72-Mach 1 - Chasing an alternator issue, turn out to be emergency brake switch wire

7173Mustangs.com

Help Support 7173Mustangs.com:

bdennis

Well-known member
Joined
Apr 3, 2021
Messages
181
Reaction score
64
Location
Melbourne, Australia
Hey guys,
Just thought I would share an issue I had on my 72 Mach 1.
My car same factory with idiot lights in the dash. I have then added a mechanical temperature gauge, a mechanical oil pressure gauge and a volt meter. (Volt meter connected to the radio 12v feed)
Some months ago I rebuilt the alternator by replacing the front and rear bearings and brushes.
Not long ago, I broke the emergency brake cable, so had to replace the cable. (Probably unrelated to the issue I found but thought I would mention it).
Recently I noticed that when driving with no parker lights or head lights, the volt meter would climb to 15 or 16v. When I turned on the parkers it returned to 14v. Then when I put the headlights on, it would drop to 12v or lower.
I thought the issue may have been the alternator or the regulator. After talking to a local Auto electrician, and thinking though the issue (I have a healthy understanding of electronics) I replaced the regulator.
This did not resolve the issue!..
After reviewing a few youtube video's I set out doing some testing.
I found that with the new regulator, when the engine was at idle, all was ok with the battery voltage being at 14v. When I turned on the parkers and head lights, with the new regulator, the battery voltage would climb to17 and 18v. This, obviously was not ideal! Yet the voltmeter inside the car would drop to 10v. In some cases the indicators would stop working and the headlights would dim.
I swapped out the new regulator for the old one. It would not increase the batter voltage and would stay stable but was not able to reach 14v.

I also tried to unplug the regulator and jump pins 1 and 3. The voltage on the battery would stay at 13v.

Very odd!.

While the engine was running and head lights on with the new regulator and the volt meter in the car showing 10v, I noticed that the emergency brake was not on and the car was in park.
I pressed the emergency brake and the head lights came back to normal and the internal volt meter returned to 14v!

WTF!...

I did some more testing and I found that if I pressed the emergency brake to the first click, the issue went away. I went for a drive and found the same thing. If the emergency brake was off, then I had an issue. If it was on slightly, the issue went away!.

I had a look under and checked to make sure the head light dimmer switch wired were not bare and touching the emergency brake cable.. All good there.
I felt around at the top of the brake assembly and found a wire that was, (what I thought) stuck. I pulled the wire and found that it had a plug on the end and was green with red dots.. (on checking the wiring diagram I found this to be wire 162. Emergency brake to emergency brake warning light wire. After I removed it from the emergency brake assembly the issue went away.

As my car has standard wiring and standard dash, it does not have the emergency brake warning light.. So I figure, there is no harm in leaving it disconnected..

Issue solved!.. (I think!)

I checked the wiring diagram and the emergency brake switch / light is connected to the same 20A circuit that the rear heated back light and power window safety relay. This is fuse #7. Upon checking the fuse box, if found that the fuse is not there and one of the connectors is not in the fuse box. Given my car has the basic or standard wiring.

I will do some more testing tomorrow to see if the wire has 12V on it.. Per the wiring diagram, it should not.. So I may have masked the issue by pulling the wire from the emergency brake switch.
While the issue, sees to be fixed, I think I need to get to the root cause...

Will keep you posted.

Thoughts?
 
Last edited:
Joined
Sep 12, 2015
Messages
6,386
Reaction score
1,436
Location
SW Ontario
My Car
1971 Mustang Mach 1, M code, 4 speed.
That is some interesting reading.
A while back, I posted about adding a switch to my PARKING brake, so I have a visual reminder that the damn brake is ON. I've driven more than once with the pedal depressed. As my car, like yours is a basic car, it did not have this little luxury. By the sound of it, your car came with the switch attached, but only connected through the circuit, not to a P/B light If I understand your meaning.
Adding this light is still something I want to do, but as I'll need to pull the P/B pedal assembly to do it, not something that is high on the list.
Also, I too connected a Bosch voltmeter to the radio wire, just because it is a 12V switched wire and convenient. My gauge reads low at a little over 13 volts all the time, but the charge voltage is 14.2. I suspect the issue is the gauge itself. Maybe the radio wire was not the best choice!
 

