The Ballast Resistor in an OPUS V12 Series 1 - hot and unreliable

XJ
1

Good morning Team,
I have an enduring problem with my ballast resistor on a Series 1 V12 OPUS set up. In short the resistor network gets really hot: I’m talking so hot you can’t touch it.
This appears to make the ignition system unreliable, causing spurious, and intermittent stalls, and difficulties in starting when warm. Another symptom is that the Tachometer fails to register. I have repaced the unit many times, and it seems to work well for a few months, until the heat problem would appear to degrade the integrity of the netwrok of resistors.
My query is:
a. Do any other V12 owners have this issue;
b. Is the ballast resistor supposed to be that hot;
c. Is there a work around - 12v Coil set up/compatability with OPUS system;

Many thanks for your interest and assistance.
Kind regards,
Chris

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3

Forgot to mention I have relocated the ballast resistor away from the factory position adjacent to the coil/throttle post to the LHS wing.

4

To quote the manual, Chris;

“The length of the triple-core extruded type cable must not be altered and the cables must not be separated or replaced by loose individual cables”

The manual does not describe the consequences of breaching this caveat - nor explain ‘why’. So whether your problems is related to the repositioning or has other causes is unknown - unless someone has further inputs?

You have also focused on, and replaced, the resistor unit, which do carry coil current and will be hot - how hot is arguable. But equally; the influence of the cable set-up may be the cause of ignition problems - and the resistor heat-up may be a side effect, enhancing an existing problem? While the pick-up module in the distributor is fairly reliable - it cannot be excluded as a fault…

The obvious solution is to restore the original set-up, though you may have had experience with that - inducing the modification?

Frank
xj6 85 Sov Europe (UK/NZ)

5

The ballast resistor is supposed to get hot, but this is supposed to stay tied to its relationship with the coil. The primary reason (or good reason why) it’d get hot is that the ignition is consuming a lot of current if running at higher revs. Relatively speaking, the ballast would be cooler if the revs were higher. Inside, the ballast resistors in the path to the coil change resistance inversely proportional to temperature and that is dictated by the amount of current passing through it going into the coil. By changing its resistance, it thus limits the current going into the coil at low revs and boosts it at high revs. This stops coil overheat at low rpm and provides for a more rapid recharge of the coil at high rpm, when there is less time to do the charging. (That’s why it takes its name from the balancing action of altering where to load ballast on ships to make them better balanced in the water.)

So the ballast is expected to warm up - it is regulating the current to the coil as it is in series with it.

The OPUS fires the coil thanks to registering and processing voltage induced magnetic pulses coming into it from the pickup in the distributor. There is a relationship between the current, voltage and inductance in the wiring which Jaguar obviously didn’t want interfered with regard to the pickup of the signal into the OPUS, but so long as any connections are good and the wiring has minimal resistance, it is unlikely that the components are going to be moved out of the operating window that gets a good clean signal into the OPUS unit. That must be true as the wiring was altered quite substantially when the OPUS units were moved from the centre of the “V” to the picture frame during the production run. What they didn’t want was low voltage cables contacting high voltage spark plug cables, where the magnetic pulses generated by the spark plug discharges may put false signals onto the input, so that’d be why the advice was not to alter the cables.

If the ballast resistor has degenerated internally and now is out of spec then relocating it or replacing with off the shelf resistors won’t work. The resistors inside are made (I think) from iron, which has a reverse temperature coefficient to that of what normal resistors are made of, so will not compensate by changing its resistance for the change in current draw of the ignition coil.

The ballast and coil characteristics need to be matched to each other to correctly charge the coil.

It’s not obvious from your description what causes your misfire. A typical diagnostic is to carry a bag of frozen peas and cool down either the OPUS, coil or ballast when trying to diagnose whether heat is the cause of loss of performance or failure in one of the ignition components. Another way is to carry spare units and swap them out quickly when problems arise, to see what difference that makes.

kind regards
Marek

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6

I am a bit out of my element here. but, I offer these thoughts.

  1. the resistance can e measured in ohms. Compare the ione in question to a known good resstor? Or find the value in the manual.

  2. I am wrestling with the idea that higher rpm’s inclrease current demmand.

  3. Time does heat up a resostor. As a poor connection will heat up over time.

  4. Point and condensor igntion inluded a resistor. to reduce volts to the points.

  5. Way back. My emploer bought comany cars. Base mdels. no radios. several of us adaped older radios. 6v radio in a 12 v car! No, not feasible some of us used resistors to drop the 12 to 6. they got hot. I not pleased. I used a center tap on the battery to get the 6.

  6. Resostors merely convert volts toheat.

  7. How hot is hot, other than to hot to touch!!

  8. toohot to touch sous too hot to me.somethong is wrog.

Carlhw

7

1/ It is very hard to reliably measure resistances lower than ~5ohms with a meter, as the test leads significantly alter the result. What you can do quite easily is measure the voltage drop across the resistor. Also measure the current flowing with an inline ammeter at the same time. Do some maths to get an answer.

