ELECTRICAL PROBLEMS 1972 F100
Logistics Pro
Joined: Mar 2009
Posts: 4,860
Likes: 612
From: Minnesota
Sounds like the MSD Box is bad. The smell isn't always an indication of failure as old electronics do smell. If the box can be opened, do an inspection and see if you can find a burnt component of most likely a failed capacitor as a 30 year old device is past its expected service life.
Laughing Gas
Joined: Apr 2024
Posts: 812
Likes: 267
From: Laramie Wyoming
if you want to get rid of the bad msd box let me know.
Logistics Pro
Joined: Jun 2003
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5th Wheeling
Joined: Oct 2025
Posts: 40
Likes: 4
Removed old MSD box and removed the lid, a large capacitor and two diodes were visibly burnt . Took some time to install the new box as the hole pattern for the box is 1/4" wider and it has a separate plug and not hardwired like the old one. It is now running and all is well ... sort of
Had the ignition switch hanging below the dash to make it easier to probe the wire, while turning the key to start it contacted the dash and some sparks occurred and some smoke, truck runs fine and starts fine. But the alternator gauge is pegged on the "C". Fried the gauge? Fried the alternator? Drove it about 20 miles so far. Any ideas on the gauge problem?
Thanks for all the help so far you guys are great
Had the ignition switch hanging below the dash to make it easier to probe the wire, while turning the key to start it contacted the dash and some sparks occurred and some smoke, truck runs fine and starts fine. But the alternator gauge is pegged on the "C". Fried the gauge? Fried the alternator? Drove it about 20 miles so far. Any ideas on the gauge problem?
Thanks for all the help so far you guys are great
Fleet Mechanic
Joined: Aug 2007
Posts: 1,839
Likes: 263
From: Piney Woods of East Texas
Put a volt meter on the battery and confirm the Alt is not putting out full power. If so it will burn up a battery real quick. There is shunt resistor for the amp gauge, may check it out.
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Cargo Master
Joined: Mar 2008
Posts: 2,589
Likes: 212
From: Washington
Quote:
But the alternator gauge is pegged on the "C". Fried the gauge?
Is the AMP gauge no longer spring loaded to the center position when the engine is OFF?
Does the AMP Meter go from center to full right ("C") after you start the engine?.
Do you still have a stock 1G ALT?
The AMP Meter shunt is usually a longer Shunt Wire, with smaller wires connected on each end
Jim
But the alternator gauge is pegged on the "C". Fried the gauge?
Is the AMP gauge no longer spring loaded to the center position when the engine is OFF?
Does the AMP Meter go from center to full right ("C") after you start the engine?.
Do you still have a stock 1G ALT?
The AMP Meter shunt is usually a longer Shunt Wire, with smaller wires connected on each end
Jim
Last edited by JimsRebel; Nov 9, 2025 at 02:14 PM.
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5th Wheeling
Joined: Oct 2025
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Likes: 4
Tested battery and it read 12.15, put a 2 amp charger on it for a couple hours and got 12.54, running it reads 12.23 . Checked the ignition plug to the switch and there is a red / green possibly yellow wire that the insulation is wrinkled because its been hot, had to put my double cheates on to see it LOL
The wiring diagram on Fordification does no show a red / green wire for a 72 or the 70 scematic it does show a red / blue which I have also. I have owned this truck since 77 and the wiring is original.
The amp meter is pegged on the C and does not move, running or off. Yes it has the 1g standard style alternator. Shunt wire?? Where does it go to and from and what color? Never heard of it and don't know what to look for. Thanks for your help guys and patience. My firtst rodeo with electrical diagnosis
The wiring diagram on Fordification does no show a red / green wire for a 72 or the 70 scematic it does show a red / blue which I have also. I have owned this truck since 77 and the wiring is original.
The amp meter is pegged on the C and does not move, running or off. Yes it has the 1g standard style alternator. Shunt wire?? Where does it go to and from and what color? Never heard of it and don't know what to look for. Thanks for your help guys and patience. My firtst rodeo with electrical diagnosis
Fleet Mechanic
Joined: Aug 2007
Posts: 1,839
Likes: 263
From: Piney Woods of East Texas
With it pegged being off I'm more incline you fried it. If it moved at all it may have a impact on readings. I would disconnect the meter and see if there is any movement. If so I may consider looking at the shunt resister. A amp meter works by reading the voltage drop across the resister.
Cargo Master
Joined: Feb 2006
Posts: 3,012
Likes: 251
From: North Pole, Alaska
What is the voltage across the battery terminals while it is running? Should be 13.5-14V. If not the alternator is fried also. For the ammeter, send it to Rocketman to have it replaced with a voltmeter which is much better. Be a good time to convert to a Taurus 3G alternator.
