Citroën CX 25 mekartips -> Tändsystem -> Kondensatorer
      uppdaterad 2006-01-30

Kondensatorerna sitter monterade en vid varje tändspole.   Alan D: Frequently neglected causes of hesitation on CX25:s are open-circuit coil suppressors (=kondensatorer). The coils cause huge amounts of under-bonnet interference if they are unsuppressed, which upsets the engine control systems.

Vad är kondensatorerna bra för?
  Harry S 2005-10-11: The capacitors that goes onto the 2 coils of a 2500 efi engine: We have discussed numerous times whether this item causes running problems or whether it is just for radio interference. How does one test one of these to see if it is worth having around as a replacement?   Bob: Find a shop that has a capacitance meter (some digital voltmeters can measure capacitance). The value should be stamped (molded) onto one side. The measured value should be within 10 - 15% of the marked value.
   As to their function, they form part of a tuned circuit. The coil and the capacitor create a load of a specific impedance value to the computer. While this impedance will change slightly with rpm (frequency), the system is designed to take this into account. While the engine should run fine without the capacitor, the resulting impedance change will cause the output of the coil to be less than optimum. You would need an automotive oscilloscope to measure the difference, but it will be there.
   The capacitor will suppress a small amount of RFI, but it will do nothing about the interference generated by the high voltage side of the coil. That's what suppressor wires are for. When non-suppressor wires are used on an EI CX, the computer has a fit and will periodically reset itself, causing the engine to either hiccup or die, depending on whether it's idling.
  Harry S: Bob, thanks for this interesting info. So now, do you know what the correct rating is for our CXs?   Bob: Adrian, I measured the capacitors on my father's '87 Safari, and one measured 220 microFarads. The other was significantly higher. They're marked 2.2 microFarads, so there's a little mystery there. We'll have to measure some others to see what we find.
  Alan D: The original suppressors are 2.2 microFarads.

  Gary D M, 2001-11-28: Does anyone know if 2.2yF is equal to 2200UF? One of the capacitors for the coil broke a wire off and I was going to get one at Radio shack. The ones they had were can type with a 2200UF 50w40VDC rating. I really don't see that it is having any effect on the running of the car, but I am in the process of trying to put everything right to see what is left. Some on the list say these affect the car other (including people at car shops here, BMW etc.) So it is a toss up but couldn't hurt to replace - if I can get one.   Shane L: wouldn't be 2.2uF would it?? ie: 2.2 micro farad. Can't seem to recall off the top of what 'y' stands for. You should have mF (uncommon), uF, pF, nF etc...
  Gary D: The 2.2yF was printed on the capcitor that is on the CX. The 2200uF was on the package in the Radio shack. The CX one is a rectangular box type and the radio shack one is barrel type. Just don't know what the y and u stand for in relationship to one another. Like mm and cm. Is it micro, pico, nano,?
  Frank (NL): Well this is an easy one, Your 2.2 yf is 2.2 farad or 2200 micro farads, humm, this could'nt be the right answer to your question so the other option is probably right because, (look only at the size of the bugger). I think they meant 2.2 micro farads witch is a normal proportion. I think it will help to tell what the use is for. Commenly a 2.2uf capacitor is the right one but it is always overrated for voltage like 40V or higher while the car can produce more than 15 volts, the 50W is the size of current it can provide so it can deliver 50W of power (shortly) without burning. 50 devided by 15V ments it must sustain between 3 and 4 amps.
  Scott R: The usual capacitor for connnecting across old fashioned mechanical contact points is in the 0.22 to 0.28 uF (that's microFarads!) range. If the purpose of the CX cap is to control RF noise from an electronic ignition system, the 2200 uF electrolytic cap will not do a very good job, but the original mylar 2.2 uF one will, because of internal inductance and series resistance.
  Gábor D J: On the original one, it is not an 'ypsilon' but a Greek 'mu', it's similar but its tail is on the left side. Alt-0181 in Windows. It means micro (10^-6). Because this character was not available on typewriters and early computers, people got used to mark it with the letter 'u' instead which looks similar, only without the tail. Consequently, unless Radio Shack doesn't know what they are doing, it should denote the same micro, and in this case, this capacitor will be 2.2 mF (millifarad, 10^-3). A factor of 1,000 between the two, that is.
  Alan D: 2.2 microfarads is 2.2 microfarads and 2200 microfarads is 2200 microfarads, so the answer is 'No, they are not the same.' I have found suitable replacements from a shop that specialisec in car audio systems. Anything near 2 to 3 microfarads will serve the purpose. Those designed for use in car-engine environments (heat and vibration) are preferred to 'ordinary' radio/TV/audio types.
  Gary D: The capacitors are on the coils for noise supression. For the radio. I really have only heard on person (on the list) who thinks the condensors (capacitors) could make the car "run" different. Everyone else says it only stops static on the radio. They are only connected to the low tension input to the coils anyway not the output, but I would like to prove or disprove common knowledge on this by replacing it and noticing a difference or not.
  Alan D: It may help to know that the purpose of the two capacitors under discussion in this thread is to prevent interference from the high-energy coil discharge interfering with the rest of the engine electronics, and of course, radio reception too. It IS possible for a broken coil suppressor to cause mayhem with the ignition timing (especially on GTi-T2s for some reason) but is more likely to cause just a 'flat-spot' or a bit of hesitation at some settings of engine speed and throttle. A well set up 25 GTi is very smooth at all engine speeds and throttle states - well, mine are, are yours? Old hands will know that one of my favourite hints is to separate the ingition-sensor wires from the other cables and lay them so that they do not pick up electrical 'noise' from the coil and injector wiring - we are talking mainly about 25GTi and GtiTurbo.
  Frank (NL): The purpose of the mentioned sensors is as follows: The initial ignition timing is determined by the synchronization sensors, which are mounted close to the flywheel. one sensor responds to the passage of a slug fixed to the flywheel at the static or idle firing point for cylinders 1 and 4. The second sensor detects engine speed by the speed of the starter ring gear teeth passing it: this information is used by the computer to determine speed-related (centrifugal) advance. On 1987 models year vehicles there is only one dual function sensor, which picks up from a datum tooth on the starter ring. The vacuum sensor enables the computer to read inlet manifold vacuum and make an appropriate correction to the ignition timing. On turbo models, a crankcase-mounted knock sensor detects the onset on pre-ignition (pinking) and causes the ignition timing to be retarded until the pre-ignition stops. A warning light alerts the driver to any malfunction in this system. The knock sensor is on the right-hand end of the engine, in front of the timing cover. Each ignition coil has two HT terminals, one spark plug being connected to each terminal. The coil secondary windings are not earthed. When the computer triggers the appropriate coil to provide a spark at No 1 cylinder, a spark occurs at N0 4 as well. Sinds No 4 is the on the exhaust stroke, this spark is 'wasted'; after one rotation of the crankshaft, No 4 fires and the spark of No 1 is 'wasted'. This is a similar system to that used for many years on the two cylinder Citroen engines. There is no sensor which directly indicates the firing point for cylinders 2 and 3, but the computer triggers that coil halfway between the firing points for No's 1 and 4.
  AEI system -maintenance: Maintenance of the system is limited to inspection and renewal of the spark plugs at the specified intervals, and inspection and cleaning of the HT leads and coil towers. No direct adjustment of the ignition timing is possible.
  AEI system -fault finding: If misfiring occurs, check the condition and electrode gap of the spark plugs before suspecting other faults. Turbocharged models are particularly sensitive to the type and condition of the plugs. If the knock sensor malfunction warning light comes on, check that the knock sensor is properly seated on the crankcase. The insertion of a flat washer between the sensor and the crankcase may improve matters. After fitting the washer, tighten the sensor to the specified torque. Apart from simple continuity checks, further fault finding requires special test equipment, or a supply of known good units for testing by substitution. Consult a Citroen dealer.
  Spark plugs (all models) -removal and refitting Disconnect the HT leads from the spark plugs or extension rods by pulling the sealing bungs out of the spark plug wells. Do not pull on the HT leads themselves. Label the HT leads with their cylinder numbers if there is any possibility of confusion. On models with separate extension rods and insulator tubes, pull out the tubes and unscrew the rods. (On later models the extensions are integral with the HT leads). Blow out any dirt from around the spark plugs, using a bicycle pump or an air line. Unscrew the spark plugs. A long box spanner, or a plug socket and extension bar, will be needed for this. Remove the spark plugs from their wells. When refitting, use a piece of rubber or plastic tube to start the spark plugs in their threads. It should be possible to screw the plugs nearly all the way home by hand (i.e. without using a Tommy bar or ratchet handle on the plug socket). Do not force the plug if it is cross-threaded, or damage to the cylinder head may result. Tighten the plugs to the specified torque. In the absence of a torque wrench, tighten by no more than 90 degrees (a quarter-turn) beyond the point where the plug washer contacts the cylinder head. On models with separate extension rods and insulators, refit them Reconnect the HT leads and push the sealing bungs home.
  Well no more about the capacitors, but I give you the whole ignition system here, print it for latter reference. I hope this is of some use, it was some work 8-). Greetings again, Frank
  Cgat: These have been omitted from my car for years to no detriment... Plus the ignition timing on my car is adjustable, I can move the slug 10 degrees from standard in 2 degree increments. Had to do this when the compression ratio was reduced, so I could advance the ignition.

