequency is inevitably twice that of the xk set-up. The only thing the module does is to make and break coil’s ground connection - to spark the plugs. The ECU and tacho gets exactly the same make and break - like coil neg; connections are either grounded or has 12V…
I too would have liked to have sat in at meetings when they decided to use the AB14 for both engines. Did they modify the AB14 or the ECU/tacho or both - and how? On both engines the injectors fire once every engine revolution, both triggered by the same module with a single connection to the ECU…raising questions…
That said; you are absolutely right with the ‘Kohm’ of the resistors - but they could still be tested as I outlined. Whatever their function; they should be tested to ensure they play no part in Rasmus’ problems…
Frank
xj6 85 Sov Europe (UK/NZ)
**
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
83
I am confused. The AB14 is the amp used for the Lucas HEI ignition, which was introduced around 1980 or 1981. Prior to that, the V12 used the Lucas Opus ignition which used a different amp. So, what “xk” are we talking about?
Series III XJ6 with the 4.2 straight six “XK” engine
Cheers
DD
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
85
OK, gotcha. I am a bit perplexed why Jaguar didn’t provide the resistors on the 6-banger as well, and for the same reason. I presume the EFI system still needs a trigger signal, and I presume the tach is electronic as well. Traditionally, a tach signal comes from the - terminal on the coil, and a couple of owners who had problems with either the resistors in the AB14 or the resistors in the ballast resistor pack used with the Opus simply moved the tach wire directly to the - terminal and the tach worked fine. Could add a resistor in the line for protection against shorts if you wanted to.
And yes, the tach is different for a V12 than for a six, as using a six-cyl tach would result in readings twice the actual RPM. Until you get to the Marelli ignition V12’s, of course, as the Marelli is really two six-cyl ignition systems and the tach gets a signal from only one, so those cars use a six-cyl tach.
Rasmus is using the AB14 on his V12, ‘like’ the one used on the xk 6-cylinder engine, Kirbert - not the earlier OPUS. Which indeed was a more complicated set-up. His problem is uneven idle and a misbehaving tacho - so there is a need to sort out if his AB14 layout is a possible culprit, and why - or if his problems has other causes…
Indeed, the 6-cylinder layout was simple; two wires to the coil and the ECU and tacho directly connected to the coil ‘-’ - no resistors. Which are added to the AB14 in series with the separate wires to the ECU and tacho, for obscure reasons. Certainly, if the V12’s tacho and ECU are modified for the higher frequency of the V12’s pick-up signal to the AB14, there should be no problems with engine and tacho - and the problem is elsewhere…
Frank
xj6 85 Sov Europe (UK/NZ)
**
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
87
If the tach is misbehaving, it pretty much has to be the AB14, the tach itself, or the wiring in between. The tach could be disconnected from the AB14 and connected directly to the - terminal on the coil as a test; if that makes the tach better, the resistor in the AB14 must be bad. But that wouldn’t address the uneven idle. Sounds like a flaky GM HEI module. It’s cheap enough, might wanna just swap in a different one and see what happens.
I still haven’t really had time to go and check on some issues, but I’ll try to clarify some of the problems I’m having.
I’ll try to clean the throttle bodies once again and readjust them, I’ve also gone ahead and ordered the teflon bushings which have been discussed in other threads, I’ll also have to double check the crossover pipe and check that it is properly seated. I also have a problem with an exhaust leak at the B bank manifold, and somewhere down the pipe aswell, so after those are checked it’ll be easier to hear any vacuum leaks, now there is some hissing almost all the time near the B bank due to the leak.
The roughness is also somewhat of a strange issue, Frank raised the issue of idle advance, I’ll have to check that, I’m also afraid that it might be down to bad compression on a cylinder on the B-bank, no real facts to back that up other than a strange gut-feeling. When adjusting the spark plug gap I used a standard feeler gauge, which I thought was a little too rudimentary, so I went ahead and ordered a proper spark plug gapping ‘tool’ which will allow me to be certain that the gap is correct, so if the problem persists when disconnecting tach and checking throttle bodies I’ll go ahead and do a compression test, and recheck the spark plugs.
Sorry, thought I had already clarified.
I suspect this is what the PO did, I have a vague memory of checking continuity between the bullet connector from the amp, which is disconnected, and the wire to the resistor pack from the GM amp, which didn’t show continuity, probably indicating that the resistor has blown and that the PO simply moved the tach pickup to coil -. I’ll have to double check.
This could have something to do with the misbehaving tach, if the resistor has blown due to an over-voltage problem or other issues and the tach wire simply moved to coil -, that would now mean that the tach had no protection and therefore might have been fried.
I’ve tried five different amps, the first being the one on the car, which doesn’t show correct RPM’s, there is a video earlier in the thread where the RPM’s go to 2500, to drop to 2000, to then climb upwards, so there is no correlation between engine RPM and tach readout. I then tried two different ‘generic amps’ which worked fine, except for getting too hot and resulting in ingition failure and the car just dying afterwhich I tried an AC-Delco unit, which is the one on the car now, the delco doesn’t overheat, but has the same issue as the OEM, the tach RPM’s ‘turn’ at about 5000, and then continue downward. I then tried a HELLA amp, which also seems to have problems, I haven’t tried whether it overheats yet.
Somewhere along the amp changes a new issue appeared, the tach needle now bounces up and down at different rates/vibrates, as if the tach is getting the wrong impulses or too high impulses; I’ve tried redoing all the connections on both the coil and inside the amp, aswell as redoing the pickup connectors, all amps now have the issue of bouncing.