midlife

Shorts checker
Joined
Jan 24, 2012
Messages
4,231
Reaction score
730
Location
Tucson, AZ
My Car
No Mustangs at the moment.
Are you still using the alternator indicator lamp in the dash? That is part of the charging circuit and must have an incandescent bulb there. With an alternator indicator lamp in use, you'll need the voltage regulator plug with all 4 pins used; if only an ammeter or voltmeter, you'll need the version with 3 of 4 pins used.

Your green/red dot wire is the emergency brake sensor wire. It's connected to a lamp and is powered by ACC fused power that has the only vertical fuse.
 

bdennis

Well-known member
Joined
Apr 3, 2021
Messages
181
Reaction score
64
Location
Melbourne, Australia
Geoff,
Thanks for the updates.
It is very difficult to tell if the switch is on the emergency brake assembly. I would need to pull it out and I don't want to go that far...

Midlife...
Yes I'm still using the idiot lights.. Although I have installed the idiot light to tach conversion where it still has the idiot light globes and slots next to the tach.

After posting, I was thinking that given the fuse is missing, there should be no power on the Emergency brake wire. So I will test that later on and see if it has 12v on it or a resistance value when comparing to ground will advise.
 
Joined
Apr 24, 2020
Messages
608
Reaction score
481
Location
Pittsford, NY
My Car
My all time favorite vehicle is our 1969 Shelby GT500
Hey guys,
Just thought I would share an issue I had on my 72 Mach 1.
My car same factory with idiot lights in the dash. I have then added a mechanical temperature gauge, a mechanical oil pressure gauge and a volt meter. (Volt meter connected to the radio 12v feed)
Some months ago I rebuilt the alternator by replacing the front and rear bearings and brushes.
Not long ago, I broke the emergency brake cable, so had to replace the cable. (Probably unrelated to the issue I found but thought I would mention it).
Recently I noticed that when driving with no parker lights or head lights, the volt meter would climb to 15 or 16v. When I turned on the parkers it returned to 14v. Then when I put the headlights on, it would drop to 12v or lower.
I thought the issue may have been the alternator or the regulator. After talking to a local Auto electrician, and thinking though the issue (I have a healthy understanding of electronics) I replaced the regulator.
This did not resolve the issue!..
After reviewing a few youtube video's I set out doing some testing.
I found that with the new regulator, when the engine was at idle, all was ok with the battery voltage being at 14v. When I turned on the parkers and head lights, with the new regulator, the battery voltage would climb to17 and 18v. This, obviously was not ideal! Yet the voltmeter inside the car would drop to 10v. In some cases the indicators would stop working and the headlights would dim.
I swapped out the new regulator for the old one. It would not increase the batter voltage and would stay stable but was not able to reach 14v.

I also tried to unplug the regulator and jump pins 1 and 3. The voltage on the battery would stay at 13v.

Very odd!.

While the engine was running and head lights on with the new regulator and the volt meter in the car showing 10v, I noticed that the emergency brake was not on and the car was in park.
I pressed the emergency brake and the head lights came back to normal and the internal volt meter returned to 14v!

WTF!...

I did some more testing and I found that if I pressed the emergency brake to the first click, the issue went away. I went for a drive and found the same thing. If the emergency brake was off, then I had an issue. If it was on slightly, the issue went away!.

I had a look under and checked to make sure the head light dimmer switch wired were not bare and touching the emergency brake cable.. All good there.
I felt around at the top of the brake assembly and found a wire that was, (what I thought) stuck. I pulled the wire and found that it had a plug on the end and was green with red dots.. (on checking the wiring diagram I found this to be wire 162. Emergency brake to emergency brake warning light wire. After I removed it from the emergency brake assembly the issue went away.

