TheSamba.com :: View topic - Is this too much voltage drop at ignition switch output?

Thu Feb 18, 2010 7:11 pm

Cleaning up some wiring today, and noticed that the wire from the ignition switch that powers the coil, horns, and warning lights has 11.4 v, while the large wire from the battery to the ignition has 12.65 v. Is it normal for the ignition switch and related wiring to drop the voltage that much? Car is 74 Ghia that starts and runs fine.

Thu Feb 18, 2010 7:54 pm

If everything works, don't worry about it.

Thu Feb 18, 2010 8:36 pm

1.25v drop from the input to the output? That's crazy high in the world of electricity. I've never measured mine or any other Dub, but that's totally unacceptable by all standards that I know of. Most switches will have a voltage drop of .1 or less, even when handling a considerable amperage (within spec). On the wide side you might allow .5v, but that's still high. Try wiggling the key while you're doing the test and see if it changes, it sounds to me like your switch contacts have been overheated. Now I'm going to have to go out and test mine......!

(I teach automotive electrical classes, wrote a training manual on automotive electricity, and have run VD tests on MANY switches under load.)

Good Luck!

Thu Feb 18, 2010 8:52 pm

Bruce Amacker wrote: 1.25v drop from the input to the output? That's crazy high in the world of electricity.

That sums up my feelings on it too.

Are you taking these measurements at the switch or on the other end of the cables? Either way I would think that it needs to be fixed, just get it narrowed down as much as possible. To be fair, I am new to bug electronics, but just in general that is a huge voltage drop.

Thu Feb 18, 2010 9:20 pm

I'm testing the voltage at the fuse block and the wires that terminate there. Big red wire from battery that is input to ignition sw has 12.65 v, and wire from sw has 11.4. I doubt the wires are loosing that much, so best guess is sw contacts are not good or dirty. It would be useful if a couple of others could test their voltage at these points.

Thu Feb 18, 2010 10:02 pm

I need to rewire my entire 69, the PO did not even crimp wires correctly..I have had 3 pull out while normal driving. 1 being from the alternator..that was fun. Im scared to think how much voltage im losing considering the PO went Fromt he battery..about 1ft of wire, then its spliced together to another foot of wire, then another splice, then wire. WTF...

Thu Feb 18, 2010 10:34 pm

I have an expensive Paladin 8000 crimper, but still do a pull test on every crimp. Have had a couple that pulled out, so I don't trust them to be 100%. That's why I was checking for voltage drop today. End to end resistence is also a good test to make on every wire.

Fri Feb 19, 2010 8:29 am

My opinion is that is a pretty big voltage drop for short lengths of wires behind the dash.

BUT, unless you have a nearly new wiring harness your wiring is probably original which would make it at least 30-yrs old (my guess). For 30-yr old wiring to have increased resistance (and lower measured voltage) is to be expected.

Determine if it really is the ignition switch by testing the voltage at the switch contacts and not 2-ft away at the fuse box.

You may find the switch is good and you just need to clean up some terminals/wires in between.

Fri Feb 19, 2010 12:43 pm

I was thinkin' that if the voltage drop was really that much that it wouldn't start up.

Fri Feb 19, 2010 6:41 pm

Yes, it will still start, however not reliably once you get a couple of volts or more of VD. I have measured fire truck power switches (rated at 1000 amps) that had a full volt of drop and worked reliably. I still suggested they be replaced/updated.

It is possible the VD lies in splices near the ignition switch, but IIRC the switch comes with 1-2 feet of 10(?) gauge wire that hooks directly to the headlight switch for the power input and directly to the fuseblock for the key on output. At least that's how my '69 was when I replaced the harness last year. The power circuit feeding the ignition switch and power output to the coil both have no circuit protection on them which is a big engineering mistake from VW. This means if anything shorts out in the power feed it smokes the 10ga red feed wire and if the coil feed wire (black 14ga?) shorts out at the back of the car, the weak link is the ignition switch contacts, taking them out. That is what I suspect happened to your car- the power feed at the coil/choke coil/idle solenoid shorted, damaging the ignition switch contacts. When I did my harness, I added a 40 amp maxifuse at the power supply near the battery/starter and a 15 amp inline fuse on the ignition switch output before it connects to the fuseblock. Even though the ig sw output connects to the fuseblock, some of the circuits are unfused (do not go through a fuse, only using the fuseblock for a connection point). I strongly suggest anyone liking their 'Dub to add these circuit protection devices too....

Good Luck!

Sat Feb 20, 2010 8:32 am

I noticed the same lack of fusing on the ingition output, so may correct that as you suggested. I didn't think about a fuse closer to the source though. Exactly where did you put the 40a fuse? Not on the main battery cable?

Considering the frequency of wiring problems on VW's, when I move the battery, I'm going to put a marine type quick disconnect switch on the positive battery cable. I can't imagine anything more frustrating than see or smelling electrical smoke and not having a wrench handy to disconnect the power.

BTW Bruce, I used your example on the seat rail mod to get my seat back about 2" more. That plus a smaller steering wheel soved the knee room problem. Now if I could just find more room for my size 14 sneakers.

