Proper Volt Gauge Operation

JulianR

Member
Jun 13, 2016
11
1
13
40
Hi,

I have an 89 Fox GT Vert. Recently the stock alternator died. I replaced it with the SVE 130 amp from LMR and upgraded to the 4 ga power wire (yet to do the ground one as well in that size). The car is running underdrive pulleys. With the previous stock alt (when it was alive), the volt gauge would drop to red when I'm stopped and would climb up when I gave it gas. The headlights would dim and brighten with RPM as well. Now that I put in a new alt, 130amp, I expected that issue to go away. However, the lights still dim, and the volt gauge still moves. The alt is working, cuz at idle, I'm getting over 14 volts to the battery (using a multi-meter). I know my 2008 Toyota FJ has stock volt gauge on it-- that thing doesn't move at all. No matter what you turn on.

So, my question-- is it normal for the Fox's volt gauge to be moving, or should I expect it to be steady like the gauge on my FJ?

Thanks!
Julian
 
  • Sponsors(?)


mikestang63

SN Certified Technician
Aug 27, 2012
9,892
6,675
204
In the garage
let me guess- you rehooked up the stock power plug and wiring to the 3g per their instructions.

The only wire to reuse off the stock 2G power plug is the one for the stator. Use a dedicated 4 gauge power wire from the starter solenoid to the + post on the back of the alternator with an inline 150 amp fuse. Tape off the old power plug and tuck it away. Hopefully you did not cut any wires yet. If you did. reuse the stock power wires from the 2G, disconnect them and tape them up/cap them off. You also MUST install another dedicated 4g ground from the bock to the chassis or 2ndary ground post.

Image courtesy of jrircker

3g-rear-view-75-gif.52292.gif



That being said, stock mustang gauges are notorious for jumping and being inaccurate. Check all the grounds in the engime and also for the gauge panel
 

JulianR

Member
Jun 13, 2016
11
1
13
40
Ahh, that's what I was wondering. Thanks for the image. So I did hook up the 4 gauge wire to the new power terminal and ran that to the starter relay. However, where the LMR instructions were different from the image above is they said to connect that Field connector to the white/black wire on the regulator plug. It does make sense to connect it to the white/black on the original power plug. But are all the 3G alternators the same as far as hook up? How do I know I won't burn something up by straying from the LMR instructions? :)

Thanks!
Julian
 

mikestang63

SN Certified Technician
Aug 27, 2012
9,892
6,675
204
In the garage
I can almost guarantee you will burn something up by having your original power wires hooked up to the 3g. LMRS instuctions are plain wrong and they were told that on this board years ago. Why they continue to provide wrong instructions is anyone's guess. Wire it up exactly as the image above with an extra 4g ground and you will be fine.
 

JulianR

Member
Jun 13, 2016
11
1
13
40
Hi Mike,

Let me clarify my post. I was surprised to see the black/orange wire thickness that the stock alt used. I had no intention of re-using it and too don't get why LMR is telling people to use those wires. I am running only the 4 gauge power wire with the original power plug tucked away and taped up with electrical tape over the top.

Now, the way I hooked up the SVE 3g Alt is in the following way. I had to re-read the instructions and read a few threads, but I believe I have it done correctly. Drove to work today and all seems good.

The SVE 3g alt kit from LMR comes with a 4 gauge wire for power, a (stator?) wire-- single wire with a plastic plug, and a new voltage regulator plug harness. I think it may be possible to re-use the stock regulator plug on the new alternator, but I didn’t try. I now regret it because the yellow sense wire is part of this plug and the stock wire used on the car is thicker than the one that comes on the new harness. Somewhere I read that it’s a good idea to use the new harness cuz old ones had issues—but I think I should have kept the old one uncut.


Anyway, the setup of the alt is this. You unplug the power plug from the old alt, and cut off the white wire that goes in to it next to the thicker black/orange. You tape up this plug with the black/orange wires and hide it away. However, keep it connected on the other side to the starter relay. It turns in to a single green wire. Next on the regulator plug, all wires stay the same—whether you re-use the old one or if you use the new one, just connect the green to green, yellow to yellow if you put a new harness on. The only difference is, the white wire from the regulator harness connects to the single wire (stator?) plug that came with the kit. Then you attach the regulator plug to the regulator, (stator?) plug to the input next to the regulator. Now, for the power wire, you run the 4 gauge power wire from the larger bolt on the back of the new alt to the left side/post of the starter relay that’s located to the left of the battery on the driver fender. I cleaned and sanded my connections while I was taking them off, you should too. Also, make sure you put the nuts on tightly for good connections. Verify that the wires are tucked away from spinning/moving parts and are not touching the hot parts. Tape the wires up with electrical tape.

