Voltage Drop

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340Dart

340Dart

Obsessive Dart Disorder
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Well I finally found out why my car wasn't starting. I checked the ignition 2 wire and there was power to it. I thought WTH. But when I put a volt meter on it and cranked it over the volts dropped to 7. MSD told me to put a toggle switch on. I really don't want a toggle switch. What would cause this drop in voltage? Is it the ignition switch? Thanks for all the help from before. I just want to get this fixed right. Thanks again.

Fred B
 
I think it could be a bad connection at the bulkhead connector, open insulation somewhere along the way that may be shorting out or you could be drawing an excessive amount of current from another circuit problem somewhere. I sure there are other possibilities as to the cause. Not fun to find and fix.
 
Use your volt meter to measure the voltage drop IN the wire, by measuring the voltage in both ends of the same wire while the circuit is powered up. If you check it in sections, you'll soon find where the voltage is being dropped, by working in sequential sections down the wire.

This is different than checking resistance. Good voltage drop is 0.2 volts or so. The more drop you measure, the more is "lost" within the wire.
 
Here's a diagram that may help.
Both wires would show minimal resistance due to the clean connection in the wires, but the lower image shows how corrosion or internal wire breakage can damage every wire but one, so resistance checks won't appear any different, but the voltage drop is significant in the lower wire.
 

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NOW FOR the correct answer.

It is important to understand how the Mopar starting/ run circuits work, and that the "ignition switch" is actually SEVERAL completely separate switches in the "can" that we call the "ignition switch."

1---ACCESSORY, hot in both "run" and "acc" feeds the switched buss in the fuse panel

2--IGN RUN, or IGN1, traditionally blue, hot ONLY in run NOT active when cranking.

3--START, traditionally yellow, hot only in "start", goes ONLY from switch, through BULKHEAD and to the start relay. On cars with seat belt interlock, is in series with the reset relay for the seat belts

4--BYPASS, or IGN2, traditionally BROWN hot ONLY in start and IS SEPARATE from "start" above. It only goes one place --from switch, through the BULKHEAD and to the coil+ side of the ballast resistor.

IT IS important to note that NONE of these circuits are fused. Even though the main feed has a "fuse link" I am personally aware of cars which have burned up an entire ignition harness before the lame-assed fuse link blew!!!

TO CHECK VOLTAGE DROP.

You have TWO circuits to check, "run" and "bypass" in start

To check RUN

Turn key to "run" with engine OFF. Hook one probe of your meter to the ignition buss under the hood. ON 69/ earlier cars, the IGN terminal of the regulator is OK, on 70/ later the blue alternator field wire is OK. IN YOUR CASE you should have a single point where the old ignition, the brown bypass, and the MSD "small red" are all spliced together. This is where you want to check

Hook the other probe to battery PLUS. You are hoping, here, for a very LOW reading, the lower the better. If you read over .3V (three tenths of one volt) you have a voltage drop problem.

Now check the same thing in "start."

On NON CD ignitions, hook one probe to the coil + IN YOUR CASE with MSD leave the probe at the brown/ red msd splice, and hook the other probe to battery plus. Disable the ignition by hooking the coil high tension wire to ground. Crank the engine, and while it is cranking, check the voltage. YOU MUST CRANK THE ENGINE by using the KEY and NOT by jumpering the start relay.

Read the voltage and again, you are hoping for a low voltage, the lower the better.

If you read "too much" voltage drop in either case above, you are checking the circuit "path:"

From battery -- fuse link -- bulkhead connector (main red battery feed) -- to ammeter -- through ammeter -- to ammeter black wire -- to factory welded splice -- to ignition switch connector -- through the switch -- back out the connector -- back through the bulkhead (run is blue, start is brown) -- and to the MSD "small red"

So your top suspects are:

bad connections in the bulkhead connector. Read this MAD article for the hows and whys. Even if you do not do this bypass, this article gives a good overview of the problems and a good simplified diagram

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

If you a factory wiring harness, there are at least FIVE possible bad connections that can prevent starting/ running

RED battery feed from the fuse link to the ammeter

BLACK actually only affects charging

BLUE ignition run

BROWN ignition bypass

YELLOW "start." would cause a "non crank" or intermittent

AMMETER connections, either bad wire ends, loose nuts, or problems internal to the ammeter

FACTORY welded splice. In my lifetime, I'm aware of at least 6 of these that have failed. This is a factory splice a few inches from the ammeter terminal in the black wire, which branches off and feeds headlights, accessory "hot" bus, and the IGNITION SWITCH power feed, and one or two other things.

