Strange Tach behaviour and engine issues after distributor service '85 HE v12

New update!

I redid the connections from the distributor to the amp, however it doesn’t seem like this has made a difference, I then checked the pickup gap, which was sligtly below 0.3mm, so right at spec.
I read that a poor ground on the amp could cause issues, so I used a crocodile clip attached between battery - and the mounting stud for the amp inside the case. This seemed to make the bouncing dissappear at idle, so I put in a more permanent ring connector connected to the intake manifold as an additional ground for the amp.

When I took the car for a test drive the problem disappeared at first, however after driving for about 5 minutes I decided to do a pull from 50-100 kmh (up to 5000rpm) and after this the problem reappeared. I headed back home and after a few minutes I looked down at the rpm gauge and the problem had again disappeared.

I have ordered a new single coil, which was described in earlier posts, and hopefully that should arrive before the weekend.

This new phenomenon if the problem disappearing and reappearing is very strange, especially since it seemed to happen after a high performance acceleration, ie. Little energy absorbed by the amp; or it could be the result of alot of energy being dumped into the amp when I let of the accelerator after taking it up to 5000rpm.

When I tried connecting the tach wire to the bullet connector on the amp the tach didn’t work, so maybe the PO had some issues with this previously and has rewired the tach.

Regarding the alternator: I believe the regulator is bad and therefore is charging too low, the battery is never depleted, it just goes from the 14v region after charging down to 12.5v. I’ll get a new alternator, but I haven’t looked enough to find one priced right and with correct fitments. I know there have been previous posts about putting on a Land rover alternator so I’ll probably end up going with that.

His mama was an American…

I see two possible issues.

  1. The insifr of the coil high tension tower is not bright and shiny.
  2. THEpodt and nut on the coil are not bright and shiny.

A vacump pump and gusager are useful tools.AKA MIYTTY Vac. CHECK THE REGULATOR WITH ONE,

cARL

It does gate confusing, doesn’t it? :slight_smile:

“AB14” is just a basic design designation that, alone, doesn’t indicate the sub-variations …nor indicate that there would even be any sub-variations. For example, if it said “AB14 Type 3” a person could reasonably assume there is also a Type 2 and Type 1.

If you were to look at a randomly collected pile of AB14 amps you’d see that the label will (usually) have other identifiers, alpha-numeric (like “4873A” or some such), which specifies the variant.

Jaguar used several part numbers for these amps, presumably due to modest design changes…but I think all of them will say “AB14” on the label.

Cheers
DD

If it hasn’t been mentioned already…

Check your engine-to-body ground and/or add a redundant ground.

Check the instrument panel grounds as well. Two wires on tabs on the I/P backing plate near the base of the speedo and tach.

Never hurts to check the “+” voltage junction posts on the firewall.

You might end up fixing the problem…including your low voltage problem !

Cheers
DD

I was about to say the same thing.

**
Did you check for distributor axle wobble, Rasmus - wobble may influence the gap, causing ign amp irregularities…

I think you are doing a good job; you may have more than one fault, or at least an obscure one. And in such a situation it is as important to find out what is working to specs as what is not - and to fix the latter as you go along. But do ensure, to the extent possible, that things are out of specs before swapping parts…

**
The load on the module relates to the the current going through the coil, which is a rather complicated with the CE system, which varies dwell angle for constant spark energy. The module itself does not absorb energy beyond that…
**

Frank
xj6 85 Sov Europe (UK/NZ)

So the new coil arrived today, I installed it but the engine seemed to run rougher, and the tach issue seemed worse. Cleaned the mounting points and reinstalled the old coil, idle seemed smoother, but the issue with tach wobble returned at idle.

The issue is now present at idle, but when I rev the engine and hold it at ex. 3000rpm it is completely smooth and the tach is completely still.
I then tried installing a HELLA amp, and the idle was worse and tach wobble increased.

I also noticed that the tach now ‘turns’ at 5000rpm, the same way that it did with the old amp which turned at 2500rpm.

I checked the pickup wobble, the axle is solid, but there is some flex/wobble on the pickup itself. It doesn’t really change the distance between the star wheel and the pickup when flexed though.

I added an extra ground wire between the pedestal and battery -, but the idle tach wobble seemed to get even worse, might just have been a coincidence though.

Next step would probably be to check the ground points inside the dash itself, any tips on doing this? Also adding a second ground and checking the + firewall points as mentioned by Doug!

Mine goes to 80C-85C, have not had the GM fail. I tried an aftermarket one, it failed within a week at those temps.

The “+” junction posts are easy. Disconnect the battery and remove the nuts on the junction posts to clean the posts and cable ends. The nuts are some arcane size but I’ve always managed to find something in my tool box to remove them.