As my car has standard wiring and standard dash, it does not have the emergency brake warning light.. So I figure, there is no harm in leaving it disconnected..

Issue solved!.. (I think!)

I checked the wiring diagram and the emergency brake switch / light is connected to the same 20A circuit that the rear heated back light and power window safety relay. This is fuse #7. Upon checking the fuse box, if found that the fuse is not there and one of the connectors is not in the fuse box. Given my car has the basic or standard wiring.

I will do some more testing tomorrow to see if the wire has 12V on it.. Per the wiring diagram, it should not.. So I may have masked the issue by pulling the wire from the emergency brake switch.
While the issue, sees to be fixed, I think I need to get to the root cause...

Will keep you posted.

Thoughts?
Wow! I, for one, sincerely appreciate the clear description of what you were running into, and how you managed to resolve the apparent overcharging situation. I have not looked at the schematic, as it sounds like you already have a schematic and know you way around the electrical system on these vintage Mustangs. But, I may look at a schematic for the circuit(s) you mwntion, just to see if I can conjure up some thoughts. That said, I have found (as I am sure you have also) found that when I see strange electrical situation like this I begin to look for bad/intermittently open ground circuits.

My electrical schematics for First Generation Mustangs/Cougars is from Forel Publications. I have their set of colorized wiring schematics for 1965 - 1973 Mustangs in PDF file format. Their pricing is very reasonable. The 65 - 71 schematics are of limited value, but better than nothing. For 1972 the schematics are better with some very logically laid out diagrams. But, their 1973 schematics are really nice, and includes ground and connector locations and connector plug shape diagrams not in the prior year's schematics. There are some differences in 1973 that do not always coincide with 72 and earlier schematics (difference in fuse block layout and distribution of power to some circuits, 73 has an electric choke not seen or used in prior years. Circuit 640 for the Throttle Position Solenoid is routed/connected a little differently in 73 vs prior years). However, the grounds and connector location plug shape info is fairly consistent across 71-73, and nice to have. This is their web site:


There are some custom circuit schematics created by Ken Pattison that offer a really nice and clear alternative look at 71 (and later) Mustang electrical circuits. I know Ken has put a lot of work into creating his own circuit schematic designs. I am not comfortable with overtly offering his works to others without conferring with him first, as it is possible he would like to receive some compensation for his efforts (not at all unreasonable given the quality of his schematics). I can say this, I have found his schematics to be refreshingly clear, and he has corrected some of the errors and unclear details found in other schematics. I have, however, attached a sample of some of his schematic work to give folks an idea of what they expect with his schematics. If I did not find them of value and useful I would not have bothered mentioning what he has produced. They really are top notch, and for anyone who is looking to create a library of some really nice schematics, his schematics deserve a place in any such collection.

Ken is a member of this forum, and it is even possible he may chime in with a reply to my comment made here. If there is any interest in learning more about Ken's alternative electrical schematics please let me know with a reply to my comment, or email me at [email protected].

1665594467883.png
 
Last edited:
Joined
Apr 24, 2020
Messages
608
Reaction score
481
Location
Pittsford, NY
My Car
My all time favorite vehicle is our 1969 Shelby GT500
Are you still using the alternator indicator lamp in the dash? That is part of the charging circuit and must have an incandescent bulb there. With an alternator indicator lamp in use, you'll need the voltage regulator plug with all 4 pins used; if only an ammeter or voltmeter, you'll need the version with 3 of 4 pins used.

Your green/red dot wire is the emergency brake sensor wire. It's connected to a lamp and is powered by ACC fused power that has the only vertical fuse.
Midlife, you continue to astound me with your deep knowledge and understanding of these Mustang circuitry issues. I am not one who tends to succumb to Hero Worship. But, if I were you would stand in good company, as one of the other folks I admire is General Chuck Yeager (for different reasons). I always enjoy learning ever more little tidbits from your contributions. Even brief posts from you, like this one, are laden with information that is worth me saving for possible future use.
 

bdennis

Well-known member
Joined
Apr 3, 2021
Messages
181
Reaction score
64
Location
Melbourne, Australia
Thanks for the info. I have the 72 diagram from Forel. They are very helpful. The colour part of the 72 set that I have is missing the last 2 pages that has the lights section so I just use the black and white sections. The part of Kens diagram you have shared look good.