SWITCH CONNECTOR. If the IGN switch connector gets hot/ damaged/ burned, you won't get full voltage to/ through the switch, or depending, OUT of the switch

SWITCH. The switch contacts themselves can become damaged. Since there are several separate contacts, this means it can manifest in several ways.



http://www.madelectrical.com/electricaltech/amp-gauges.shtml

amp-ga18.jpg
 
67Dart273 I did just like you said and tested in run and start. I got .5 volts on both tests. I checked the wiring and it seemed OK. I already bypassed the brown wire from the bulk connecter before so its not going through there. I had my buddy wiggle the key to see if that did anything. Sure enough if fired but that was it. Is this my problem? I tried doing what the tech at MSD told me. I put the red wire from the box on 12 volt source and it fires right up if I jump it at the starter relay. It seems I'm not getting anything out of the ignition switch. I hope this is it.

Fred B
 
OK, you can easily check. Really, your top suspects are

the bulkhead connector

the ignition switch connector

the switch itself.

Just check all of them the same way. 1/2 volt is too much. It won't keep the car from running if it is ALWAYS 1/2 volt, but that may change, because a bad connection that causes that voltage drop usually is corroded, loose, and heats up, etc. So sometimes it may get worse, or lose connection all together.

OK----you've checked out in the engine bay, now check at the ignition switch connector, because that's easier than probing the interior side of the bulkhead connector. You can access the ignition switch connector under the column, and "back probe" both sides of the connector.

You can have low voltage coming in on the battery feed TO the switch connector, drop INSIDE that connector, drop in the switch itself, or in the connector again going "out" to the ignition feed

So check voltage with the key in "run" on both sided of the connector at the battery feed. If voltage is down TO the connector, you have a bulkhead connector problem, etc.

If voltage is the same at the switch side of the connector at BOTH the battery infeed, and the "ignition run" terminal, then you have no problem in the switch.

I've forgotten---what model year is your car?
 
340Dart said:
The steering column and wiring harness are from a 71 Dart. And thanks for your help.

Fred B
Click to expand...

I was goin' to look up the wiring diagram. What do you have for a diagram/ shop manual?
 
Ok I checked at the connector. I have 12 volts at both ends of the connector at the red wire. I didn't know how to check the red for .3 or less. The other ones it was .1 and .2 volts at both ends. I don't remember what color the wires were because my meter broke and it was freezing so I didn't get to check them again. So the wires in the engine compartment were .5 volts. I already have the red and brown wires bypassed from the bulk head from before. Does this mean I still have a problem at the bulk head?

Fred B
 
It could be right in the blue wire, that is, the bulkhead terminal that feeds that wire through.

Did you check this with some load? If everything is "off" there is no load, and nothing to cause the drop.

Think of this as a line on a map, and you have a leaking fuel tank, LOL

You start at the battery, and end at the load. In the case of the blue run wire, the load, with the key in "run" is the ignition system (orginally, in the case of MSD, you are only triggering an internal relay) the alternator field on 70/ later cars, and the regulator IGN and field circuit, any year. On later "smog" cars, some have electric chokes, etc, that might add to.

So your path is battery -- fuse link-- through the bulkhead (red wire) -- to and through the ammeter -- out the black ammeter wire -- to the in harness splice -- to the ignition switch connector -- through the switch -- out the switch connector, now on the dark blue "run" wire -- back out the bulkhead on the blue -- to the ignition buss.

The brown is a simpler circuit, because it comes from the switch (in start) --through the bulkhead -- to ignition, so it must be checked "in start."

