Engine Coil Resistance Advice
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Thread: Engine Coil Resistance Advice

  1. #1
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    Default Engine Coil Resistance Advice

    Hi,
    I am wondering is anyone can tell me how to do a resistance check on the secondary windings of the engine coils in my 85' Series 2 CX 25 IE. Where do I put the red and black leads from the multimeter to take the reading? Ill attach a link to a photo of the engine coil so you can all see what type it is.

    Cheers, Moe.

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  2. #2
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    Just where those two wires are going in to test the primary side and the lead post to the positive side of the wires for the secondary


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    Sorry, I don't know what you mean by 'lead post to the positive side of the wires for the secondary ', can you rephrase that? Thanks

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    IMG_1555065845.517354.jpg


    Garage C5 X7 3008 XTE
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    Fix it right the first time

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    IMG_2929.jpg
    Thanks for the diagram, so in this annotated version of my original picture, the red arrow is pointing to the lead post and the yellow arrow is pointing to the positive pin, is that correct?
    cheers.

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    Hi looking at your picture, the coil looks like a double ended coil in other words your vehicle would have two of these coils and no distributor if that is the case the secondary is isolated from the primary and secondary resistance is measured between the two plug lead terminals on the coil.
    Late series CX used both distributor single coil and dual coil ignition.

    Let us know the detail on your CX.

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    Hi Hydrac,

    Yes my CX doesn't have a distributor cap, its a dual coil ignition.
    So I need to measure the resistance between the two plug lead terminals on the coils? where would these be? If you could draw some arrows on my picture that would be good.

    Cheers. sorry for the late reply, I've been away,

    Moe.

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    Hello again,
    Your picture shows the two plug lead terminals quite clearly, the two secondary terminals ( i.e. output to plugs ) are one above the other on the left had side of the coil, and in your picture the upper one indicated by the Orange arrow and the one below that with the same rubber cover is the other terminal.

    To remove the spark plug lead simply grasp the rubber cover arrowed and with a slight twisting motion pull it from the coil ( avoid pulling on the blue spark plug lead ) do the same on the lower one and you will see either two posts looking like the top of a spark plug or in some aftermarket coils a socket just like a conventional coil.

    What happens in this type of coil system is that each coil fires two spark plugs with one plug firing on the exhaust stroke and one plug firing on the ignition stroke. With an electronic ignition system this is a very common arrangement and almost is universal on modern cars as it eliminates the Achilles heel of earlier ignition systems, the distributor.

    Cannot remember the coil resistance but it should be say 250 - 300 ohms resistance does not give a good indication of coil condition, it requires more sophisticated equipment capable of measuring inductance or impedance.

    Hope this helps and am wondering do you have an ignition problem or are you just curious.
    Last edited by Hydrac; 25th April 2019 at 07:32 PM.

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    Thanks Hydrac,

    that was well explained. So I got a reading of 3.6 when the multimeter was set to 20k, so that is 3,600 ohms correct? I actually don't have a manual for my specific Cx model, and from what I can see online, the secondary winding resistance seems to change from model to model. However since both coils have the same reading and are within the expected range, I'm guessing they are alright.

    Yes I am having trouble with ignition. I bought the car for 200$ with no background info on it (the owner had died I think). So I put in a new battery and the the motor turned over but wouldn't start. So I checked for spark but didn't see anything (hopefully I did this correctly). Then I checked the fuel pump relay (which works when I hold the contacts closed with a peg), the injectors (which receive the correct volts), and now the coil resistance seems to be fine.

    Any hunches as to what might be going wrong?
    Cheers, Moe.

    PS. Just an afterthought, the spark plug lead which goes between the peg with the orange arrow and the spark plug closest to the coils has a loose connection, could that be the problem?

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    Hi
    Re resistance yes 3.6k is correct, my apologies I left a 0 off in my reply and yes there is a wide variation in this figure and coil reliability is not a problem I've encountered in this model and it is reasonable to assume they are ok.

    Ignition first, to check for spark just remove one plug reconnect the lead make sure the body of the plug is in good contact with the engine and crank you should see a spark, you probably did something like this but just thought I should confirm.

    Re the fuel pump relay I'm not sure I understand that bit, the fuel pump relay is a little different to a normal relay it supplies 12v to the fuel pump the electronic ignition unit and the EFI unit. Did you mean that when you hold the relay closed there is 12v on the injectors?

    When you turn the key to start the fuel pump relay should close and the fuel pump with 12v across it should run, have you or can you check this and of course if the pump does run you should have fuel under pressure in the fuel rail.

    You do need to be careful working on these cars as the electronics are not as robust as later types and spares are not that common and are expensive.

    Re the spark plug lead if the coil has a post as distinct from a socket the leads you have are I think the wrong type in that they have one male and one female end, the correct lead would have two female ends not a major problem now as if the lead is a little loose on the post you could close the connector a little for a better fit. The leads themselves should have a resistance of say 4kohms + or -

    I have a complete circuit diagram for this vehicle diagram contact me if you want one.

    Meanwhile I'll await the latest news on progress

  11. #11
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    Hi,

    Re the fuel pump relay, I could hear that the fuel pump wasn't running when i turned the key in the ignition, so I pulled the blue cover off the relay and closed contacts together manually, which made the fuel pump run, so to hold the contacts together I used a peg from the clothes line! And yes I think that relay has to be working to get the volts to read at the injectors.

