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basic electrics

Discussion in 'Anything and everything' started by grolls, Nov 22, 2004.

  1. grolls

    grolls Active Forum User

    when working on your car please remember to take the proper precausions for your own personal safety

    I could sit and list many mistakes that people have done over the years but heres a few classics to learn by.

    1. mate jump started his car from his wifes car, wifes car running when connected jump leads on car battery that was flat, large spark from jump leads resulting in the flat battery exploding thus throwing acid into his face and eyes....3 months off work...nasty!
    ( i will cover the correct way to jump start a car later, the safer way)
    2. iv'e been told this one a few times now, remember gold/silver jewelry conducts electricity very well, so remember that when sticking your hand on an electrical item such as a starter motor like this poor sod did, remove it, his wedding ring shorted out on the battery feed to the starter and welded itself there. as you could imagine the ring was glowing red hot and he couldn't move his hand away....i was told he lost the use of that finger!


    Always remember to fuse an electrical item as close to the power source as possible!

    electricity can kill you, yes even a 12 volt system!

    poorly wired/insulated cables can cause fires!

    Im sorry but it has to be said that neither the club or i can be held accountable for any repairs you make to your car and any information you get from this site is your responibility to check that the information given is correct.
    Last edited by a moderator: Nov 25, 2004
  2. grolls

    grolls Active Forum User


    if you had a garden hose and then stood on the pipe the water flowing would slow down because you are adding resistance to the flow. electrics work in the same way. on a car (any electrics) we measure this in "ohms"

    what determins the resistance is

    1. the size. ie width of cable

    2.lenth of cable

    3.substance of cable (ie made from steel,brass,silver,gold etc)

    4.the temperature of the cable.
  3. grolls

    grolls Active Forum User

    to check a coil.

    first make sure unit disconected.

    to do any checks some meters may need to be zero'd(read your instruction booklet) but if not available hold both the red and the black leads together with the meter set on the ohms scale and the meter should read zero ohms.

    primary windings. ie; the two bolt on/push on terminals either side of the big h/t lead in the middle.

    a rough guide would be the folowing.

    an un ballasted ignition system= not 240/260 =3-6 ohms( output voltage 18-20k volts)

    a ballasted system= 1.2-3 ohms 240/260( output voltage 28-31k volts)
    measure at the coil and dont include the ballast resister normally 1.5 ohms)

    an electronic system= .3- 1.2 ohms or less=( output voltage 61k volts PLUS)
    Last edited by a moderator: Nov 23, 2004
  4. grolls

    grolls Active Forum User

    coill secondary windings, from the middle pin (h/t lead to distributor) to the coil - or number 1

    all these systems should read between 3-14k ohms regardless if electronic/points etc.

    oh by the way the ohms scale on a meter looks like a balloon inflated but stapled at the sides!

    to check the above you must make sure you are on the correct scale; ie if you wish to measure the primary scale your meter must be on a lower scale ie less that 20/30 ohms or you will read incorectly.
    if you wish to check the secondary windings your meter must be on a minimum of 20k-25k scale or you will read incorectly.

    you must still check that the "tower " ie where the king/main lead comes from the coil to distributor has no cracks or this could cause a misfire.
  5. grolls

    grolls Active Forum User


    now for all those people that say converting to an electronic system does make a performance difference, ie fitting a 280zx distributor and coil to a 240/260

    you can see by the power difference that the coils put out;ie double!..just make sure if you upgrage your system that you have the correct plugs and good quality h/t leads to get the maximum benefit.

    even cars with coil over plugs( a coil for each plug that is bolted directly over the plug, thus eliminating the need for h/t leads,distributor, distributor cap,rotor arm) can be checked in the same mannor.
    a word of warning
    some new systems that are now on the market clean the spark plugs when the car is switched off,( anytime up to 2 mins. later all the coils are triggered sending a huge burst of electricity down to each plug)

    this is done to ensure the car starts and runs perfectly on the next start up........if your going to check your coils or change your plugs on a modern car wait for atleast 5 mins or the pace maker go wobly due to the electric shock you will recieve.
    Last edited by a moderator: Nov 24, 2004
  6. grolls

    grolls Active Forum User

    spark plug leads(h/t)

    to check an h/t lead, meter must still be on the OHMS scale.but of course you must remember that such things like dirt,oil,damp,poorly routed leads and poor insulation can have a major role in a misfire espacially when under load...if in doubt.....replace.

