MAD ammeter bypass question

-

1MeanA

Well-Known Member
Joined
Jul 22, 2009
Messages
1,729
Reaction score
1,628
Location
Edmonton
I was wondering what the electrical gurus think about my plan. Basically I just want to reduce the amps going through the firewall. Will this accomplish that goal safely?
  • I have a stock 340 Dart with a new engine harness.
  • I plan on keeping the ammeter functional at the moment
  • I am going to add a 10 AWG bypass wire from the alternator to the battery per the MAD instructions.
  • If I am correct this will reduce the amps running through the bulkhead although I don't know by how much.
  • The instructions show the use of 10 AWG wire with a 14 AWG fusible link which is basically a slow blow 15 amp fuse but I was thinking about adding an ATO fuse (15-30 amps?) like the Little Fuse MAX30 instead. Nuisance fuse blows during high charge events would render the bypass useless so I would like to prevent that.
Thoughts?
 
No10 may be too large to keep ammeter (somewhat) functional. "Experiment" or longer wire. But you cannot "just do that." You need a fuse/ breaker for one, and for second, you STILL MUST repair the ammeter/ bulkhead connector/ other connections in that circuit if they are at all compromised
 
The connectors are all fine. Also the current through the gauge will be less than stock. It’s just the fuse type and size I am mostly concerned about.
 
I'd go with 30 but have you done anything to protect the original wiring? The fuse link is darn poor protection, and I have an annoying old story about that

In the early 70's some girls slammed into the rear of my 70RR while stopped in traffic. The Plymouth dealer gave me a beater loaner Plymouth Valiant loaner. One cold morning (yes, in San Diego) the CPO came in and said something, turns out the alternator had froze and the ragged belt was making smoke and noise. I shut it down, took a rag and unfortunately wiggled the pulley. Turned out a diode had fallen out of the mount, down inside, and when I moved the pulley. I got to stand there and watch the underhood harness burn down, and after about 5 seconds, it finally blew the link.
 
If you use a 10 or even 12 gauge wire your ammeter isn't going to do much of anything. But it's fine, I ran my car like that for years.

You are correct, adding the shunt wire will greatly reduce the load on your bulkhead. A fuse is an important thing to include, but I don't know if I would use the MAX30 in a stock looking engine bay; I would probably go with a fusible link. With that said, I used a newer style fuse, but my setup is way far from stock.

It's probably easiest just to watch the DDG video from a couple weeks ago; he covers just about everything:

 
If you use a 10 or even 12 gauge wire your ammeter isn't going to do much of anything. But it's fine, I ran my car like that for years.

You are correct, adding the shunt wire will greatly reduce the load on your bulkhead. A fuse is an important thing to include, but I don't know if I would use the MAX30 in a stock looking engine bay; I would probably go with a fusible link. With that said, I used a newer style fuse, but my setup is way far from stock.

It's probably easiest just to watch the DDG video from a couple weeks ago; he covers just about everything:

QUOTE]
I’ll give it a watch. I was going to go with the fusible link until I looked into links vs fuses. I’m not sure why the MAD instruction recommends a #14 link which is only 15 amps. Maybe it will survive?
 
Last edited:
@1MeanA , I did the whole MAD upgrade, including the ammeter bypass, and I wondered the same thing, why not just use a fuse instead of the fusible link? It actually would have been easier to wire in a couple of fuses than fusible links.
My guess would be to keep a more factory looking appearance.
 
Last edited:
I’ll give it a watch. I was going to go with the fusible link until I looked into links vs fuses. I’m not sure why the MAD instruction recommends a #14 link which is only 15 amps. Maybe it will survive?

I think in most cases 14 awg will support 15 - 20 amps on a short run of wire. The fusible link is intended to protect wiring that's 2 sizes smaller than the link, and since 10 awg supports 30 amps it's right in line with the needs. Your factory fusible link is probably 16 awg.
 