    Thanks for the further info, hopefully Ill have time to have a look at some of those things you mentioned on the weekend.
    Cheers,
    Moe.

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    Hi Marion
    You have been lucky buying that car for a bargain price. Doubly lucky to get a good learning experience

    You know once you try the easy things and nothing happens you do need to get down to the basics. There is a procedure to do logical checks to get a dead car started. Just guessing is OK if you are a mechanic with a few years under your belt, but otherwise a waste of time.
    I suggest that the early electronic ecu controlled cars would probably be hardest as there are no codes to point you at things, so first principles are the best. Some old text books from the trade might be found in a TAFE library. You have no history to guide you in this case so check each step for yourself. Just logic really. But the analogue ECU systems were a bit more resistant to misuse that the digital ones IMHO.
    The engine must be mechanically working OK. The pistons go up and down, the camshaft is timed correctly and the valves work in the correct way. Then a compression test will see if the mechanical things are OK.

    Then spark must work and at the correct timing. So the sensors must be OK and the ecu working correctly. Some of the old systems had codes which could be read out by the light flashing in a pattern after shorting a pin. See if the manuals tell you how to do that.
    The fuel must be pumped when switching it on and when running. It is usually switched off after a short time till the engine starts. The pump may even run off the oil pressure sensor. A pressure gauge reading on the fuel rail will tell you what is happening. But the injectors will 'fire' even if there is no fuel to them. The injectors must be 'fired' by the ECU. Probably the timing is not critical. You can check the injector pulse existence with a home made LED light and wire, tester. This can also be used to check the pulse to the coils is there.

    How is that for a start to check. What is the efi system called, a motronic or what ? The air flow meter type ? The old text books probably give some schematics on these old systems to check for operation.
    Jaahn
    Last edited by jaahn; 1st May 2019 at 10:56 PM.
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    hi Moe
    I have a workshop manual for a CX, all models 1975-1988 Haynes. If interested send me a PM. Postage cost.
    It says the fuel system is a LE2 Jetronic EFI system, that was a common Bosch system which you can google for information. http://www.dethomaso.fr/article-desc...-65776977.html
    http://www.cardiagnostics.be/-now/Ed...n%20Manual.pdf
    The spark is controlled by a seperate ECU, probably Bosch also.
    I think both of these would be analogue systems not digital.
    Jaahn
    Last edited by jaahn; 2nd May 2019 at 06:53 AM.

  14. #14
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    Hi Jaahn,

    Thanks for taking the time to offer some advice. Those links look very useful.

    I confirmed that there is no spark at the plugs today. Since the coils are working, would it be a good idea to try getting a new sparkplug to see if that makes a difference?

    If not, I guess I'll be going back to basics as you suggested.

    And yes Ive only been able to find manuals for older cx's so if you have a manual for my model I would be very interested. Ill PM you.

    Cheers, Moe.

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    Hi Moe
    I do not think you have proved the coils are working yet. The resistance readings only prove there is a connection through each winding. It does not show whether the high tension will short out internally or externally under load. It also does not prove the coils are being driven by the ECU. Those plug leads look suspect too. So forget new plugs just yet.
    Basics need to be checked. Sorry.
    Jaahn
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    I'm going to be controversial... On my latest CX, I came to the conclusion that my coil(s) were shot because of a lack of spark but it turned out that the plugs had been contaminated & were tracking (shorting the current) so no spark. New plugs and all was good. Then I found the pesky fuel-regulator had been leaking fuel straight into the manifold (which was why the plugs were so messed up). Plugs are cheap - give them a go but make sure you always earth them nicely to the block (I use a jump-lead); it's gonna hurt a lot if you're holding one of those and YOU end up completing the electrical circuit...
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    Hi eddie
    Well you could be right and new plugs are probably a good thing anyway. I do not think it is a likely answer but who knows from my keyboard.
    The problem with double ended coils is that you must test them properly. You must have a spark plug in both ends when you test them and both plugs must be properly grounded. If one is not able to fire the circuit is broken and neither will fire or if the leads are crap or not working the test is not valid. Actually you can test them by flicking the positive on to the primary and it should spark but I do not think Moe knows enough to do that yet. Basic test !
    Jaahn
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    Jaahn is 100% correct - BOTH plugs (and leads) per coil must be working or the test will fail.

    That's because the electrical path is not like normal cars (body-plug-lead-coil-body).

    Instead it is:-

    body-plug 'A'-lead-coil-lead-plug 'B'-body

    Break any part of that circuit and it simply won't work.

    New plugs & test each plug (carefully) although if one works then the other should also work.
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    Hi again,
    To check for spark the simplest way would be to remove the extensions from two plugs and refit them to a pair of plug leads from the same coil and position them ends facing each other say 4mm apart hold them firmly in that position on an insulating surface with insulation tape or Sellotape and crank the motor, there will be spark or not.

    Whilst cranking there is a threshold voltage below which the ECU will not operate so be mindful of battery charge.

    One important thing is do not disconnect the primary side of the coils with power applied.

    The coils are fed from the same 12v rail as the fuel relay via a different set of switch contacts, there are from memory 3 sets of contacts in the switch.

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