2/ Higher rpm do not increase current demand. The ballast resistance alters with temperature. The temperature is related to the average current flow over time.

At high rpm, the coil risks not being fully charged as there may not be enough time between ignition events. If so, the average cumulative amount of current falls, the coil runs cooler AND the ballast resistor resistance drops. That means the coil now gets a higher charging current at high rpm to compensate for the fact that it wasn’t getting fully charged, so it’s a self adjusting system which tries to keep the amount of charge put into the coil constant whether the rpm was high or low. (Conversely, the low rpm, the coil gets fully charged, that makes the ballast get hotter, its resistance goes up and that means the coil gets less charging current.)

Essentially, it lessens the effect of variation in dwell time on how much charge is put into the coil. It’s a neat self adjusting system.

3/ Avoid making poor connections…

4/ We don’t have points on this car.

5/ 6/ 7/

8/ 'Allo 'allo. Somethong may indeed be wrog if the ballast ends up at a temperature it wasn’t designed to be, Rene, mon brave. If it gets very hot for the wrong reason, the coil gets less charging current than it wants.

kind regards
Marek

8

The idea of using heat variable resistance for regulating coil charge is foreign to me, Marek…

It’s too slow reacting to have anything but negative effects. Generally, resistors are measure/rated at 20C and the variations with temp is unwanted - heat is wasted energy. Basically, heat sinks and oil (coil) is used to remove heat…

Principally, we want as much energy delivered to sparks as possible - any restrictions are basically unwanted. The limiting factors are coil impedance (during coil charging) and indeed resistance - external (or built-in) resistors sometimes added for general current reduction.

While the dwell angle may be constant (unless varied by the amp as in the CE system) the dwell time varies with rpms - but we still want adequate spark energy at all times. But such are electrics; at full song the V12 must deliver some 600 sparks a second and can do so…

Certainly, resistors and coil gets hot, but heat variations are far to slow to be used for use as a regulator compared to engine reaction to gas pedal movement. Equally, either component may fail due to overheating, local or general, as may any electric/electronic device. Which basically means that something is wrong - otherwise such events would be very frequent.

Incidentally, all temp variations must be assessed with reference to degrees Kelvin, of course.

Frank
xj6 85 Sov Europe (UK/NZ)

9

Gotta love us old pharte diagnosticians!

I used bags of frozen corn…:joy:

10

I disagree with almost all of this… and here’s why.

It’s been around since at least 1965, at least in America it is (Ford Barracuda)

Do not confuse the surface temperature of a large heatsink with the thing causing it to heat up. The surface temperature due to dissipation of heat by the heatsink is a time averaged out effect but the thing generating the heat might be very rapid up and down spikes in heating put into the little resistor embedded in the middle of the package by the fluctuations of current. You haven’t observed the current being drawn, let alone measured it -you are only commenting on the resulting surface temperature of something that the resistor was bolted to.

The variation is wanted - the clue was in the title - it’s a ballasted resistor, not just a resistor. The material chosen for the resistor is deliberately chosen so because it has a high positive temperature coefficient and that’s what gives it the ballast property.

I’d want to deliver as little energy as possible subject to it being enough to light off the mixture. I’d not want to generate excess heat for no good reason.

I agree !! (See /8 above)

Really?

One way or another the system is drawing too much current and heating up too much and it isn’t obvious if there is an unwanted partial short (path to ground), a faulty coil, a faulty ballast or poor wiring.

kind regards
Marek

1 Like
11

Well, this is disappointing. Based on the title of the thread I thought it was going to be about my ex girlfriend.

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12

I bet she hated having to shift her weight whenever you filled up with petrol

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13

[quote=“MarekH, post:10, topic:430245, full:true”]
I disagree with almost all of this… and here’s why.

We all have our opinions, Marek…

My opinion is that temp variations due to heat is unsuitable. My opinion is that we want as much spark energy as possible - voltage bridge the plug gaps, energy ignites the fuel. Restrictions are a result of components having resistances, which limits current flow and cause heating - which must be controlled, not the other wy around.

The heating of primary and secondary coil wires is not instantaneous, nor does frozen peas applied externally cool the coil wires instantaneously - it takes time. I don’t share the idea that ignition will not work properly without the use of heat variable resistance - though of course, resistances are unavoidable…

In principle; the influence of heat on resistance variations are easily done as you outlined in a previous post;

Connect coil neg to ground via an ammeter and turn ign ‘on’ - read current. Initially current should be as defined by coil (and eventual external resistor) resistance - then the influence of heat is shown by drop in current. And, additionally, overall hot resistance can be easily as a back-up. Of course, the coil will quickly heat up - so circumspection is called for.

The same test can of course be carried out on any resistor - the ‘ballast’ resistor (like ballast in a boat) is called that because it generally lowers current flow, not because its resistance value varies. Essentially, digital ohmmeters can easily detect resistance within 0,1 ohms - which is why injector coil resistance can be checked to the correct 2,4 ohms.

Current is only flowing through the coil while the coil neg is grounded (points or amp) - as soon as ground is broken the coil ‘rests’ (cools/not heats) though spark energy is released through the secondary winding. So main heating occurs during coil grounding - which varies with rpms, but increases in frequency as rpms increase.