The instrument panel grounds can be reached by removing the tach and speedo. Or, perhaps by removing the under-dash trim panel and looking upwards.

Picture the tach and speedo as clocks. One ground tab will be near the 7:00 o’clock position next to the tach and the other will be at about the 5:00 o’clock next to the speedo. These provide a ground for the metal instrument panel backing which, in turn, grounds (at some of) the instruments. I can’t remember, off hand, if the tach is grounded in this fashion…but it might be. If any doubt, run a redundant ground the the metal panel.

On my Series III the battery “-” cable attaches to the body of the car right about where the diagonal engine bay brace meets the RH inner fender. I added and extra ground cable from this point to the engine …at the air injection pump bracket as I recall although I’m having a bit of brain fade on that exact detail at the moment. I can check later.

Cheers
DD

**
There are two issues here, Rasmus; uneven idle and tacho misbehaviour - maybe related, and maybe not…

With the odd tacho readings; it is virtually useless, is deceptive - and and may be defective. You have used this tacho for setting ign advance, which may or may not give a correct outcome. I suggest you disconnect the tacho wire, white/slate blue, from coil negative, and concentrate on engine behavior, idling or driving - before dealing with the tacho itself as a separate problem. And there is also the odd chance that the tacho itself is a cause…

The ign amp is there to run the ignition system - the tacho is just a tack-on. Since the tacho doesn’t faithfully follow engine rpms anyway; it is pointless using it. Listening to, and ‘feeling’, the engine is more appropriate for pursuing this as an engine problem - doing one fault area at a time.

The ignition and coil arrangement may be unrelated to the uneven idle - caused by some other engine problem. Even with serious engine problems; engine invariably smooths out at higher rpms - though the fault(s) of course remains. If the uneven idle is unsolvable - you can then pursue the tacho problem as separate issue.

The single coil set-up is simpler, but if two coils are used, they should be of the spec type - and they should at least have the same primary wiring resistance. A previous post indicated that they were not? And the coils themselves are innocent of the uneven idle - they may fail at higher revs but ‘any’ coil should idle the V12 smooth as silk…

Frank
xj6 85 Sov Europe (UK/NZ)
**

I took the dash apart and cleaned the grounding for the instruments in the dash, I also cleaned the positive connections and all connections on the tach itself. No difference. I even added alligator clips between known good grounds and the ground connections in the dash but this didn’t change anything either.
The coils both measure 0.8 ohm, it must have been a measurement error when checking the first time.
When driving as normal the engine is very smooth and has plenty of power, regardless of whether the tach is bouncing or not, it is only at idle when the engine is lumpy and seems to correlate somewhat with the tach bounce.

I am afraid it is not really feasible for me to install a permanent second ground strap since I’ve had the car inspected with this setup with a power shutoff switch mounted between battery - and the ground point; this is mandated by my insurance company, which allows me to get insurance for 150€/year instead of over 200€/month (!!).
To try and diagnose the issue I have added temporary ground straps between several points, among others the battery - and the intake manifold, and between the ground point on the fender and the air injection bracket. This makes no difference.

I have tried using three different amps, the original GM amp has little on idle, but reverses at 2500 rpm, as described in earlier posts. The new ACDelco amp has quite a bit of bouncing at idle, and seems to reverse at 5000rpm, and a HELLA amp I’ve tried suffers very heavily from bouncing at idle, I haven’t tried to rev with the HELLA.

I have also noticed that the tach sometimes bounces on startup, between 0 and like 100kmh when cranking, this only happens sometimes.

I let the car run for a few minutes while just sitting in the car and trying to analyze the rev gauge jumps, it seems like it does so whenever the engine ‘stutters’ almost as if the spark is faster than otherwise. It doesn’t feel like a misfire, but rather like the spark is too fast, or too soon. The ‘fast-fire’ is always accompanied by a rev bounce, but not the other way around - ie. the rev bounces more than the engine ‘fast fires’.

I also replaced the fuel pump and fuel regulator, which resulted in what feels like a somewhat smoother idle. But I’d like to clarify that the idle isn’t necessarily uneven, but rather plagued with the occasional misfire which shakes the car a bit - unsure whether related to low compression, bad spark, or bad fueling.
I also managed to inflict this when I dropped the FPR on the fender…



I am sure you understand how satisfied I feel with my car purchase right now…
Nevertheless the search still continues. I’ll try to list some of the facts so far:

The tachometer is showing the wrong RPM:s depending on which ignition amp is used when accelerating. Using the original GM amp the tach is showing 2500 rpm, after which it ‘turns’ down to 2000 after which it continues to climb normally.
Using a new AcDelco amp the tach initially climbs to 5000 rpm after which it turns, unsure whether it ever turns back up.
The tachometer is also ‘bouncing’ up and down about 100-200 rpm at idle, however remains steady when engine is held at higher RPM:s. This issue is consistent with three different amps, however to different extents.
There also seems to be a slight misfire at idle, and some kind of issue which results in what feels to be the engine firing faster, this seems to be linked to the tachometer bouncing.

