TheSamba.com :: View topic - Electrical gremlins, coil voltage drop when starting.

Tue Jan 15, 2013 11:12 am

Hey guys, turns out on my 73 sb 1600 dp, the voltage on the #15 wire coming from the fairly new ignition switch drops to about half a volt when i try and start the car. Very hard to start the car the way it is. Im checking all of the grounds and trying to figure out what would cause such a drop other than a fault in the switch or my wiring :roll: ty in advance

Tue Jan 15, 2013 11:56 am

I'm assuming that 0.5 volt is measured at the coil #15. Go back and measure the voltage coming off the switch, and at the wire connector at the fuse box, see if those are close to 12 volts, or also 0.5 volts.

I'm also assuming that you didn't change any wires back at the coil, just on your new ignition switch.

Tue Jan 15, 2013 12:12 pm

You cannot discount the possibility of a failed new part. Happens all
the time. I got a new ignition switch for my VW once, and it lasted
less than a week.

Tue Jan 15, 2013 12:25 pm

I was reading off of the switch and getting .5 volts lol. Whenever I disconnect the starter wire from the starter and test the switch it works fine, it seems that the load the starter solenoid or starter places on my car causes the drop. Also the problem has persisted for a lot longer than any changes ive made unfortunately :(

Tue Jan 15, 2013 8:10 pm

Do a search for "Hard Start Relay." Then replace your ignition switch (electrical portion) and install a hard start relay under your rear seat at the same time. It will make your switch last much longer because it will significantly reduce the amount of current that your switch has to move.

Without a relay, the switch will have to flow ~22 amps through its tiny contacts to engage the starter solenoid.

With a relay, the switch will have to only flow about .16 amps.

This means that even a cheap Chinese ignition switch should last a significant amount of time. Your starter will turn even faster than ever and your battery will still be able to handle cranking the engine when voltage drops below 11 volts.

Wed Jan 16, 2013 2:24 pm

that sounds like the best solution. I'm gonna get on it! tyvm

Wed Jan 16, 2013 6:51 pm

Learn and perform a voltage drop test. Do it between the battery positive post and the push-on terminal (#50) of the starter. Also do the same between the starter housing (a ground source) and the battery's negative post. This will test this circuit while under a "load".
You can also do this test betwen the battery positive post and the terminal 15 on the coil.
Of course, this is assuming you have a good battery that isn't falling on its face with the load of the starter.
The bottom line is you can test any electrical (non-ECU) with this test.
Hope this helps, Bill
PS: google "load test" on you tube.

Sun Dec 07, 2014 4:55 pm

Hello All, 71 SB. When I check the voltage at the switch/fuse panel I get 12V to the coil but when I check the voltage at the engine bay, I get around 9-10V drop to the positive side of the coil. Engine had been running before but slowly started to stall and not start at all. I checked ground (starter turns) and replaced the points and condenser. It will fire sometimes but not stay running. I ran a test wire directly from the switch to the coil to eliminate the possibility of a bad wire in the harness. Can anyone help me out with this electrical gremlin, thanks.

Sun Dec 07, 2014 6:18 pm

If you do in fact have a true 9-10v drop to the coil, then there is a problem. For kicks and giggles when it does quit remove the carb top and verify the carb float bowl has gas inside. You can actually run a jumper wire from the license plate lite connector just above the carb and run that to the term 15 (+) side of the coil. You can disconnect the original black factory wire to eliminate that possibly faulty circuit during this troubleshooting. You have so many possible variables here with either the fuel system and/or the ignition system. Is the coil, points and condenser Bosch items?
Where are you at?
Hope this helps, Bill.

Sun Dec 07, 2014 6:37 pm

Thanks rockerarm yes I agree there are variables. I'm trying to isolate it down to maybe a ground parasitic drain or a bad connection at the starter or something. I'm scratching my head with this. It is clearly a electrical issue. Fuel is good and points are Bosch.

Sun Dec 07, 2014 6:49 pm

Before you go crazy, do a voltage reading at the battery when cranking. See if thats what you are reading at coil. A better way to check voltage drop is to put DVOM on low scale(2v) and put one lead on pos(+) of the battery and other on (pos) side of coil and see what voltage is. Rich

Sun Dec 07, 2014 7:19 pm

richardc thanks, with the voltmeter and test light there is no drain between the post and battery cable. But as you suggested while cranking there is a drop on the pos battery side and coil. Seems that when I ground the VOM to the engine and connect the pos probe to the coil wire there is a drop. Can the ground at the engine block be effecting/interfering the ignition circuit? Confusing because the starter works (good ground) or not?

