Electrical Troubleshooting

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Someone wrote -

I have a '69 bug which I recently overhauled. I took off the body and the instructions didn't have anything about unhooking any wiring. I managed to yank several wires off a small box under my seat and I can't seem to get the back on the right order. This is not the regulator, it is smaller and has four posts on it. I can only find 3 wires not hooked up under the seat. I also can't find anything on any wiring diagram referencing this box. Do you know what it is and how I can hook it up??? My car does nothing when I turn the key, not even any clicking. The starter is hooked up correctly and is brand new.

In responding to this question, Rob gave some great electrical troubleshooting advice -

Could the “box” you’re referring to be a plug or relay for a rear window demister? It might also be a "hard-start" relay - more on that later.

The only other "box" under the seat of a ‘69 bug should be the regulator, and since that and the wiring is attached to the body, not the pan, it should not have been affected by the body-off.

A couple of quick checks to fault-find the electrical system:

There should be a thickish red lead coming from the engine bay to the rear side of the regulator - that's from the generator.

There should be a similar thick red lead running from the battery to the front of the regulator. That's the lead which supplies power to the car. Another red lead is joined to this one (on the front of the regulator) and runs forward to the light switch (which is used by VW as a junction box). Have a look at the light switch from inside the luggage area - that lead comes in from the left side of the car. It then splits into two red leads on the light switch, one runs direct to the fuse panel (for "ignition off" power for interior light etc) and another runs to the steering column, then back up to the fuse panel (for "ignition on" power).

With a volt meter (or 12v test globe) test the battery + post and earth (body). This makes sure the battery strap has a good connection with the body. Got power there? OK - now test the joined red lead at the front of the regulator and the body. Got power there? (if not, check the battery terminal and red lead attached to that). OK, now we have power at the regulator, go to the luggage compartment and check the OTHER end of this lead on the light switch (the one from the left side of the car), and the body (earth). Got power there? Now check the other two red leads on the switch, and earth. If you have power all the way up to this point, you should be able to turn on lights, horn, interior light, brake lights etc provided the fuses are OK.

Now you need to make sure you have 12v to the ignition system and sparks coming out, so grab a tester (screw driver with light thingy in the handle) or any 12v bulb and try this -

With the ignition switched on, connect one side of the bulb to the thin black wire on the + side of the coil (black wires in VW-speak means "has power with the ignition switch on"). The other side of the bulb goes to the engine case. You should have the bulb glowing. If so, the coil is getting power like it should. If not, you'll have to find out why not before you can go any further. If using a screw driver type tester, connect the clip to any convenient engine case component or the tinware and touch the tip of the tester on the black wire terminal - it should glow (ignition on remember).

If you have power, then check that the points open as you rotate the crankshaft (by hand). If not you'll never get a spark (it's the points opening which creates the voltage spike through the coil to produce the spark.)

If the points are opening, then the next test is to pull the centre wire off the distributor (leaving the other end on the coil) and point the loose end close to the engine case or other handy bare metal around the engine (use insulated pliers to hold the wire - an 18000 volt shock is no fun!). Now get someone to crank the engine using the key - you should see a series of sparks jump the gap. If you see the sparks, then the coil is good; if not, you'll have to replace the coil.

Now check the plug wires. Reconnect the centre wire into the distributor and pull each plug wire off the plug in turn and try the spark test. You'll get fewer sparks (one forth as many for each cylinder compared to the distributor centre wire) but should still get them. You can use a spare spark plug for this test if you like -- rest the outer side on the engine metal and connect the plug wire to the top of the plug.

If you have sparks at the plugs, then you know you have no problem there. If you have sparks with the centre-wire test from the distributor but not at the plugs then it's either the plugs or the plug wires at fault.

If you have power to the coil, to the distributor, through the points and through the wires to the spark plugs, and if the plugs are firing as they should, the engine should be ready to start.

There is a faint chance the "box" you refer to under the rear seat is a hard-start relay for the starter motor (some folks think this will fix an ignition switch problem), so now PUT THE CAR IN NEUTRAL and apply the hand brake, and crawl under the right rear wheel so you can see the starter motor. Short across from the thin wire to the main starter cable terminal. If the starter cranks over, then you've got power where it should be. If you don't get any starter movement, get a long length of normal wire (doesn't have to be very heavy) and link the battery + post directly to the small wire connector on the starter. If the starter turns, then there's a break in the solenoid circuit (which SHOULD run directly from the ignition but might be running through a hard-start relay), and you've narrowed the problem.