    normal type ie car with distributor

    with the wire unpluged from the car connect the meter to each end of the plug lead. you should be getting a reading of approx. 15k ohms per metre max

    distributorless ignition coil(ie; coil pack bolted direct to engine)

    these leads you shouldn't get more than 30k ohms per metre lenth.

    incase your wondering why spark plug leads have a high resistance the answer lies in the coil. if the path for the spark was very easy from the coil to the plug the coil wouldn't build up a nice big juicy spark before sending it down the cable. It would be leaking power down the cable as it was trying to build up the spark in the windings causing a misfire.

    k=thousands of ohms.

    do this when meter connected to item you are checking!
    just an easy way to ensure you are on the right scale, set your meter to the maximum on the scale setting ie; millions then slowly turn the dial on your meter to reduce the setting untill you see a figure. keep going until the figure dissapears. then go up one click on the meter and this should show the correct should now see the figure in its correct scale ie; tens,hundreds,thousands or millions
    Last edited by a moderator: Nov 24, 2004
  7. grolls

    grolls Active Forum User

    what is voltage?

    voltage is pressure, its the pressure that pushes electrons down the cable thus suppling power to what ever part needs power.

    we measure this in volts(DC if were measuring a car electrical system, AC for car sensors i will post about sensors later)

    if you were to connect a multimeter in parallel to a car battery, ie 1 lead to each terminal, if the meter showed 12volts this would mean you have 12 volts of pressure.
  8. grolls

    grolls Active Forum User

    multimeter safety

    i know all meters have a fuse in the back of the meter but often this fuse is expensive,hard to get and may not blow fast enough if you make a mistake.

    my advice would be to cut BOTH of your multimeter leads and put a low amperage fuse in line. i have 7.5 amp fuses on mine.

    solder any connections you make and do a continuity check. ie switch meter on and set to the lowest ohms scale you have then put both the red and black leads together and it should read zero ohms. After youv'e done this you can feel sure that the meter will be ok.

    The reasons behind doing both leads are 1. to get an equal balance between the leads and 2. its no good protecting the positive lead if you get a voltage spike up the negative lead...damage done!
  9. grolls

    grolls Active Forum User

    charging systems

    this is a very difficult post as there are many variables such as bad connections and poor battery condition will all have an effect on the charge and amp rate as will a worn or poorly adjusted fan/drive; if a battery is faulty(a duff cell as an example) you may see either an over or under charge. if anyone would like me to cover the old british type ACR..early ford,bl,vaux etc or dynamo's .i will be more than happy too just pm me.

    all of the early jap alternators will quite happily push out upto15.5 volts when cold, this normally would drop to14.5 ish once the battery and associated electrics have settled down after 20 mins or so. this is ok providing you have a decient QUALITY battery...if you use a cheap standard lead acid type battery dont expect a five year life expancy because they dont like the high my opinion you should use the jap style batteries (the ones with the little glass viewing eye) or a calcium battery for the best results.. how to check an alternator will be in the next post.
  10. grolls

    grolls Active Forum User


    generally on most jap. alternators you will have 4 terminals, these are marked,

    A orB, for the battery live feed (normally a 10mm nut holds down the thick wire)

    E, for an extra earth( normally an 8mm nut or bolt holds this in place if used)

    N, phase tap (normally the wire connector is held in place by a plastic clip along with the F, wire, in the shape of the letter" T")

    F, field....see above.

    Firstly unlike the old british type of alternator if you charge light bulb has blown the jap. style will still charge, but be warned if the bulb on the dashboard comes on this could also mean an over charge is taking place. dont just assume its not charging.