As stated earlier the engine also seems to have a slight misfire or other issue which makes the engine ‘shake’ or stutter, it is unclear what that issue is, and I’m afraid I might need an oscilloscope to see where the issue is, if it is fueling or ingnition related.
Seems to have been done by the PO, as stated earlier.
Just to clarify, I have the v12 AB14 installed, which is correctly wired according to the diagrams supplied by Doug and myself, the only difference being that the tach has been moved from the connector with resistor, to the coil-, which should’t really cause any issues.
The severity and nature of engine roughness can be hard to describe via the written word.
Is it severe enough that a passenger would notice? Is the engine visibly shaking at idle?
Back in the day, when these cars were new, road test editors commented of an occasion tremble at idle. Both my V12s idle smoothly but neither can be described as absolutely like silk at all times. Imperfection minor enough that it would probably escape notice by anyone not sensitive to such things and/or attuned to the car.
Listening to the exhaust at the tailpipes can be useful. If you have a putt-putt sound then your misfire fault can and should be traced down. And it shouldn’t be too hard (famous last words) to do so.
Have we talked about dirty injectors yet in this thread?
I’d say so, atleast I would notice. The engine is sometimes shaking, not always when the idle is rough. I’ve tried to capture it on video, but since my camera has auto stabilization you can’t see anything. I’d like to say that the severity of the shaking is about the same as you’d expect from a misfire, usually it goes away as soon as you press the accelerator even a bit, but once or twice it has continued shaking/misfiring when accelerating until I get it up to about 2000rpm, after which it stops. I’ll try to see if I can get the shaking on video. But you can definitely see it when looking at the engine, and feel it in the cabin.
I can’t really say I do sadly, maybe occasionally, but not really, when holding up a towel or similar behind the exhaust I can’t really see any abnormalities.
I am also unsure that the engine is running on all 12 when at idle, it could well run on 11 or 10, which could cause the shaking/uneven idle as one bank is struggling with the other.
Not yet I don’t think, considering the car stood for almost 10 years, with only the occasional start, this could of course be a contributing factor, I have used several bottles of injector cleaners etc. but never really noticed a difference. I know there is a kit which you can purchase with the filters and all included, but it is quite expensive - but definitely an option.
FWIW, every oldie car I’ve owned has benefited from professional injector cleaning…done either at a shop using flushing equipment or removing the injectors for professional servicing. In every case the idle quality was improved.
Would there be any huge benefit to professionally cleaning the injectors? Looks like professional cleaning would cost about 300 euro, whereas a kit with hoses etc. would be about 100 euro. I’d probably have to buy the hose kit either way so cleaning would be four times more expensive than just replacing filters and seals.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
93
From what I hear, yes! Supposedly a reputable shop will clean the injectors and then flow check each one, and then swap some injectors with ones from their stash to give you a matched set that all flow about the same. That last step would be worth the price IMHO.
It’ quite possible that those two facts are related Rasmus.
If for example you have a leak in the intake of one cylinder, that cylinder would run very lean, probably misfiring, and the other 5 of the bank rich as the ECU will detect the lean mixture and add more fuel.
I would first sort out the leaks before anything else.
In principle, Aristides; valve problems should show up on the vacuum gauge. However; the special problem of the V12 is that engine management inputs, including vacuum, are taken from one bank…
So unless the relevant data is checked for both banks; one is in the dark. Like the coolant temp is read from one bank - while the two banks’ cooling is 'semiseparate…
Having the injectors professionally cleaned, on suspicion, is of course throwing money at a problem, Rasmus - which is not necessarily wasted money…
However, this entails removing the injectors - which gives the occasion to test their function before sending them away.
But I still ‘insist’ you test the compression for evenness on all cylinders. It’s a lot of work on a V12, but at the very worst; it will reveal anomalies that cannot be remedied by external actions. And at the very best; it will reveal an engine in good order - making external work worth while!
As an aside; we suggested disconnecting the tacho completely - to eliminate any influence a defective tacho may have on the engine’s running. And the resistors inside the AB14 is easily tested - as suggested. Incidentally; the resistors are in series with the tacho and ECU - and can be connected anywhere along the respective wires…
**
There is an anomaly here…somewhere, Kirbert. The resistors restricts the current flow to miiliamps - either necessary for ECU operation or safety; directly connected to coil neg, there is no such restrictions. However, depending on ECU construction; it may not use current flow, but ‘no current’ voltage - which is independent of resistance.
Since the 6 operates directly from coil ‘-’ the two ECUs may be identical in this respec…?
The primary mixture control comes from the pressure sensor in the ECU…which takes the vacuum reading from the balance tube that joins the the two inlet manifolds together.
If the vacuum tube functions as it should, Doug - and what if there is a substantial difference between the two manifold vacuums. I still think the vacuum should be checked on each bank separately - for any valve/gasket fault? And he still hasn’t reported on ECU vacuum…?
Yes, the vacuum should be checked on general principles if nothing else…although doing so sometimes creates cause-versus-effect quandaries
I’m not at all sure how low vacuum would have to drop before the mixture became so overly rich that it would cause an idle quality problem.
The engine might well enjoy a slightly richer mixture at idle; they often do.
Heavy exhaust fumes and/or sooty spark plugs would certainly be indicators of detrimental over-rich mixture. An exhaust analyzer would be ideal…but probably not in the cards in this case.