If the box is a relay (they usually have a little circuit diagram on the side) then you can try removing it from the circuit. There would be one wire from the wiring loom near the regulator (that bring power from the starter section of the key switch) and one heading through the body to the small contact on the starter motor. The other lead should connect to the battery + post. And possibly an earth wire - connected to the body somewhere. If you can see the wiring looks like that - just connect the lead from the harness near the regulator, directly to the lead running down to the starter. That might just fix the problem.

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Dave was experiencing a problem with his battery continually going flat. -

Now for a little mystery. The car's been sitting for about two weeks without being driven, and the battery's gone flat again. The "Oil" light barely comes on; the "Alt" light not at all. The interior light was set to be on when the door is open; perhaps the door isn't closing all the way, I don't know. The light came on very dimly, but the ignition wouldn't turn over the starter motor at all.

Not being an electrician, a dumb question: Is it possible to run through the system with a VOM somehow to detect shorts? Although I think if there were a direct short the battery would run down a lot faster, like within just a few hours (or less). I've gone 3-4 days at a time between outings the last little while with no problem, but each time the car has sat idle for on the order of two weeks or more the battery as turned up flat.

"Speedy Jim" wrote, regarding testing for electrical drain from the battery -

Take the positive cable off the battery.

Connect the VOM meter from the cable to the positive post.

If set to DC Volts, the meter will respond to the slightest drain. If it shows a drain, switch to DC milliamps. Any reading over, say, 5 ma is suspect.

From that point you can disconnect things to find any culprit.

Rob added -

Make sure that you first test the door switches and if they prove OK, remove the light globe so you can test the other circuits without having to continually close that door to disable the internal light.

If it's taking up to 2 weeks or so to drain the battery the current will not be large (maybe as little as 200 milliamps (.2 amps) or so) so if it's a light globe shorting out it may not even be enough to make it glow.

The headlight switch on VWs is used as a junction box for the main power supply. You could also try pulling the thick red wires off the light switch one at a time (there should be three). The one tracking in from the left side of the car is the one from the alternator (comes through the floor of the luggage area just under the left side hood hinge as part of the wiring loom there). If removing that fixes it, the problem has to be in the alternator (that red wire brings all power to the front of the car - [and then back to the engine ignition and rear lights]).

Still draining? Then put the first red wire back on remove the red wire which disappears down through a hole in the luggage area floor above the ignition switch - this one takes power to most of the operating circuits in the car which work with the ignition on (lights, ignition, wipers, backup lights etc).

Still draining? Then put the second wire back and try the third wire, which leads off the switch to the fuse block. This supplies power to the non ignition switch circuits like the horn, interior lights and maybe radio. If the radio is an after market job it may have it's own in-wire fuse behind the radio - pull that and check for current drain too.

Don't forget to pull the separate fuses for backup lights and rear window demister too. This might help pin down the area if nothing else.

Then remove one fuse at a time on the main fuse block and test check the multimeter - this might narrow it down further. If not, remove each light in the instrument cluster one at a time and repeat the test (the current drain might be through a light but not enough to make it glow).

Dave tracked the problem down as follows -

This morning I went out to see if I could find the electrical "leak" using my volt/ohm/amp meter (VOM), using the instructions above provided by Speedy Jim and Rob. Here are the results -

With all the fuses in, the ammeter showed a 2.5mA drain.

With all the fuses out, the ammeter read 0mA.

The reading for all of the fuses, except one, was 0mA. Only with fuse #8 in the holder is there any electrical "leakage," and that at 2.5mA.

So - mystery solved! The things attached to fuse #8 are the emergency flasher, the cigarette lighter, and (I discovered after removing the fuse) the radio. I suspect it's the radio that's drawing the juice.

Oh -- the wire that connects the radio directly to the battery also pulls 0.68mA (I'm getting good at reading my meter! :-)

Rob responded -

That's not so bad - sounds about right for a radio memory. One of these days I'll treat myself to a good digital multimeter - I have a $9.95 cheapy which works fine but has limited settings - 10v and then 50v for example - not so good for accurate 12 or 24 volt readings.

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Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.

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Have fun fixing your VW - just keep them fweeming, OK?

Last revised 4 May 2004.