    The AorB post is nice and simple, this a direct feed to the battery normally via the starter motor cable.

    The E terminal is also nice and easy. E = earth.. normally a wire strap that bolts to the alternator adjusting bracket or engine block.

    The N terminal denotes A PHASE TAP. this is a low volt supply to the regulator( 1 volt on tickover rising to about 8 volts when engine revs increase. basically this turns the charge light off and controls the field circuit for the alternator.

    The F terninal supplys the alternator with a 12 volt feed from the regulator.

    Wiring colours on most regulators....but not all! consult your wiring diagram if you have one.


    Firstly make sure all terminals are CLEAN AND TIGHT(the biggest problem with electrical faults)

    firstly with a multimeter set to VOLTS DC lower range 20 volt scale, check what you are getting at the battery, you should see 15.5 volts max. then dropping off when its run for a while to about 14.5.

    if you do not have this do take the same measurement directly from the back of the alternator (engine running so mind your fingers) your just making sure you do not have a broken cable or fuse link gone when checking here.

    now checks with engine off.

    next you need to to a resistance check on the alternator to earth, set your multimeter to the ohms scale" low range" and if you hold one multimeter cable to the engine and the other to the E terminal on the back of the alternator a good connection should show less than 5 ohms. do the same on the regulator earth, you should also show less than 5 ohms.

    engine running.

    just as a quick check if you are getting 5 to eight volts when engine running above tickover on the N LEAD ON THE ALTERNATOR, then your fault will lie either with a faulty regulator or a bad connection upto the regulator
  11. grolls

    grolls Active Forum User


    This type are fitted to early vw,ford,bl,vauxhall...etc.

    easy to identify and just as easy to check as you only have 3 wires in the back of the unit and two of those are joined together.

    normally these alternators have a single 3 pin plug with a spring clip holding it in place.

    the two large wires go directly to the battery via the starter motor cables. expect to see 13,8 volts DC at the battery when the car revved. (these are the two that are joined together in the harness)

    the single wire (small) takes a live feed from the charge light bulb from the dash board. if you earth out the small wire (remember to use a fused wire 5 amp would do) the bulb should light up. if not you have a power feed problem to the bulb or a blown bulb...either way the alternator will not charge unless this bulb works.

    the alternator earths itself via the mounting bolts.
  12. grolls

    grolls Active Forum User


    most multimeters do not have anywhere near the power rating needed to check amp rates for alternators. please note that if you have lets say a 90 amp alternator fitted to your car the alternator would only supply that sort of power if the battery was very low in power and then you put your lights,heater,rear demister,etc. if your battery was fully charged and you had no heater,lights, etc. on expect to see less than half that .

    there are two cheap and easy ways to do this, one is by a clip on amp meter (very crude but effective anyway) the other is by way of a digital amp clamp) these are very accurate and they are very easy to use. i will cover the useage of these in the next post
  13. grolls

    grolls Active Forum User


    a great bit of kit and fairly cheap to buy.these clamp round a cable to measure amp draw. i would recomend this bit of kit!

    a few rules you must follow.

    1. you can only check one lead at a time = the reason being if you were to put two cables in the clamp and both those cables were carrying 10 amps each lets say, all the meter would read is zero amps as one wire cancels out the other, if done one at a time you would see the correct amperage of both.

    2.most clamps have a directional arrow on the side telling you either towards or away from the battery. wrong way round= wrong reading.

    3. most clamps have to be zero'd by the wire you are going to test as they will pick up readings from nearby wiring or other electrical items.

    As i said in the previous post an alternator will only put out maximum amps if the battery was very low on power , you can see this happen with the clamp.

    simply put the clamp on the battery lead and see what you are getting.

    most alternators will show only 10-25 amps max when the battery is fully charged and you had no load (lights, heater etc)

    Of course if the battery is faulty or you had a poor connection or a slack fan belt this will have an adverse effect on the charge rate( again most electrical problems are due to poor or loose connections)

    To test to see if your alternator is doing its job, run the engine for 10 mins and take a reading with all lights,heater etc. off
    then turn off the car and leave the headlights on for ten mins or so, then restart the car and switch all the electrics on, the difference should be very noticable.
  14. grolls

    grolls Active Forum User


    You may laugh at such an easy subject but the damage done can be rather painful to the wallet.