Thanks all. I will have to decide which way to go.....probably not my biggest problem these days :) I'm not too worried about aesthetics as I can hide most of the wire and the fuse fairly well. The idea of running all the current through the firewall has never made sense to me. I read up a bit on fuses vs fusible links since its not an area I have delved into much. Fusible links, although crude devices, have some advantages in certain applications like when the load might be high momentarily. They also have less connection points. Perhaps the slow blow fuses provide the same sort of protection. For example the MAX20 fuse protects for current over 20 amps but can take 30 amps for 20 seconds. That seems like it should work.
 
I drill through the bulkhead and run wire straight through and eliminate that fire hazard, I started doing this after motorhome caught fire, Joe
 
The idea of running all the current through the firewall has never made sense to me
That's a misunderstanding.
MAD is a poor attempt to redirect the recharging current.
If you want to know how much current is going through the ammeter and wiring to or from the battery at any time, just look at the ammeter.

Once the car is running power comes from the alternator. Some goes to charge the battery (if it needs it) and the rest goes to whatever electric items are running. This can be anywhere from 5 amps (ignition & field) to around 25 amps (lights, heater, wipers on). If you want to reduce the maximum current the wires, and especially the connections at the firewall will see, then install a headlight relay harness. That's 9-10 amps that will never go through that connector (unless your alternator fails and you're running on battery). The relay harness will also address the increased load on the headlight circuit caused by replacement headlights drawing more amps than the original 6012 lamps.
 
That's a misunderstanding.
MAD is a poor attempt to redirect the recharging current.
If you want to know how much current is going through the ammeter and wiring to or from the battery at any time, just look at the ammeter.

Once the car is running power comes from the alternator. Some goes to charge the battery (if it needs it) and the rest goes to whatever electric items are running. This can be anywhere from 5 amps (ignition & field) to around 25 amps (lights, heater, wipers on). If you want to reduce the maximum current the wires, and especially the connections at the firewall will see, then install a headlight relay harness. That's 9-10 amps that will never go through that connector (unless your alternator fails and you're running on battery). The relay harness will also address the increased load on the headlight circuit caused by replacement headlights drawing more amps than the original 6012 lamps.
I did understand that the headlight relay is a great way to reduce the current. Up north here I will rarely need them and I think they will remain stock. However, in a situation where the battery needs charging, then there will be higher current draw as you mention. I've checked out all the connections and the engine harness is new so I probably don't need an ammeter bypass or this shunt but while I am redoing everything why not?
 
One reason is that the ammeter will no longer function so you will know longer know whether and how much the battery is charging or discharging.
A voltmeter can indicate whether the alternator is producing power at the regulated voltage (depending a bit on where its measuring). So we can assume the battery is charging properly when we see 14ish Volts on meter. That makes it resaonable substitute. But just as the ammeter can't directly indicate system voltage, the voltmeter can't tell us if the battery is actually charging or or overcharging. There are advantage s to each.

MAD's method is poorly thought out. It sends power to do everything all the way to the battery, then through a bunch of connectors and extra fusible links. So that's a long path with lots of connections. And if there is a battery short, now there are two links in parallel ??? so that means twice as long to burn through. On older vehicles the purpose of the alternator is to run the car, any accessories, and recharge the battery after starting.

From Chrysler's 1960 Master Tech booklet.
View attachment 1715892982

The wiring scheme changes slightly over the years but the overall strategy always used the ammeter in the same manner. It only shows battery discharge and charging. The 1976 a-bodies got a remote shunted ammeter. Instead of using a wide metal plate in the ammeter, most of the current flowed in parallel through a wire of known length in the engine bay.


This is a typical standard wiring scheme. Of course details vary with years and options.
View attachment 1715892983
Power to run the car is supplied by either the battery or the alternator.
If the alternator can not provide sufficient energy at approximately 14 Volts, its output voltage will drop.
The battery begins to assist when the its voltage and the alternator's are equal.
It takes over when its voltage is higher.

