Misfire

  • T3 Ignition checks - Ignition coils

    On a VW T3, from the factory there were 4 different ignitions coils fitted throughout the production run, one of them was only used in some African countries on models fitted with points style ignition and are unlikely to see them here so they will be glossed over.

    Early vans were air cooled, the 1.6l "CT" engine and the 2.0l "CU" engine, they used what we call the early coil, it had DIN connection for the king lead, when the WBX joined the ranks then they too used the early coil. In July 1984 they swapped from the early DIN coil to the later "sawtooth" style connection at Chassis number WV2 ZZZ 25 Z EH 155 001

    The later coils as fitted to everything else, these could either have a green label on them or a grey label. ETKA [VW's Electronic Parts Catalogue] is a little confusing regarding what vehicles have the green and which have the grey label, in fact it is contradictory, we feel the best way to get the right coil is just to fit what was on there originally.

    A faulty ignition coil could cause a few different problems, we find that they start to give flat spots while driving, this is usually a sign they are past their best. They can be checked with a multimeter set to ohms (more on that later).

    An ignition coil is basically a step up transformer encased in steel and filled with oil, there are 2 circuits, a primary and a secondary, the primary is the low voltage side, the secondary is where the high voltage is induced. 12v is supplied to the primary circuit and around 35000v is induced in the secondary coil which is then fed via the king lead to the distributor cap where it is dished up to the correct cylinder.

    The primary circuit, this is the low voltage side, the outer terminals on the body. One is marked "15", this is ignition live [Black wire]. The other terminal is marked "1", this is what's called the tachometric signal, this is the switched earth supplied by your ignition module to turn the coil on and off [Green wire].

    The secondary circuit is between terminals 1 and 4, the green wire terminal and the HT output to the distributor cap.

    Obviously you disconnect the electrical connections from the coil before you get busy with your Ohmmeter as you want to measure the resistance of the coil and not the rest of the ignition system too!

    The expected resistances are listed in the table below.

      VW No Bosch No Fitted to Primary Secondary    
    Early 211 905 115 B 0 221 122 023 CT, CU, DF, DG .52Ω to .76Ω 2.4kΩ to 3.5kΩ Up to 24 E 155 000
    Late – Green label 211 905 115 D 0 221 122 349 DF,DG,DH,GW,DJ,MV,SS,SR .5Ω to .8Ω 2.4kΩ to 3.5kΩ From 24 E 155 001
    Late – Grey label 191 905 115 A/B/C 0 221 122 399 DF,DG,SP,DJ .6Ω to .8Ω 6.9kΩ to 8.5kΩ    

    DSC_0066 Testing the secondary resistance with an Ohmmeter

    After testing, if you find the Ohms readings are out of specification then it's time to change the coil, it's as simple as that!

    Early coil here

    Late coil - Green label here

    Late coil - Grey label here

  • T3 Ignition checks - HT Leads

    HT leads, simple concept, it's a wire that carries the high voltage produced by the ignition coil and sends it to the distributor cap and then on to the spark plugs.

    Whenever we get a T3 with a petrol engine in for rough running this is one of the first tests we do, check the resistance of the HT leads.

    The T3 HT leads are made up from a few components, the connection to the distributor cap, the connection to the ignition coil, the spark plug connector and the cable itself.

    Each connector has some built in resistance to help with radio interference, generally the ignition coil and the distributor cap connectors generally have a 1KΩ [one thousand ohms] and spark plug connectors have 5KΩ [five thousand ohms]

    The cable itself has a copper core and should have no resistance.

    This means a king lead [lead from coil to cap] should have a total resistance of 2KΩ [1KΩ + 1KΩ + 0Ω=2KΩ]

    Easy.

    Then, each HT lead [Cap to spark plug] should have a resistance of 6KΩ [1KΩ for the cap connector, 0Ω in the cable and 5KΩ in the spark plug connector]

    Checking is simple with your multimeter, select the correct scale for measuring 6KΩ and test each lead individually. You are not looking for exactly 2KΩ for the king lead, but somewhere very close, ditto for the HT leads, you probably won't achieve 6KΩ exactly but they should generally all be even and around that figure.

    VW made the leads in the manner so that each spark plug receives an even voltage, if one lead has a higher electrical resistance then the it's associated spark plug will receive a lower voltage and result in rough running.

    We often find vans here with "Silicone" leads and simple crimped brass connectors, Silicone HT lead is usually resisted along it's length meaning that the longer the cable the greater the resistance and as you know with a T3 engine you have 2 short leads and 2 long leads, which if you absorbed what was said above will result in uneven voltages at the spark plug! We do not advise people use silicone leads on T3 models.

     

    DSC_0076 Checking HT lead, this one is a little high for us and will be replaced.

    DSC_0075 Checking HT lead, this one is open circuit and the source of the misfire on this particular vehicle.

    It is also worth visually checking the leads to make sure the electrical connectors are in good condition as sometime the saw tooth ratchet mechanism for the later pin type connectors fail and cause loose connections.

    Cap and coil connectors from left to right Cap and coil connectors from left to right DIN 90°, DIN straight, Sawtooth 90°, straight sawtooth and in the foreground is a cheap none resisted item found on cheaper leads.

    You can find HT leads here for..

    Aircooled 2.0l CU vans here.

    Early T3 1.9l with DIN type connections here.

    Late T3 1.9l and 2.1l with saw tooth connections here.

    Even Oettinger WBX6 here.

    You will also find a range of HT lead components for making your own here

     

2 Item(s)