Sun Dec 07, 2014 7:56 pm

ridindubs wrote: Hello All, 71 SB. When I check the voltage at the switch/fuse panel I get 12V to the coil but when I check the voltage at the engine bay, I get around 9-10V drop to the positive side of the coil.
<...>
I ran a test wire directly from the switch to the coil to eliminate the possibility of a bad wire in the harness.
Just to be clear, you are getting voltage reading of 12v at the ignition switch/fuse box with the ignition switch ON (confirm which # fuses; some are battery powered others are ignition switch powered).
But if you walk to the engine you get only 9v-10v at the ignition coil #15 terminal with the igniton ON and the engine OFF.

That sounds like you have old wires and the resistance in the black #15 wire between the ignition switch and the ignition coil is causing a voltage drop. Have you tried cleaning up the electrical contacts with sandpaper/wire brush? You may also need to trim the ends off the wire to get to wire that is not corroded. Over time the ends (even under the insulation) get oxidized and the resistance increases. You can clip off the last inch of wire and reach "good copper". Strip and crimp on a new end assuming you have enough length. This is not unusual for 30+ year old wire.

What happened with the test wire that you ran from the ignition switch to the coil... what was the voltage reading at the coil? Did this test wire fix the problem?

ridindubs wrote: Engine had been running before but slowly started to stall and not start at all.
When you say "not start"...
Do you mean the engine will not crank?
Of do you mean that the engine cranks normally but the engine never fires up?
Test the voltage at the ignition coil WHILE you are cranking the engine. Less than 10v and the coil will not fire reliably. You want the voltage no less than 12.0v.

As suggested above, check you battery voltage (measured between the two battery posts). You want 12.6v when everything is OFF. No less than 11.0v while the starter is cranking the engine.

Sun Dec 07, 2014 8:21 pm

Ashman, yes thanks. You are correct when I walk to the engine I get 9-10V from #15 with the ignition on and the engine off. Appreciate the advice on the wires but this is why I used a good/new test wire, with the same result. To answer your questions: Engine cranks but not firing up (voltage drop).

Sun Dec 07, 2014 8:58 pm

"Same results" with your test wire means you have 12v at the front and 10v at the rear?
What gauge test wire did you run from the front? It should be 14AWG or thicker.
Where are you getting power from at the front? Which fuse#?
As a separate test, run the test wire from the battery post and confirm the voltage difference.

If the voltage source at the front remains at 12v while the voltage at the rear is constantly 2v lower there is a problem with too much resistance in the length of wire. You can measure your test wire. A good wire should have nearly zero resistance.

One last thing. Are you taking your voltage measurement at the coil #15 terminal with the black #15 wire conncted, or are you taking the reading from the end of the disconnected black wire?

Sun Dec 07, 2014 9:20 pm

Voltage measurement reading I did both connected at the coil and from the end of the wire. Yes 12V at the front directly from the switch and 10V at the rear. The wire is 14AWG. I have ran the test wire from both the battery and directly from the switch even bypassing the fuse. Test wire resistance is zero, it is new.

Mon Dec 08, 2014 1:26 am

Can you check the following and report back your readings:
Voltage at the battery posts (measure to the 1st decimal position)? 12.6v is optimum.

If you are running a generator + VR under the rear seat, test the voltage at B+ terminal of the VR?

Voltage at the fuse box?
Two fuses (#8 & #9) should have power from the battery. Two (#11 & #12) should be powered from the ignition switch. The difference between these two should be small. If the voltage is different by more than a couple of 1/10ths of a volt that is too much.

Voltage at the #15 terminal of the ignition coil. Test with the ignition ON and while you are cranking the engine. If you can get your engine started test the voltage while the engine is running.

Test the resistance between the #15 (+) and #1 (-) terminals of the ignition coil with everything disconnected. This will measure the primary resistance which should be in the 3-4ohm range.

Resistance between the green points wire and ground. With the points open this should be infinite. Rotate the engine until the points are closed. Resistance to ground should be zero.

Finally, using a set of jumper cables, connect a known good battery to yours. Now double check all of the above readings and see if there is a change. If there is a noticeable voltage change it is an indicator you have a weak battery.
As you test further from the battery, you should measure slight voltage decreases. But more than 1.0v loss is too much.

Mon Dec 08, 2014 11:33 am

Ashman, thanks for helping me. Ok so after going crazy for a minute and taking a break I recomposed myself and went back to the basics. I checked everything (air, fuel & ignition). So after checking all my electrical connections and realizing (not to mention) my electric fuel pump being suspect. It had sounded like it was on and pumping but was not. I hit the carb with some ether and it fired right up! So I guess this was a bit of a electrical/fuel issue (joke/lol) anyway just glad you and the rest of samba are only a click away for troubleshooting our dubs, Thanks!

Tue Dec 09, 2014 2:49 pm

i see this kind of thing a lot when dealing with exterior low voltage circuits, bad connections/wiring causes a no load proper voltage but as soon as you apply load the voltage drops to nothing. Bad wire, bad ground,bad connection, or bad coil. You already tested wire, ohm out ign switch, search test coil, test grounds.