Sat Feb 20, 2010 3:51 pm

donmurray wrote: I noticed the same lack of fusing on the ignition output, so may correct that as you suggested. I didn't think about a fuse closer to the source though. Exactly where did you put the 40a fuse? Not on the main battery cable?

My battery has been moved to under the RR seat so my power supply to the voltage regulator has changed: the power feed harness was tapped off the starter hot stud with a 10ga eyelet, run along the bottom of the "firewall", attached to the area above the bellhousing with cable clamps and self-drilling screws. It would have been easy to put a fuseholder in here, but accessing the fuse to replace it would have obviously been a bitch. For the fuse I ran a 10ga wire from the starter stud forward, through a grommet to the area under the seat where the fuseholder is located. From there, the power feed goes back through the same grommet, back near the starter, and continues its new path above the bellhousing, through a grommet into the engine compartment forward of the VR, and to the VR. I could have put the fuseholder near the VR but then about 3 feet of wire would have no circuit protection. If it blows now, it's easy to change under the rear seat.

You might ask why I tapped off the starter hot stud when this power feed is merely inches from the hot post on the battery- why didn't I just pull my power from the battery? Anything in the vicinity if the battery has a tendency to corrode, so it will last longer tapping from the starter stud. I do have the (sealed) fuseholder wrapped in plastic to prevent corrosion, and all splices are heat shrinked. I really should figure a way to vent the battery outside the cabin, but haven't gotten around to it yet. Late sealed batteries don't really vent many fumes these days, anyway, especially if the charging system is working properly and the battery is not worked hard.

I recommend using a 40A sealed Maxifuse for two reasons- you can buy 40A regular (ATO/ATC "blade") fuseholders, but we have a problem with 40A regular fuseholders melting the terminals (without blowing the fuse) on IH trucks running a 40A fuseholder feeding the ECM. I replace these with a 40A Maxifuse holder with excellent results. I used a current clamp and an oscilloscope to determine what the amp draw was on that circuit, and it reached about 28 amps when the starter was engaged. That's why I chose a 40 amp fuse. It's been that way a year now with no problems.

donmurray wrote:

Considering the frequency of wiring problems on VW's, when I move the battery, I'm going to put a marine type quick disconnect switch on the positive battery cable. I can't imagine anything more frustrating than see or smelling electrical smoke and not having a wrench handy to disconnect the power.

I have one on my other car, but there isn't enough clearance in this car- it looks like it would rub on the seat. I think I have a 65 series battery, maybe changing to a side post would allow a shutoff to be installed. PS, don't put it on the positive side, put it on the negative. It won't fit the positive post, and the positive side corrodes much more than the negative side.

donmurray wrote:
BTW Bruce, I used your example on the seat rail mod to get my seat back about 2" more. That plus a smaller steering wheel solved the knee room problem. Now if I could just find more room for my size 14 sneakers.

Yea, my stompers are 14-15s, too. It is a bit challenging, but not bad. For longer drives I take my shoes off.

Good Luck!

Thu Apr 08, 2010 1:11 pm

What's the standard battery cable gauge for the battery to starter connection?

Here's a battery disconnect switch that is small and not too expensive. surface mount, and the connector posts are long enough to accomodate more than one lug, which could be useful if you want to put the switch on the positive side.

http://www.westmarine.com/webapp/wcs/stores/servle...assNum=297

Thu Apr 08, 2010 4:48 pm

4 gauge is the common size stocked at FLAPS and plenty big enough for most 'Dubs, anything bigger than that is probably overkill. WalMart has an inexpensive plastic marine shutoff switch for a few bucks, have you seen them? It has a removable plastic key for security. I use them on my trailers that have 12v battery powered electric winches on them. The studs are long enough to accommodate several lugs. Harbor Freight has them also: http://search.harborfreight.com/cpisearch/web/search.do?keyword=battery+switch

Battery Cut-Off Switch 92688-0VGA $4.99

Heavy Duty Battery Cutoff Switch 66789-0VGA $7.99

BTW, back to your original post, I measured my ignition switch voltage drop at .24v with the engine running. I don't know how old the switch is but it looks like it has been replaced somewhere along the way.

Good Luck!

Thu Apr 08, 2010 5:18 pm

its amazing how much voltage is lost through the shitty little contacts in even a brand new german switch

i put a new switch and barrel in mine and even then my volt meter used to only show 12.8V with the engine running and only 11.6 when off

i used a relay to switch the ignition circuit on and the difference it made was amazing
volt meter sits steady at 13.8V now and my windscreen wipers work so much faster, my charge light no longer fainty glows with my dash fan going with the lights on either

when i still had a vw engine the voltage to my coil jumped nearly 1.5volts too

Thu Apr 08, 2010 8:45 pm

Thanks Bruce, I'll look at the Walmart switch.

I now have a digital volt gauge installed reading on the ignition output side, and with the engine running it indicates 13.7. With engine off it shows 11.5, with a multi meter reading of 12.5 on the battery lead. Any idea why the ignition output voltage is so different between engine off and engine running?