Ideally, the coil should only be grounded for the time required to fully charge the coil - which, crudely, is constant as per a constant dwell angle. In idle the grounding is longer than necessary, creating unnecessary heat. The Lucas CE system somewhat counters this by varying the dwell angle with rpms - but it is still fairly primitive…

Unfortunately, neither of our opinions are of much help to Chris - the system is what it is whatever our respective opinions…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

14

My “opinion” is that the original poster needs to find either:-

1/ a lower than expected resistance path to earth (or anywhere lower than “12v” that is) at one of the ballast connections or in a component after the ballast. This will cause higher current flow. That’ll come out as heat somewhere.
2/ a higher than expected resistance along the path to earth through the coil when hot. That means there will be less current flow into the coil when hot, leading to incomplete charging, leading to misfire.

Failing that, you’ll have to look at the OPUS.

There used to lots of good information about OPUS on the old ReOPUS website which you should be able to find using a wayback machine to access archived internet pages. Roger Bywater’s aj16 engineering website also explains how the ignition works. There are also a circuit diagram for OPUS in my old photo pages of the jag-lovers.org website. The car’s repair manual has the OPUS ballast resistor values and connections and troubleshooting information.

If you have confidence in your multimeter, or can hook it up to a wall bench supply that gives a constant current, then you can measure the ballast connections cold and then repeat the exercise after leaving the ballast in the oven for a while. The resistance through the ballast to the coil connection should be higher when hot. The diagram in the manual gives the ranges it ought to be - it is unclear whether that is a production tolerance or the cold versus hot variation. The ReOPUS site does note that there were different part numbers for ballasts. I presume some catered for different tachos in the top connection so I wouldn’t assume they are all completely interchangeable without measuring them first.

Measuring current through a coil isn’t simple. It is an inductive load so the current at turn on is higher than the steady state it drops to. Just because a meter resolves to 0.1ohms on the display doesn’t mean it can accurately measure 1ohm to within 0.1ohms.

The fact that the problem keeps being solved with new ballasts but recurs (and I presume the OPUS and the coil are unchanged) suggests the common factor here may not be the ballast.

The tacho not registering ought not be related to the ballast as (if the s1 v12 XJ is the same as the s3 Etype) the top connection is just a straight through link. If that is the case with this ballast, then it means the ballast can be ruled out as a cause of tacho failure by simply linking the top two connections. That is also a handy way of flushing out whether the OPUS is at fault.

kind regards
Marek

2 Likes
15

My girlfriend in California was also like that. I was posted to San Diego FLEASWTRACENPAC Point Loma learning all about the AN/SQS-23D-G, at the ripe old age of 20. With an Australian accent, not good looks, I had a fabulous time in down town San Diego, and environs. The drive in movies, in a Datsun 120Y, was a new level of contortion not to be attempted by anyone above 25. No names, no pack drill, but my companion was a very well endowed Irish/American with strawberry blonde hair. Memories!

16

To everyone who participated my sincere thanks. I never really got an answer to two of my questions re: a. The 12v Coil and an REOPUS system; or
b. Does anyone else have a very hot (too hot to place a digit on it for more than a few seconds) ballast resistor.
Notwithstanding, as a Navy Weapons and Electrical Engineer, I enjoyed the technical discussion immensley. I have replaced the ballast resistor with one of my spares, relocated the ballast resistor to the front radiator support, and installed a new battery (the previous one having been subject to a posting to Darwin, where a battery lasts aproximately one year).
The Daimler is running well, and has not emabrassed me at round abouts, or generally when driving in suburbia.

17

So we just have to wait to see if the specific problem has been permanently fixed, Chris…?

Incidentally, what about the temp of the ballast resistor now? An IR would be useful…

Frank
xj6 85 Sov Europe (UK/NZ)

Frank
xj6 85 Sov Europe (UK/NZ)

18

Well good news Team, relocation of ballast resistor to radiator support has worked a treat. Resistor heat sink is barely warm, probably due to being isolated from the engine bay, and subject to masses of cool air from the front grill. I haven’t put an IR temperature guage on the resistor but it is, as mentioned very cool. This has improved performance, and I’m assuming it will prolong service life. Car starts, immediately hot or cold, and runs smoothly, without any idle ‘misses’ or hesitation under acceleration. I must admit the new battery was probably responsible for some of these improvements.

19

Well done. Just been watching the ‘Harrys Garage’ XJ12C latest video in which he has bitten the bullet on his recently rebuilt poor running V12 and fitted new Injection system as well as previously new Ignition system.

Quoted cost for the pair I think just under $20,000. Thats in addition I think to the $20,000+ engine rebuild. Though apparently he will get 5mpg more from the engine, it’ll take a long while to get that back.

So, you know, if moving the ballast doesn’t quite fix it, then all you need is the simple application of very large sums of money.

I am reminded why I have a relatively simple straight six with carbs and a $300 new distributor and electronic ignition kit.

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Money will fix most things…