The issue first appeared after the distributor had been taken out of the vehicle and serviced, initially the problem was discovered when the engine was set to rev to 3000 rpm for timing, however the tach was observed not following engine RPM. I suspect the issue was already present before the distributor was taken out for servicing, however this is not verified. After replacing the amp with an aftermarket model the tach seemed to follow engine rpm, and the timing was set, during the test drive the car suffered from ignition failure, probably related to amp overheating, a new aftermarket amp was used but this also overheated. When a new AcDelco amp was installed the amp no longer seemed to suffer from overheating, however the issue of tachometer bouncing now occured. This issue now appears regardless of amp used.

So far the following parts have been checked:

  • Replacement coil DAC6093, which didn’t solve the issue, old coils measured within spec.
  • New spark plugs, gapped correctly.
  • Distributor mechanical advance greased and working.
  • Vacuum advance hose replaced, now working.
  • All connections from coil reshoed on coil end.
  • Ignition amp replaced, non GM/AcDelco amps suffer from overheating.
  • Grounding points inside dash- and in engine compartment cleaned and remounted.
  • Connector between distributor pickup and ignition amp redone with new spade connectors.
  • New fuel pump and B-bank FPR installed.
  • Spark plug wire resistance measured, all within spec.
  • Distributor pickup gap measured, within spec.
  • Engine timing set to 18* BTDC @ 3000 rpm, possibly wrong since tach is suspected showing incorrect RPM:s.

My thoughts on how to continue:

  • Take out the distributor from the car and re-service it in order to stabilize the magnetic pickup and possibly notice something ‘out of place’.
  • Remeasure spark plug wires.
  • Figure out a way to test the magnetic pickup in order to decide whether it should be replaced.
  • Possibly purchase a full AB14 ignition module.

So, does anyone know of a way to test the magnetic pickup? I suspect you’d have to use an oscilloscope, however I don’t have access to one as of writing this post.
Did the straight 6 engine have the same AB14 unit as the v12? I haven’t been able to find a scrapyard with a v12 here in Sweden, but several straight 6 cars, and I’m hoping to be able to get one cheaply from there.
The ‘fast firing’ issue being related to the tach bounce seems pretty reasonable, as far as I understand the tach is just using the same impulses as the coil, a fast impulse would lead to the tach jumping thinking the engine was at ex. 5000 rpm instead of 800 rpm, but it wouldn’t have time to get up enough to display the interpreted rpm, I’d need an oscilloscope to verify this however.

As always, any ideas of ways forward?

Rasmus,

again, without any first-hand experience or knowledge in the 12 cylinder setup, but: As Frank pointed at: what’s really wrong with your engine running or performance? It pretty much sounds like nothing. Certainly, while sitting in a car just waiting for misbehaviour in the idling, the engine will eventually do so. It would rather surprise me if it didn’t. Take the car out for serious rides over some weeks and check whether just anything seems amiss - then continue the repair-and-modify route. If not, pull out this tach and have it overhauled at around 200 EUR - or get another one. I’m confident your problems will disappear.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Rasmus,

You can add a ground strap between the engine and the chassis, that’s where the problem is usually, and leave your shutoff switch as is.
Measure the resistance between the battery (-) and engine.

Yes, you might find something.
Check the pickup cables and plug all the way to the amp.

Yes again, but not easy.
There was a recent thread, I can’t remember if it was in the XJS list or the V12 engine, about the pickup and the aftermarket replacement.

I find it very curious that you have different results with the three different ignition modules…
Maybe you are extremely unlucky and all three are faulty?
I think the AB14 module is the same with the straight six engine, if they are cheap in the scrapyard it might be worth a try.
If it works you will know where the problem is, but it if doesn’t it will not be conclusive either.

Did you ever checked the shielded wire that goes from the amp to the ECU? It’s a known cause of misery.
I would also thoroughly check al the wiring involved, you might have an intermittent short or brake somewhere.

I think that it’s either the amp either the pickup and their wiring, not much else is involved on the timing of the spark.
Nevertheless, as Frank suggested, do your tests with the tach disconnected as to eliminate any possibility of a faulty tach interfering with the ignition.

Good luck!