Something I forgot to mention is "have you" added any heavy loads to the ignition run circuit, like, for example, are you trying to power an electric pump (without a relay) directly off this buss?

So back you your "road map," establish a load on the circuit you are checking. In the case of the "run" circuit, turning on the key will cause the alternator field circuit to draw a little, not much. So just check each point in the path I mentioned above.

The EASY way to check drop directly is to make a big long no14 clip lead and clip it to one probe of your meter. As you stick the remaining probe into the various points of the "path," you will directly read voltage drop, the lower the better.

Let's say (because the MSD doesn't draw much) that you clip an additional load to ignition, like an old headlamp. You can clip in onto the blue alternator field connection, example.

So you check at the bulkhead in the engine bay. You think "could the bulkhead terminal be bad?" But it's difficult to crawl inside and get to that terminal, so what's easier? Let's check at the switch connector. Maybe there's minimal drop there, so what's between? The bulkhead. Now you MUST crawl up in and probe the interior side of the bulkhead. Yet another thing is the "wiggle" test. Clip your meter out on the load end and wiggle the bulkhead connector. If it changes much, that means something. Ok, BUT is it the blue, or the battery feed (red?) Simple. Go to the IGN switch feed (because it's easy) and recheck while wiggling the bulkhead If it still changes, it's probably the red wire coming through the bulkhead.

ANOTHER easy access place is the alternator stud. With your "ignition run" load in place, you SHOULD have no real drop on the black wire from the alternator. If you compare the feed into the IGN switch to the alternator stud, "what's in between" is the bulkhead connector (black), "in harness splice" and bulkhead connector (red) So by disconnecting/ connecting your "ignition run" load, if that drop CHANGES much at the alternator, you have ELIMINATED the following:

Blue wire from the load to the switch, and up from the switch to the "in harness splice."

The ONLY way, by the way, to check that splice is to pull the cluster, and cut/ untape the black ammeter wire down into the harness until you get to the splice.

Hope I'm not confusing you further. When you post voltage readings you must post exactly where the probes are connected, and exactly what is on/ off in the system.
 
I had the key in the run position. I didn't connect anything to the ignition circuit. I will try and test more. It just might be the blue wire. Whenever it warms up I will try and test like you said. And thanks for your help. I will let you know and tell you where I connect the probes and whats on and off.

Fred B
 
Not to contradict any of the above, it's the result of lots of experience, but one quick way that I've used to check the bulkhead connector is to FIRST disconnect the battery ground cable, them simply unplug the bulkhead connector from under the hood. Look for any damaged or corroded brass parts. If they all seem OK, spray some WD40 into the connector on the firewall and plug and unplug the engine side several times. Reconnect the battery.

Maybe that will clear your problem, but THIS SHOULD NOT BE TAKEN AS A FIX!!! It might narrow down the problem area(s) a bit. All the electrical connections and all the ground leads need to be clean and tight! If you find burned parts in the bulkhead connector, you have your next project laid out for you.

ATB

BC
 
Well I bypassed the bulk head connector. All wires are spliced together. Checked wires going from ignition switch to connector. All is good. Turned the key and she fired right up. Starts every time. Thanks for all the replies.

Fred B
 
great job! i wouldnt stop there now that you have it working... i'd make it dependable.

first you should make sure your block is grounded to the firewall well and to the frame.

also if you google "mad electric" and go through all the writeups there he has lots (tons) of tips.

all those connectors in the bulkhead can be cleaned. you can use electronics cleaner in a can (watch your eyes!), and also you can clean them manually one by one but really the main two to worry about are two fat wires... the one that goes to the alternator and the other main feed line.

also make sure your connectors on the alternator are in good contact and clean.
 
My 73 Duster has finally succumbed to the dreaded corroded spade terminals poking through the firewall. I bypassed the alternator terminal about three years ago to fix a charging problem, now the car will not run in gear with the headlights on. Reached under the dash and touched the main battery feed terminal with car running and it was too hot to touch.

Think i will bypass the blower motor wire too while I have it apart.

40yrs was a pretty good run and it will be a cheap repair sans the back ache afterwards. This must be a very common problem with these cars now.
 
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