These capacitors again...
  Alan D, 2002-12-31: I may have given the wrong impression in one of my postings about the T2 problem. The capacitor was THE cause of the problem. It was, however, exacerbated by the sensor cables being bunched with the coil and injection harnesses behind the engine. It might not have been so neck-breaking otherwise. When they were separated the engine became as smooth as only a CX turbo engine can be.   Scott R: Has anyone tried shielding the sensor leads? That should make them far less noise sensitive. You can get braid that will expand and slip over the connector. Connect it earth, best at the engine end. Regards from an analog electronics engineer!
  Bob R: What was the failure mode of the capacitor? To prove the point I made earlier, I disconnected both capacitors on my fathers '85 GTI and the engine started and ran perfectly. While a capacitors' initial mode of failure may be to short, I maintain that this condition will not persist, as the current in the circuit will generally be sufficient to cause either the point of failure or one of the leads to burn open. In over 20 years working in the electronics field, I cannot recall ever replacing a shorted capacitor.
  Gary M: Bob, that's the test I was hoping someone with a healthy CX would try. Who knows when something is wrong with a car which one of the thirty things fixed or touched really made the CX purrrrrrr, but take one that is purrrrrring already and test it by seeing what will make it mad - that's component testing! I would still like to get a couple and have it the way it is supposed to be.
  Alan D: Bob, as you suggest, the capacitor was O/C - capacitors in low-energy circuits will sometimes fail to short.   Bob R: Right you are, Alan. As usual, I dropped a word in my last post. I meant to say that the initial failure mode is SOMETIMES a short! For those suffering with a car that likes to cut out or otherwise misbehave, take note! The most common causes of ignition misbehavior on a CX are cross-talk between the leads to the coils and the flywheel pickups, and RF interference from improper (very low resistance) spark plug wires. If you have a problem after ensuring that these issues have been resolved, remove the ignition computer and open the case to inspect the circuit board. We have seen one that had significant damage to the printed circuit.

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