**
Which can be caused by dist axle wobble, Rasmus - and engine ‘stutter’ may also be caused by a ‘slow’ spark…

Remember that the axle ‘sideways’ influences pick-up gap, while ‘fore-and-aft’ changes timing. But it may be somewhat rash to ascribe all uneven idle to one single cause? But tacho reacts faster than the engine to such timing variations…

Actually, an oldfashioned way to tinker with ign timing, particularly with a vernier device available, is to use rpms and manifold vacuum. Ie, adjusting idle timing for max idle and vacuum. It’s not really a recommendable procedure; the spec timing is set for overall driving - and using vacuum is just valid in idle, of course. But timing adjustments/variations does influence idle rpms. As a matter of curiosity - what happens to idle (rpms). and timing if you disconnect the dist vacuum hose…?

Still, the tacho should not be sensitive to type of ign module used, if it is working correctly; it implies some issues with the tacho itself. Have you tried disconnecting the tacho lead?

I sort of always, until boring, advise a compression test when irregular engine idle is encountered. If compression is uneven enough, it will cause uneven idle - and external work to fully solve the problem is then futile. And, of course, if the engine itself is OK - external work is worth while…

Incidentally; the 6 cylinders’ AB14s is the same as the V12s; all AB14s seems created equal, using the same ign modules - as Doug has expressed…

‘We’ xk owners tends to accept some uneven idle as the nature of the beast - but the V12 is expected to be dead smooth…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

Guys, the AB14 as used on the 6 cylinder cars is different that as used in the V12 cars. As used on the V12 cars the AB14 includes a tach feed and an EFI trigger. On a six cylinder car these are external connections made at the coil

The first pic is V12. The the 6 cylinder for comparison.

Cheers
DD

At this point I certainly agree.

Get the engine running right and then worry about the tach. And, it might occur that fixing the engine also fixes the tach. Or might not.

Cheers
DD

Thanks again for the tips.

Battery fully charged, issue still occurs, I read on another forum that someone suggested that a bad alternator might be the cause to a similar issue. I’ll try to disconnect the alternator and run off just the battery to see if the issue disappears, I will order a new alternator, but that will have to wait a few weeks.

As a matter of curiosity - what happens to idle (rpms). and timing if you disconnect the dist vacuum hose…?

Idle is unchanged, which seems reasonable since the advance is running off the ported vacuum which is basically zero at idle, I haven’t checked the timing with the advance connected.

How should I go about diagnosing the poor idle?
The symptoms are:

  • Sometimes the idle is hunting, especially when warm, about 100-200rpm.
  • Suspected misfire on B bank, hard to tell.
  • Engine shakes quite a bit after starting during idle.
  • Runs just fine when cruising, and has plenty of performance and acceleration. No abnormalities when driving, only at idle.

As stated earlier the FPR for the B-bank and the fuel pump are changed, fuel filter was changed three months ago when I purchased the car. There has always been a poor idle when I’ve owned the car.
I got a bunch of parts from the PO who got them from his PO, there were a few spark plugs, a few spark plug leads, an A-bank FPR, and an OEM Jaguar intake temperature sensor (EAC2863).
I suspect all spark plug leads have been replaced. since all on the car seem relatively new and measure within spec.
As stated earlier I also changed all the spark plugs, serviced the advance weights and set the timing, the problems appeared after this.

I spoke to my father who’s helped me a bit with diagnosing, he mentioned that he noticed that all the contacts on one half of the distributor cap were black, whereas the other side seemed fine. I can’t remember him telling me this before, but this could be due to the cap sitting wrong on the distributor, or being warped; I’ll have to check when I continue diagnosis.

I have also changed the intake gaskets about 2 months ago, and adjusted the throttle butterfly valves and installed new throttle bushings. I have also checked the AAV and it functions properly.

Next steps I guess would be to check the fuel pressure, try to find vacuum leaks, and check the compression, also check the distributor and possibly order a new cap and rotor.

Very good suggestion

Cheers
DD

Double check your throttle bade gaps on both sides…with linkages attached. Are they the same? Throttle bodies clean?

Also, since your digging, check the the ECU in the boot is getting a strong vacuum signal from the engine. A low vacuum signal will skew the mixture rich. A rich mixture at idle might cause a rolling/hunting/oscillating idle.

Along the same lines make sure that the enrichment switch isn’t coming into play at idle. Depending on spec/market/year this might be a vacuum switch, electric switch, or both.

Does your car have oxygen sensors?

Checked, how?

A problem here can cause a hunting idle…although usually much more severe than you’ve described

Good on the fuel pressure test just on general principles. Low pressure might result in a too lean mixture. High pressure, too rich. Either could cause idle problems.

Vacuum leaks would normally result in a higher than normal idle and/or the idle speed adjustment screw being ineffective…but wouldn’t change the idle quality. That is, wouldn’t cause roughness. The FI on the HE V12 doesn’t measure incoming air. A vacuum leak is seen, essentially, as the same as opening the throttle a bit…just another source of incoming air.

Cheers
DD