Approx. 1 volt drop between battery and fuse box

[Bill Crowell]

This question concerns my 1978 D-300, but I hope nobody objects because it has basically the same electrical system as other Mopars.

With the ignition in the "run" position but with the engine off, I get about a 1 volt drop between the battery and the fuse box. I guess I should try removing the fuses to see if removing one of them makes the voltage drop go away, but before I do, can anybody tell me if this sounds anywhere near normal?

I can make exact measurements at the battery and fuse box if necessary.

Does the ECU really load the system enough to cause that much voltage drop, or must something else be drawing current when it shouldn't? Or maybe a bad connection at the bulkhead connector?

Thank you very much for your thoughts.

 

[67Dart273]

That is absolutely NOT normal, and likely will cause a battery overcharge condition. My "usual instructions" for testing is to

Either find the "switch" side of the ballast or the alternator blue field wire. Connect one lead of your meter to that, connect the other to battery POSITIVE and turn the key to "run" with engine stopped. With the meter set appropriately, you are hoping for a VERY low reading, less than .3V (3/10 of one volt) ANY drop in this circuit ADDS to the voltage regulator setpoint and raised charging voltage.

Let's say that you in fact read 1 volt in this test. With the engine running and charging, and if the VR is set properly for nominal 14V, you will have FIFTEEN volts at the battery, while in operation.


I've posted many times on this, and the most likely candidates are the primary power distribution

In "sort of" order the suspects are

Bulkhead connector, fuse link, ammeter, right at the fuse box IE the wire to the fuse clips buss, and the "welded splice."

Start by reading this article if you have not

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

The example photo they show

looks EXACTLY like "several" pickups I saw in that age range where things like winches (some people call 'em wenches) or snow plow hoist had been installed

On some years, the dash construction is PLASTIC and the ammeter is a sandwich of parts held together by the tension of the stud nuts THROUGH that plastic. So if the ammeter gets a 'little warm" from high current, the plastic softens, and the connection gets EVEN WORSE which then gets downright HOT

Refer to their simplified diagram on the page:

Follow the "current path" from the battery.....starter relay junction........fuse link.......bulkhead connector......ammeter......welded splice......fuse box

Each and every place named above is a possible trouble place. You must go along each path, while under load, and check what your drop is and see where you are losing power.

PLEASE read the article carefully. It contains a lot of good info. No, lol, I did not write it.

Bad bulkhead connector terminals and the ammeter are covered.

Additionally, you can have drop to the voltage regulator / ignition circuit through the ignition switch and switch connector. One "way around" this is to use the ignition "run" feed to trigger a "Bosch" style relay and feed the ignition / VR off that

WELDED SPLICE. This is rare but does and has happened. In my lifetime, I've been aware of perhaps eight of these failed. There is a splice up under the dash in the black ammeter wire. If you suspect it, you must cut open the harness for a few inches, starting at the ammeter, and find it, and inspect it. Again this is RARE. Find and fix "everything else" and keep it in mind.

FUSE PANEL. For circuits (not ignition or VR) that go through the fuse panel, keep in mind the riveted wire connections and condition / tension /rust /corrosion of the fuse clips.

 

[Bill Crowell]

Thanks very much, 67Dart273. That was really helpful, and so was the article that you mentioned. I really appreciate it.

 

[jos51700]

Tony Defeo says that there's nothing better than an original, OEM harness...

 

[67Dart273]

Tony Defeo says that there's nothing better than an original, OEM harness...

We don't need to start in on this and if you do it will get ugly. I'm not taking either side, by the way

 

[Mopar Tim]

And where ever there is a voltage drop, there is heat associated with the loss. The more loss the more heat. Ohms law. An undesigned voltage drop will eventually fail. A cascading effect.

 

[TylerW]

I haven't had an old Mopar yet that didn't have a voltage drop between the battery and fusebox. Nothing to really worry about. If wires start to smoke, that's when I would worry.

If you want to be proactive before having an issue, follow the instructions and diagram the Professor up there ripped off from Mad Electrical for his lecture.

I like Tony. He's helped me out more than once

 

[RustyRatRod]

I haven't had an old Mopar yet that didn't have a voltage drop between the battery and fusebox. Nothing to really worry about. If wires start to smoke, that's when I would worry.

If you want to be proactive before having an issue, follow the instructions and diagram the Professor up there ripped off from Mad Electrical for his lecture.

I like Tony. He's helped me out more than once

I think you should try to be a little more condescending.

 

[67Dart273]

Rusty I just knew there was a reason o' Ty there was on my ignore list but I'd forgotten. Now, it seems, I might've had a reason.

So, Ty, ol UNbuddy listen the hell up. I didn't rip off a damn thing from Mad Electrical. In fact I give them credit EVERY SINGLE TIME I mention them, which in some ways surely generates more traffic for their website.

It's merely that what they have there are pretty fair ideas for factory wiring. It saves me one HELL of a lot of re-typing. It doesn't even apply to me as I rewired my car and won't even have either a firewall connector or an ammeter.

So you see, Ty, ol UNbuddy, you have nothing to say, as usual. Nothing, at all

 

[barbee6043]

back years ago when I could afford to restore a car and do a nice job, after dropping that fresh engine back in ( back then a B body), I always bought a repop engine wiring harness from the guy and his wife that made them, ( they always had an ad in MCG)... I forget their name, maybe still do..... main reason I did was the original harness was usually somewhere hacked up, the plastic covering the wire was hard, cracked and broke open in many spots. plus it made a restored car look " finished"..
I ask this question from you guys that are auto electrical knowledable, ( example: 67 Dart 273 for instance.), is that plastic covering that copper wire having those cracked places, does that really pose a problem with current flow? I realize most of the electrical problems we find is probably from the bulkhead connection to the interior.
I admit, i havn't bought a new harness since maybe 2003 or so. I get my herd running and driving with the old harness, and yes, seems like all my electrical grimlins always seem to stem from the bulkhead inside.