Chrysler had some better ways to address large power draws, some of these heavy duty wiring schemes were only offered to fleet buyers, but others came as a standard option for everyone. The one we've seen the most on FABO came on cars equiped with electric defrost grids on the rear glass, starting may around '72.

In 1975 Chrysler split the alternator feed on the engine compartment side for all A-bodies. So that reduced the load on the alternator output wire going through the firewall connector.
In '63 and '65 they used a ring terminal connector instead of a push on 1/4" spade. But even those can be melted given enough current.

that's prob more than you wanted but hope it provides some insight
 
High charge events are hard on everything; the wiring, the connectors, the alternator, and the battery itself.
There's a recommended max charging rate for lead acid batteries that can be estimated from the amp-hr rating of the battery.
 
Does nobody here follow what @72RoadRunnerGTX has posted about this bogus / red-herring ammeter bypass?
First Ive heard of it. I dont claim to be an electrical guru, or know near as much as some of the other members on here, but I did recognize my wiring frying/smoking/fizzing just as described may happen and was directed to that bypass by some very knowledgeable people whom Ive come to trust on here.
 
I believe this was the first video posted about it:



I recommend that people have a look at this thread:

 
When there is a problem, you need to find the cause.
If there is a leak in the windshield by the instrument cluster - then the connections will get damaged.
Or if there is a high current for extended length of time.
Or there are people who didn't realize there was a dead short and fried wires.
Posted about this dozens of times. What can I say. Insanity is powerful. People prefer instant answers.
A broken clock is right twice a day.
My suggestion is to learn how the system works, and that provides a tremendous power in being able to troubleshoot.
 
Posted about this dozens of times.

Most auto ammeters are is a metal bar with two studs pressed in. The magnetic field from the moving electrons makes the needle move.
There's really nothing to it. No switches, no transistors or resistors. As long as the studs are well connected it can handle a normal battery charging with no resistance (and therefore no heating).

It's not usually the ammeter that fails first when someone ignores the fact that the meter is showing excessive charging or discharging. But when that circuit does see high current for long periods of time, stuff starts to fail. If the meter's studs get loose, then there is resistance and its all downhill from there.

MAD's 'fix' is send the power to the battery first and everything else second. This creates a really long path, with more connections. It now requires the all of the current flowing from the alternator to go through at least one fusible link. Every link is a restriction.

Many people tackle this task with limited or no experience or tools, nor have the interest in buying good electrical crimpers and terminals etc. needed. So poor design and poor workmanship 'fix' a problem that didn't exist. Then they get in real trouble when stuff fails. Boo.

Redfish (who worked at Chrysler dealerships) has written that a common problem in early 70s models is a leak: mentioned that here
[Found!] - Ammeter Gauage for 74 Duster
and here
Electrical gremlin

Here's photo of a '71. Is that an ammeter fault or a corrosion problem at the connector?
1738894419194.png


Here's an example where the OP ignored the ammeter while breaking in his engine.
Here's photos of the melted wire connectors, including a ammeter connections.
1738894382201.png


Here I snipped a bunch of posts from threads on the IFSJA (full size Jeep) forum where people got themselves in trouble following various ammeter delete ideas.

We've had at least two people post that when they, or their dad, with many years working service at Chrysler dealerships never encountered this supposed massive ammeter fireball problem. Here's one please remove


As mentioned earlier, use the ammeter (if there is one) to monitor battery charging - especially after a jump start.
 
Last edited:
Not a fan of ammeters. The OEM system is a poor design IMO. The Bulkhead and low alternator output at idle are the killers on the systems.

Anyone that says an ammeter isn't ever or won't be a problem is fibbing to you. Seen plenty of issues with them and wiring connected to them in 50 years of this stuff.

Your car, do what you like. Want to run an ammeter fine, want to run a voltmeter, fine.

Pick your parts, pay your money.
 
-
Back
Top