volts at coil

[sleeper bird]

iv got an issue iv been trying to resolve.first my bug is a 1979 that was converted to carb(previous owner)it will start for a min and just die.its getting fuel but i notice when checking the volts at the coil which is new i get 9.5 volts is this correct?seems it should be 12 volts to me.

[sleeper bird]

PS glad to be a part of your community

[JRKman]

Should be battery voltage. Read the voltage at the battery.
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1969 Type I
House paint red
1835cc single port:
Scat C25, ICT's, SVDA

[sleeper bird]

yea probably should have mentioned that, battery is 12.5 volts.Might see what voltage reads at the coil when cranking,will post results.Thanks for the reply

[Tim Donahoe]

Are you testing the correct post on the coil? Don't check the post that has the condenser wore. Test the post that has wires going to your carburetor.

Tim
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Let's do the Time Warp again!

Richard O'Brien

[sleeper bird]

Just went out there and with the assistance of my helper (my ten year old daughter)with ignition on 9.5-10.5 volts when cranking it drops to 4-5.7volts.she wants to run but its almost like its starving for fuel,iv got a new fuel pump and filter and a new carb i know its getting fuel.

[Cusser]

[ashman40]

9.5v at the coil is a HUGE voltage drop! I'm surprised your coil is even firing at all.

In addition to Cusser's three measurements, please also check the voltage at these points:
4. Voltage at the fuses that have constant battery voltage. These fuses have 12v+ when the ignition switch is OFF.

5. Voltage at the fuses that have ignition switched voltage. These fuses have 12v+ when the ignition switch is turned ON.


The voltage at the ignition switched fuses are at the other end of the long wire that runs to the ignition coil and will give you an idea of the voltage BEFORE the long wire run, and how much voltage may be being lost.
As a test, run a jumper wire from the alternator B+ post to the ignition coil #15 terminal. This will give you direct access to full battery voltage. If this fixes your starting/running problem you know it is due to the voltage level reaching the coil.


In my case (and maybe yours) the old black #15 wire running to the ignition coil is oxidized and creating resistance. This diverts current from passing down the wire and the voltage suffers.
A couple options are available if this is your problem:

1) Clean up all connections between the battery and the ignition coil. Take a wire brush and remove any oxidation on all connections. If necessary, snip off the ends of the wire, strip the insulation off to expose clean wire and crimp on new terminals at the ends of the wires. This will reduce the resistance and increase the amount of current that makes it all the way to the coil.

2) Run a new #15 wire from the ignition switched fuses to the coil. This is not easy as you want to follow the path the wiring harness follows. It is not impossible but difficult. If you go this route, run a few extra wires in case you need them in the future for other things like a tachometer.

3) Add a relay in the engine compartment. The small Bosch-style relays will work with as little as 6v. You can use the existing 9.5v to energize the relay which will pass 12v from the alternator B+ to the ignition coil #15. This was what I did, but there is a workaround you need to also put into place... You will need to add a small diode on the blue GEN light wire so that voltage only flows from the GEN lamp to the D+ at the alternator. You want to prevent voltage coming FROM the D+ making it back to the #15 circuit. Without this diode, voltage from the D+ will keep the relay powered even after you turn off the ignition switch and the engine will continue to run because the relay will continue to power the coil.

_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Cusser]

Also post a few good clear photos of your engine compartment, and that also helps us determine if yours has a generator or an alternator if you're not sure.
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1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297

[sleeper bird]

[sleeper bird]

[rockerarm]

Hi and WELCOME.
What you need to do is called a voltage drop/loss test. You need to test the entire circuit then if a noticeable drop exists you narrow down the testing to isolate the fault. This is how it is done.

With a DVOM assure you have a fully charged up battery and it appears you do have one.

With a DVOM and a couple of jumper leads, hook up the positive from the DVOM to the battery positive post. The DVOM negative will hook up to the coil term #15/positive. Turn meter to dc volts to about the 20v scale. Turn ign on or start up car and document the meters reading. This is the voltage drop/loss of that entire circuit. ideally you should see less than 1v drop. The standard for this is .2v drop per connection. On this circuit you have multiple connections and two long lengths of wire.

With the proper wire schematic you can follow along and see all the connections involved here. You test sub-sections of this circuit by this method:
DVOM positive to battery positive (as before) and DVOM negative hooked up to the red wire at bottom of ignition switch (term 30), test and document.
Next step, hook DVOM positive to red wire at bottom of ign switch and DVOM negative to black wire at bottom of ign switch (term 15), test and document.
Final step, DVOM positive to the black wire at bottom of ign switch and DVOM negative to the coil positive terminal, test and document.

You performed 4 tests here. First test was for the entire circuit and the following three tests were the same circuit broken down into sub circuits.
I have witnessed a 2v drop from the battery to the front of the car on late model bugs and have witnessed a noticeable drop across the ignition switch which nowadays has shown to have reliability concerns.

This test method is the gold standard for testing automotive circuits. When you learn and understand its validity you will be leaps n bounds above many others who refuse to grasp its concept.

Here is a good video explaining the concept. You will notice he also checks the negative/ground circuit.
https://www.youtube.com/watch?v=WMXENKujYtc&t=328s

Hope this helps, Bill.

[Tim Donahoe]

Sleeper, yes, the number 15 (positive side) is the side of the coil you check for voltage.

The positive side usually has three wires attached to it. One goes to the carburetor and piggybacks to your choke and carburetor electro-magnetic cut-off. One goes to your transmission. One goes to your ignition. This is the side of the coil you wish to test for voltage.

If you've been testing the other side of the coil--the one that usually has just one green wire going to your distributor (the condenser, actually)--you will have gotten maybe 9 volts, or less.

So make sure you're testing the positive #15 post.

If you're lucky, maybe you've just been trying to test the wrong post.

However, if you've been testing the correct post, you better listen to the wisdom imparted by the others in this thread.

Tim
_________________
Let's do the Time Warp again!

Richard O'Brien

[ashman40]

[Abscate]

A good number for a voltage drop T the Terminal 15 coil terminal is o.1-0.2 volts. A cheaper voltmeter won't be that accurate so it might be tough to measure that small a change, but anything over 0.5 volt means bad wiring

These are static, not running, not cranking numbers
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.ssS!

[sleeper bird]

[rockerarm]

[rockerarm]

[VW_Jimbo]

[Meiang]

I assume the engine would run after it was converted from fuel injection to carburation. Pretty obvious but no one has asked that yet. If so, from when it was running to the current situation what changes have taken place.
Basic checks:
1) no load voltage between the +ve and -ve terminals of the battery is 12.5-12.8v or there abouts.
2) the earth strap from the battery to the body is in good condition and the connections are tight and free from corrosion.
3) as per 2 but from the battery +ve terminal to the starter motor.

You indicate a 5v drop from static to cranking at the coil terminal. For this to occur it is likely the battery is not delivering correct voltage and needs replacing. No load outside of the starting circuit (which I can think of) could cause a voltage drop of this magnitude.

Carryout 3 tests after charging the battery to a nominal full state charge.
1) Measure voltage across the battery terminals with the ignition off.
2) Measure voltage across the bettery terminals with the ignition turned on.
3) Measure voltage across the battery terminals when cranking the engine over.

As mentioned previously.
under test 1 - voltage should be 12.5-12.8v
under test 2 - voltage should be 12.0-12.5v
under test 3 - voltage should be above 10v

If the above 3 tests show the battery is OK then the problem lies elsewhere in the cars wiring system.