Last edited by RichS2659; Nov 9, 2025 at 05:16 PM.
Cargo Master
Joined: Mar 2008
Posts: 2,589
Likes: 212
From: Washington
The AMP Meter should be spring load to the center position, without power applied.
You might try removing the negative battery terminal, but would agree it sounds like it is bad. You might try tapping on it, after all, it's probably broken anyway.
To bring the alternator ONLINE, you need to have KEY ON power on the "S" connection of the voltage regulator. This (green?) wire comes from the ignition switch usually.
Regarding the Shunt Wire, the was a thread recently where a guy with a 79 bronco was have a similar issue. While not the same year, there's a lot of good info and links regarding how everything works. Ask if you have more questions.
It rare to find a truck with original wiring, especially between the alternator and battery area.
https://www.ford-trucks.com/forums/1795249-ammeter.html
Jim
You might try removing the negative battery terminal, but would agree it sounds like it is bad. You might try tapping on it, after all, it's probably broken anyway.
To bring the alternator ONLINE, you need to have KEY ON power on the "S" connection of the voltage regulator. This (green?) wire comes from the ignition switch usually.
Regarding the Shunt Wire, the was a thread recently where a guy with a 79 bronco was have a similar issue. While not the same year, there's a lot of good info and links regarding how everything works. Ask if you have more questions.
It rare to find a truck with original wiring, especially between the alternator and battery area.
https://www.ford-trucks.com/forums/1795249-ammeter.html
Jim
Thread Starter
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5th Wheeling
Joined: Oct 2025
Posts: 40
Likes: 4
the red / yellow wire out of the switch is the resistance wire for the points and is the 12v connection to the red msd wire. This wire is fried and is what I had my old msd wired to, looks like I need to overlay a wire to the switch, also going into the same spade connection at the switch there is a brown wire, neither wire is in the wiring diagrams that I can find. ??? Is it possible to remove the spade from the ignition connector and replace the wire in that spade connector??
Running I had 12.23 at the battery, so looks like the alternator is fried. Don't have any wires on the back of the amp gauge just a plastic cover over the pins, looks like the power is routed through the large connector to the cluster. Looked at the post on the bronco alternator and it shows a pic of a plug with three wires blk/ylw, red and yellow that is exactly what I have. I'm guessing that the red or yellow is the shunt wire?
Bought this truck used in 1977 with 60k miles, and I guarantee that the wiring is factory original. Not sure how to proceed on the amp gauge
Running I had 12.23 at the battery, so looks like the alternator is fried. Don't have any wires on the back of the amp gauge just a plastic cover over the pins, looks like the power is routed through the large connector to the cluster. Looked at the post on the bronco alternator and it shows a pic of a plug with three wires blk/ylw, red and yellow that is exactly what I have. I'm guessing that the red or yellow is the shunt wire?
Bought this truck used in 1977 with 60k miles, and I guarantee that the wiring is factory original. Not sure how to proceed on the amp gauge
Fleet Mechanic
Joined: Aug 2007
Posts: 1,839
Likes: 263
From: Piney Woods of East Texas
the red / yellow wire out of the switch is the resistance wire for the points and is the 12v connection to the red msd wire. This wire is fried and is what I had my old msd wired to, looks like I need to overlay a wire to the switch, also going into the same spade connection at the switch there is a brown wire, neither wire is in the wiring diagrams that I can find. ??? Is it possible to remove the spade from the ignition connector and replace the wire in that spade connector??
Running I had 12.23 at the battery, so looks like the alternator is fried. Don't have any wires on the back of the amp gauge just a plastic cover over the pins, looks like the power is routed through the large connector to the cluster. Looked at the post on the bronco alternator and it shows a pic of a plug with three wires blk/ylw, red and yellow that is exactly what I have. I'm guessing that the red or yellow is the shunt wire?
Bought this truck used in 1977 with 60k miles, and I guarantee that the wiring is factory original. Not sure how to proceed on the amp gauge
Running I had 12.23 at the battery, so looks like the alternator is fried. Don't have any wires on the back of the amp gauge just a plastic cover over the pins, looks like the power is routed through the large connector to the cluster. Looked at the post on the bronco alternator and it shows a pic of a plug with three wires blk/ylw, red and yellow that is exactly what I have. I'm guessing that the red or yellow is the shunt wire?
Bought this truck used in 1977 with 60k miles, and I guarantee that the wiring is factory original. Not sure how to proceed on the amp gauge