Joel, is your ignition relay powering the coil or the starter? Your results seem a mystery to me. If the relay is power to the coil it shouldn't have much effect because coils, at least when running, don't usually draw much current. If the relay is controlling the starter it should only have effect when the starter is engaged. What am I missing?

Fri Apr 09, 2010 8:09 am

I’m not sure the way you’re hooking up the voltmeter is correct. Put one lead of your DVOM on the input side of your ignition switch, which on most VW’s is the red fat wire attached to the headlight switch. Put the other lead on the output side of your ignition switch, which is commonly the first two fuses in the fuse box fed by a black 12/14 gauge wire. Start the engine, the meter should read .5v or under with the engine running.

As far as VBAT goes, it should measure 12.5-12.6v after the car sits overnight. The reason you need to let the car sit overnight is there will be a surface charge on the battery which takes time to dissipate (or you could run the lights and wipers for 5 minutes to remove the surface charge). If you check your VBAT just after running the engine, it will show a false high voltage of 13 or so. When you run the engine, your VBAT should be 13.5-14.5v above idle (generators don’t charge at an idle). Because VW mechanical regulators are not as reliable as modern electronic regulators, I commonly monitor my charging voltage with a little voltmeter I plug into the cigarette lighter I installed (hidden) under the glovebox:

http://cgi.ebay.com/Vector-VEC008-Digital-LCD-Volt...25595ecdc3

Why you are showing 11.5v at your ignition switch output confuses me, as there should be no voltage here at all with the key off. Is your battery running dead overnight? Perhaps you are not connected correctly.

Good Luck!

Fri Apr 09, 2010 8:19 am

It sounds like Joel has added a relay just after the ignition switch and before the fuse box. That should provide good clean current for all electrical systems except for headlights, wipers and E-flashers which get power from a battery cricuit (#30). The rest all run off the #15 circuit in some way (at least all the ones Joel listed).

In my case, I installed a relay in the engine compartment to get max current to the coil. It was getting down to 10v over all the old wiring.

I have a caution when installing a relay to boost current/voltage in the ignition circuit... don't create a "sneak circuit".

The coil stops working below around 9v. The carb's idle cutoff solenoid also stops working at around 9v.

"So what?" you may be asking since turning off the ignition switch cuts all power to the ignition circuit and the voltage goes to zero, all of these things should shut off and turn the engine off too.... not always.

At the moment you turn off the stock ignition, the #15 circuit from the ignition switch stops providing power to the #15 fuse box circuit and the ignition coil.... but the alternator/generator are still turning and making voltage thru the blue field wire that runs to the Gen light in the dash. This Gen light is also connected to a common 12v+ wire that comes from the #15 circuit of the fuse box. The same connections that the ignition switch powers. So the alternator/generator is now providing the electricity to the #15 circuit thru the Gen light and depending on where you installed your ignition relay... there may be enough voltage present to keep the relay closed (Bosch-style relays only need 7.5v to remain energized).

On my car, the Gen light would glow and the engine would continue to run. If I stepped on the brakes or turned anything on, the voltage drop would cause the relay to power off and the engine would die. But it was very scary to see the engine continue to run without a key in the ignition.

My fix was to add a 1A diode (1N4001) to the fuse box side of the Gen/Turn/Oil lights. This would allow + power to flow from the fuse box towards the lights, but not flow backwards. Now everything works as it should.

Good detailed explanation here:
http://www.mgexperience.net/article/mgb/ignition-run-on.pdf

Fri Apr 09, 2010 10:15 am

Bruce, Think we're using the word "output" differently. By output, I mean power from the ign switch when the sw is on. My digital gauge has the power wire connected to this ign output, and the gauge is grounded to a ground buss. Then with the ign sw on, but engine not started, the gauge reads about 11.5. After starting, and off idle, the gauge reads 13.7.

When testing the ign sw with a hand held meter, I ground the multi meter to the body and put the red probe on the input wire from the battery and get 12.5. Red probe on the ign output wire and get 11.5. This done with engine off. The numbers from the gauge and the multimeter are same, engine running or not. I just don't understand why the ign sw would have a 1v drop with the engine off, and none with it running. With the sw on and engine not running the coil must be pulling the sw output voltage down. I will test with the coil wire unplugged. I'll be back!!!

Fri Apr 09, 2010 6:01 pm

Ashman is right, im running a nice thick wire straight from the battery to the fusebox powering the ignition circuit and just using the ignition switch to control the relay

The problem is, that by the time the power goes from the battery at the back to the ignition switch in the front, through the crappy little contacts in it then all the way to the back again for the coil, plus add the load of windscreen wipers, turnsignals, radios etc thats where the voltage drop comes from

by using a relay im sending strong battery voltage straight to the fusebox so theres no voltage lost through the ignition switch and it also takes the load off it the switch which will stop it burning out over time
most cars built in the last 20 or so years are setup like this with an ignition relay

i always keep a spare relay in the glovebox in the event should it ever fail, they're only the size of a match box

oh yeah, i also run a relay for the starter solenoid (aka hard start relay) i think those are a mandatory item in an old vw