I need to do some more digging around to make sure it is the emergency brake wire. Per the diagram, it should not have an impact. I may have moved something else that was causing the issue.
I will advise once more investigation is complete.
 
Last edited:

midlife

Shorts checker
Joined
Jan 24, 2012
Messages
4,231
Reaction score
730
Location
Tucson, AZ
My Car
No Mustangs at the moment.
Midlife, you continue to astound me with your deep knowledge and understanding of these Mustang circuitry issues. I am not one who tends to succumb to Hero Worship. But, if I were you would stand in good company, as one of the other folks I admire is General Chuck Yeager (for different reasons). I always enjoy learning ever more little tidbits from your contributions. Even brief posts from you, like this one, are laden with information that is worth me saving for possible future use.
Your $20 is now in the mail *G*
Thanks for the kind words, but when one specializes in a niche field for 14 years, one knows a great deal about very little!!!
 

bdennis

Well-known member
Joined
Apr 3, 2021
Messages
181
Reaction score
64
Location
Melbourne, Australia
Update..
I completed some more testing and can confirm that the emergency brake wire does not have power to it and is not connected to ground. So it cant have been causing the short...
The short has gone away now so the only thing I can think if is a wire in the main loom was touching and rubbed through on the brake mechanism. Or, the hid beam switch was touching the emergency brake line.
I have now moved the high beam switch wires so they are nowhere near the emergency brake cable outer.
I have moved the wires from above the emergency brake mechanism and tied away the emergency brake.
I am still seeing 15v when the head lights are on when running from the alternator, but this is about 2v above the battery voltage.
I have tested the system with both the old regulator and the new one and completed the plug off regulator test and the pin 1 to pin 3 regulator wire test.
The new regulator is definitely better as the old one does not increase the voltage to the battery when running. The new one does.
I am still keen to test the whole system with another alternator to see what happens. So I will loan one from a mate a see.

I will keep you all posted.
 
Joined
Apr 25, 2020
Messages
211
Reaction score
86
Location
NJ
My Car
73 Mach 1, Q code, 4 speed, fully optioned
I see you are in Australia. Is the car right hand drive? If so, how are the pedals (gas, brake, (clutch if manual), parking arranged? Also where is the high beam light switch mounted? A photo of the arrangement, if right hand drive, would be great. If right hand drive, wiring harness routing has to be different than with left hand drive (US and Canada) cars which might effect how we (mostly US members) think about your issue.
 

bdennis

Well-known member
Joined
Apr 3, 2021
Messages
181
Reaction score
64
Location
Melbourne, Australia
Update to my alternator issue.
I changed the volt meter that I added under the dash for a speco one. Not because the one I had was fautly, but more top match the mechanical Oil pressure and temp gages I had added. The new gauge is more responsive and flickers above 14v and goes to 16v when driving. When I turn on the parkers and or headlights the issue settles down.
I replaced the regulator with a new one and the issue persisted. I more wanted to find the problem rather than just pulling the lights on each time.

I finally found the problem.. Bad earth!.. basically, the black / negative cable from the batter goes direct to the side of the block. But there was no earth wire from the block and or there was no body earth contact that is supposed to be 1/2way along the battery negative that bolts to the body below the voltage regulator. (see here - https://7173mustangs.com/threads/fuddling-electrical-failure-72-convertible.41626/post-417663)

I added a extra wire between the battery terminal and the body and job done.. Problem fixed.. How simple was that!

Look look at replacing the cables with factory looking ones in the future but I'm glad the issue is fixed.
 
Last edited:
Top