    First things first.
    1. a flat battery gives off explosive gasses
    2. car electronics dont like power surges
    3.a battery has gone flat for a reason

    listed below is the safest way to jump start any car be it a classic or modern without damaging your wallet or your pride !

    1. make sure you have BOTH sets of keys in your pocket. ie; the car that has the flat battery and the car that has a charged battery.

    REASON......2 things here.

    1. this makes sure no car is running thus preventing any power surges to any electronics normally causing an expensive ECU replacement.

    2. you will feel really silly when the car that has a flat battery locks all the doors with the keys inside due to the central locking activating when all of a sudden the battery has a little power!


    connect the red lead, positive to both cars batteries(no order) assuming cars not touching.

    NEXT. connect the earth lead to the flat battery cars engine(not battery)

    NEXT. connect the other end of the earth lead to the supply car via the engine also.

    the reason for this is to reduce any risk of a spark anywhere near a battery that is giving off any gasses, hence reducing the risk of explosion from the battery and yes it does happen. A face full of acid hurts like hell!

    NEXT. if the flat battery is 100% flat just leave both leads connected for a good 15 mins. this will give the battery a good chance to pick itself up a bit before starting the doner car.

    NEXT. start the doner car up.
    if its a modern car it will charge at its maximum rate on tick over, if an old car you may have to increase the revs to about 1500 to get a decient charge rate..

    NEXT. you should run the doner car for at least 15 mins to get some power into the flat battery BEFORE attempting to start.

    NEXT.let both cars run for a few mins. then switch both cars off before removing jump leads. this also prevents any voltage spikes to electronic parts.

    NOTE. if the doner car will not start after this try again or take the battery off and charge or replace assuming the alternator is ok.

    just a little note; the dearest bill i have seen after a jump start was nearly £1500 ....jeeze, he only left the park light on for 24 hours!!!!!!!!!
  15. grolls

    grolls Active Forum User



    before you do this test make sure of two things.

    1. keep the keys in your pocket incase the doors lock

    2. if you have a coded stereo, do you know the code?

    Most meters have a facility for upto a 10 amp draw test.

    Make 100% sure all lights, ignition etc all off.(do not attempt to start car or you will cause damage to your meter if not fused as previously recomended, 7.5 amp fuses on leads)

    make sure meter is on before this test and set right.

    1. remove either pos. or neg. terminal to do this test.

    2.connect meter, the earth lead from your meter to battery terminal(disconnected one only )
    3 the 10 amp socket (red lead) connect to the lead you have removed.

    this puts your meter directly inline "in series " ie; any power being used buy any part of the car will show on the meter. yes this does the entire car at once.

    you should see zero amps on the scale... an interior light or boot light will show approx. o.30 of an amp. dependant on size(s) of bulbs.

    if you do have a zero amps draw shown, open the drivers door so the interior light comes on.(dont do any more than this)

    then check the meter with the light on.
    the reason for this is to make 100% sure you connected it right no light = not done it right or flat battery or blown fuse on meter. then close door( blown fuse on meter = large drain..check very carefuly).

    if you are showing an amp draw , check all lights, boot,glove box, etc. if all off, start pulling fuses out one by one till the meter drops to zero. then you would have found the faulty circuit!

    allarms will take 2 to 3 weeks to flattern a battery

    modern cars can take upto 20 mins for electronic proccessors to power down.

    dont forget that you have an electric clock and a stereo memory, these will consume very little power, probably o.o9 amps or less approx.

    Last edited by a moderator: Nov 27, 2004
  16. grolls

    grolls Active Forum User


    This ones nice and easy.

    1.remember to check one wire at a time. should see a zero amp draw from the battery.

