Both were cleaned and a new throttle bushing was installed on the one side where it was bad. They were first adjusted to 0.05mm independently, then I measured the set screw as described in a post I read on the forum which resulted in me adjusting one side very slightly, however it was still at 0.05mm when measured. Both linkage arms are slightly loose, as should be, and actuate the throttle at the same time.
Will do! I have read quite a bit about bad vacuum signals to ecu but haven’t gone ahead and checked mine.
Where is this switch located? I haven’t ready anything about it prior.
Not sure, the PO has done some exhaust work, and when under the car it seems like something might be missing, or it might just be a half-ass exhaust leak fix. It’s a -85 v12 HE from the UK, so maybe one of you regulars know; it doesn’t have cats.
The electric switch (when fitted) is on the throttle capstan, RH side. It actuates at about 75% throttle opening.
The vacuum switch (when fitted) is a blue/white device with two wires attached to the rear of the RH intake manifold. It actuates at about 3" of vacuum as I recall.
On cars with both, they’re wired in series.
The purpose is to richen the mixture about 10% under heavy load and/or large throttle opening.
Crucially, I can’t remember at the moment if these switches are normally open or normally closed. I’ll try to narrow that down!
Normally open. They close to enrichen the mixture.
Testing the vacuum switch must be done with the engine running. If the engine is not running then the vacuum will obviously be zero and the switch will be closed !
Is it the trigger with the green wires? I was told this was a kickdown switch, but given your answer I’ll check whether there is any difference unplugged.
I don’t have any good pictures to look at the rear manifold to see if there is any vacuum switch, I suspect it might be the vacuum switch that has a T from the vacuum feed to the ECU? I think it might be the switch seen in the ‘rear’ of this picture with a vacuum to it and two wires from the top of it. If so I’ll test with that unplugged aswell!
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Even on a 6 cylinder; 100 - 200 rpms cyclic variations in idle would raise eyebrows, Rasmus, implying cylinder imballance - fuel or compression. Doug’s advice on checking vacuum at the ECU is urgent - it should read manifold vacuum. On a V12 the two banks should work in perfect harmony; if one bank is is lagging/misfiring it’s pulled by the other, and unevenness in one or both banks may cause/enhance cyclic rpm variations.
What are the rpms during the ‘shaking’ state? With a cold engine the idle should be 300 -400 rpms higher than your hot set idle - which is what? Shaking at low rpms implies too much advance - what is your idle advance?
All engines will run perfectly at high rpms - even with idle issues caused by gross errors.
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As Doug says; these are issues to be revisited - you seemed to have ‘done something’ there, and the advance is somewhat suspicious. Point is that the distributor is follows the engine, not the other way around - but distributor functions and settings sure influences the engine’s behaviour…
in your position I’d try to find an old-school engine tester allowing you to visualize spark and ideally put the 12 sparks over each other to identify issues.
Such testers also allow to disable individual cylinders and thereby identify compression or mixture problems.
A twelve is certainly harder to manage than a six, but should be doable with the right tools and - sometimes - the right person helping you to make full use of them.
Now start with mechanical engine issues such as compression and make sure there is no vacuum leak. Then comes ignition and spark and finally fuel and injection.
Keep cool and do just one job after the other - eventually you’ll find one - or several - causes for malfunction
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You have a very valid point, Doug; the modules are identical but their connections and connectors differs, as is seen in the picture. Ie, having the original connectors and wires, V12 or 6, any sourced AB14 could be used…? It seems that the AB14’s bodies are identical - except for some difference in the wire grommet?
Some thoughts; the prime purpose of the module is to make and break the coil negative. On the V12 it does that 6 times per engine revolution - on the xk it does so 3 times. However, the ECU triggers the injector only once every revolution - both on the 6 and the V12. On the 6, the ECU fires the injectors at every third pulse, and while the V12 ECU may count every 6 pulse - it leaves the tacho in limbo. Directly connected to the coil ‘-’ it would read twice the revolutions - unless the tacho is different between the 6 and 12…
Which arguably it is not. So the special connector shown in your V12 AB14 set-up somehow half the number of pulses to the ECU and tacho…?
Which would explain the strange behaviour of Rasmus’s tacho. So the V12 does need the special connector, and 4 wires to work - whether or not one or two coils is used. So the question is; does he have this connector? In my take; if he has not - he may have twice the fueling, explaining the uneven idle. And twice the tacho reading, explaining the odd tacho behavior - which may have repercussions for ignition timing set, with its own problems.
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Plain piggyback does the same as connecting the ECU and tacho wires to the coil neg, Doug…
The problem is that the module delivers 6 ignition pulses per revolution for the V12, and 3 for the xk. Which is OK if the respective ECUs and tachos are designed for that - counting revolutions correctly i both cases; 6 triggers for the V12 and 3 for the xk. I don’t know…
If specific, the same connections on the coil can be used for both engines…
The RPM’s aren’t necessarily steady when the engine is stuttering or shaking, the engine seems like it’s almost dying then getting more fuel and so forth, so the engine is hunting idle when warm. I read something about Mercedes using a similar fuel injected systems as the pre-he v12, and often times it was related to an issue where the engine would surge (high idle) and then the fuel would be cut off, where by the engine RPM’s would fall and the ECU would inject more fuel and so forth. So IMO this suggest some type of ECU problem, could be related to not receiving vacuum, I haven’t had time to check whether the ECU gets vacuum or not. I’ll have to check my idle advance, if I remember it was something like 5-8* BTDC, which I realise now is way too much, I’ll have to double check, but I suspect the springs are stretched; I’ll have to check if new ones are available if stretched, but five minutes of googling wasn’t really promising.
Warm idle is about 750, cold 1500, can’t quite remember.
I don’t think so, the v12 tach must be different than the i6, the difference between the i6 and v12 ab14 seems to be the resistor block, I remember reading that the purpose of the resistors is reducing noice. I suspect that the resistor-block has malfunctioned and that the PO has changed the tach feed from resistor block feed to coil feed, I’ll have to double check if the resistance is correct, but I have a vague memory of checking the resistor pack and there being no continuity between the oe tach feed from amp and the amp negative feed, I’ll double check. Seems to be a normal 10k resistor, Doug and I have both referenced schematics above.
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This is the overrun fuel cut-off, Rasmus; cutting fueling at 1400 rpms with the throttle closed - resuming as rpms drops below 1400 rpms. This is fitted to ‘European’ xk engines - I’m not sure it is fitted to the V12. This fault is very characteristic, caused by a maladjusted microswitch - not similar to your symptoms.
No specs are given for V12 HE idle - one pre-HE mentioned is 5 deg btdc, but the HE may need less? Set at the spec 18 deg at 3000 rpms - the idle advance is sort of academic…
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The purpose of the injector resistors is to protect the ECU in case the an injector shorts out, they serve no other purpose. However, on the xk engine the coil ‘-’ connection to the ECU is physically routed via the resistor pack as a mere interconnecting point - no relation to the resistor packs function. While irrelevant for your question; it illustrates that physical wiring cannot be read from wiring diagrams…
In principal it should be continuity between the white/slate blue at the coil/ignition amp and the tacho connection. There is indeed a 6 - 8 Kohm resistor indicated the diagram in the wire connection from the coil/amp to the ECU - and there may be a similar one for the tacho. Possibly related to the difference between the xk and the V12…? Ie, that such resistors are required for the V12 tacho and ECU to counter the higher pick-up trigger frequency. And the special connector for the V12 ‘4-wire’ contain such resistors…??
It is bewildering; both engines are using the AB14
…with the same module and triggered by the same pick-up; there is nothing the manual describing how this higher ‘pulse rate’ is delt with for the V12 ECU and tacho. Whether directly converted internally to this pulse rate - orby external means…
However; it is suspected that the tacho, if faulty, may interfere with coil operation. Hence the simple check of disconnect it - just for a test…
What often happens is the AAV becomes stuck in the open position allowing the idle speed to go very high…like 1500 rpm. The ECU, seeing this high RPM with the throttle closed, cuts the fuel. The idle RPM naturally drops.
When the ECU sees the lower idle RPM, it turns the fuel back on…and the RPM then swings back up to 1500 or so. Then the fuel cut-off once again kicks in. RPM drops.
Repeat, repeat.
I don’t recall that this is happening with your engine.
A much smaller amount of idle speed hunting (100-200 RPM, lets say) can be caused by uneven throttle blade gaps and/or dirty throttle bodies. I call this a “rolling idle” or “oscillating idle”. Up/down, up/down…like a roller coaster ride.
None of what I’ve just mentioned would be called shaking or stuttering. There’s no roughness involved. Just up/down engine RPM
Cheers
DD
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
79
You could easily convert the 6 cyl AB14 into the V12 type. The only thing in the white blob is a pair of resistors, 6K and 10K IIRC.
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In which case he should have 4 wires coming out of the AB14, Doug - and resistor state can be easily tested, according to your diagram in an earlier post…
Ohming EFI wire to coil ‘-’ wire should then read 6 (or 10) ohms, tacho to coil ‘-’ wire read 10 (or 6) ohms - and EFI to tacho wire; the sum - 16 ohms. All according to Kirbert’s input - though he doesn’t state which is which - hence the (…). It may also be some tolerances - but the test will verify that the resistors are OK…
And I wish he had told us earlier…
Without the resistors the tacho and EFI will read xk ignition and rpms data - doubling fuel and rpms for the V12…
So simple a conversion - to be verified; and if OK Rasmus problems can be addressed properly…