    3. dont forget your stereo and clock will draw some power 0.09 approx.

    4.modern cars can take up to 20 mins for processors to power down.

    First remember to zero the meter by the wire you wish to check( stops interfierence from other electrical wires/motors).


    1. DO NOT take any battery leads off. meter and the clip on the clamp... anything more than 0.09 and you have a drain big enough to flattern the battery in a few days!
    as before start removing fuses till the meter drops to near enough zero to find the faulty circuit.
  17. grolls

    grolls Active Forum User

    Fuel Pump Enertia Switch


    .There are many crash sensors on the market today, some are fitted inside the car and some on the outside(under bonnet)
    just remember that if your going to fit one externally make sure its a sealed type with a rubber boot!

    Most sensors only have two wires (power in and power out)
    not all are wired directly to the fuel pump. some may be wired via a fuel pump relay.

    Not all sensors will take the sort of power that a fuel pump will draw so the safest way is to use a relay

    As to where to place the enertia switch always on a structural(metal) part of the car ie; on an A post, chassis member, inner wing etc.

    You must remember that if you have carbs fitted to your car it will take a few mins for the float bowls to empty, hence the car may stay running for a while till they empty!!!!!!!!!!

    but the pump should have stopped the Fuel Pump(most important)


    remember to fuse!

    You can use a standard 4 pin relay if you wish= nice and easy!

    on the relay you should see the following numbers( if not then pm me to see if you have one that will do!)



    the enertia switch could be fitted here, cut wires and connect an in and an out!)

    NO. 31 this is the relay earth, (enertia switch could also be fitted here, same rules as above)

    NO 87. this is the wire that will feed the pump.

    once wired in. the pannel that you have attached the sensor too, give it a really good hard thump and see it it trips the pump out

    Last edited by a moderator: Mar 6, 2007
  18. grolls

    grolls Active Forum User

    engine slow to turn over

    there are a few reasons that this may happen.

    flat battery, worn out starter,poor connections

    just testing with an ohm meter with the circuits statatic. ie not being used is not enough. i will cover volt drop in the next post.
  19. grolls

    grolls Active Forum User

    engine slow to turn over

    assuming we have a fully charged battery and its in good health.

    1. disconnect coil so we dont get a spark at the plugs.

    2. measure the voltage at the battery when turning over engine.(it should be at least 9.3 volts dc. let the battery voltage settle, ie; turn over for atleast 5 seconds before measuring)

    3. now measure the starter motor wires. ie put your multimeter negative wire on the starter motor body(earth) and the positive on the main wire that goes directly into the starter motor. you should have the same reading( you can allow upto half a volt loss due to cables and connections)

    if you have more than half a volt loss.

    1. with the meter still on volts dc, place the positive multimeter lead on the battery + and the negative on the starter motor wire +( the one that goes straight into the starter) you should see zero volts or as near as damn it when the engine is turned over.

    2 do the same with the battery negative and the earth on the starter motor body.

    3. again if you have more than half a volt showing you have a bad connection.

    4 if you have no volt drop you can assume the starter is worn out( of course assuming the engine is not flooded or seized)

    if a reading of more than half a volt is found

    leave the multimeter connected to the battery and move the negative wire to each connection between the starter and the battery ie; power feed into the starter then solenoid then the wire that feeds the starter then the battery terminal and do the same with the earth. all of a sudden you will see a difference in the meter reading and hey presto you have found the bad connection.
  20. grolls

    grolls Active Forum User

    Dug out some old notes that I can share with you all. This check sheet helps with the more modern cars, its just an insight on what to look for.:)

    1,Has Spark. injectors not working.= ECU should be ok, Main relay ok, crank sensor ok.= check live to injectors and continuaty to ECU.

    2,No spark. injectors working.= ECU C/P/S ,immobiliser , main relay ok. check live and switching/earth to ignition module.

    3,No spark, no injection. Check any ECU controlled sensor for 5 volts. No power would mean power to ECU gone/poor earth.

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