Finally, if the kit came with a new 4 gauge ground wire, I’d be done now J but I still need to buy one and hook it up possibly from the alt bracket connection bolt to the body. I hope this help someone. Disregard my last post about the white wire. In the original setup, the white wire just ties the power plug with the voltage regulator plug.

If I did this wrong, please correct me—car seems to run fine for now. J
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,316
2,649
234
74
Dublin GA
lowendmac.com
Here is the reasoning behind using only a single 4 gauge fused power feed to the alternator. If you use the two 10 gauge black/orange wires in addition to the 4 gauge wire, you have two fused power feed paths. The total current capacity of the wiring is the sum of the fused paths. The 4 gauge path is fused for 125 amps, and the two 10 gages wires are fused for 60 amps. That is a total of 185 amps, which exceeds the capacity of the alternator. Overload can occur without the fuses blowing, damaging the alternator.

The worst case scenario is that the alternator develops an internal short to ground resulting in a catastrophic failure. The initial short circuit surge current is limited by the resistance of the wiring. The current in a parallel circuit divides up according to the resistance of the branches. If the 4 gauge fuse opens up first, the two 10 gauge black/orange wires will be carrying the short circuit surge current. Depending on the time lag of the fuse links, they may open up before a fire starts or they may not.

See http://www.allaboutcircuits.com/textbook/direct-current/chpt-5/simple-parallel-circuits/

LMR does not seem to have an electrical engineer or even someone who really understands electrical theory on their staff or in an advisory position.. That's why they gave bad advice concerning the 3G installation and the 2 black/orange wires.

NAPA automotive fuse holder for 1125 amp fuse Part number BK 7821143



Here's the rest of the 3G wiring diagrams...

Starter solenoid wiring 86-91 model cars.

Connect the fused 4 gauge wire to the alternator and the battery side of the starter solenoid.
attachment.php?attachmentid=52294&stc=1&d=1192414749.gif


Starter solenoid wiring 92-93 Model cars.
attachment.php?attachmentid=53216&stc=1&d=1201020653.gif
 
Last edited:
  • Like
Reactions: mikestang63

mikestang63

SN Certified Technician
Aug 27, 2012
9,892
6,675
204
In the garage
Hi Mike,

Let me clarify my post. I was surprised to see the black/orange wire thickness that the stock alt used. I had no intention of re-using it and too don't get why LMR is telling people to use those wires. I am running only the 4 gauge power wire with the original power plug tucked away and taped up with electrical tape over the top.

Now, the way I hooked up the SVE 3g Alt is in the following way. I had to re-read the instructions and read a few threads, but I believe I have it done correctly. Drove to work today and all seems good.

The SVE 3g alt kit from LMR comes with a 4 gauge wire for power, a (stator?) wire-- single wire with a plastic plug, and a new voltage regulator plug harness. I think it may be possible to re-use the stock regulator plug on the new alternator, but I didn’t try. I now regret it because the yellow sense wire is part of this plug and the stock wire used on the car is thicker than the one that comes on the new harness. Somewhere I read that it’s a good idea to use the new harness cuz old ones had issues—but I think I should have kept the old one uncut.


Anyway, the setup of the alt is this. You unplug the power plug from the old alt, and cut off the white wire that goes in to it next to the thicker black/orange. You tape up this plug with the black/orange wires and hide it away. However, keep it connected on the other side to the starter relay. It turns in to a single green wire. Next on the regulator plug, all wires stay the same—whether you re-use the old one or if you use the new one, just connect the green to green, yellow to yellow if you put a new harness on. The only difference is, the white wire from the regulator harness connects to the single wire (stator?) plug that came with the kit. Then you attach the regulator plug to the regulator, (stator?) plug to the input next to the regulator. Now, for the power wire, you run the 4 gauge power wire from the larger bolt on the back of the new alt to the left side/post of the starter relay that’s located to the left of the battery on the driver fender. I cleaned and sanded my connections while I was taking them off, you should too. Also, make sure you put the nuts on tightly for good connections. Verify that the wires are tucked away from spinning/moving parts and are not touching the hot parts. Tape the wires up with electrical tape.

Finally, if the kit came with a new 4 gauge ground wire, I’d be done now J but I still need to buy one and hook it up possibly from the alt bracket connection bolt to the body. I hope this help someone. Disregard my last post about the white wire. In the original setup, the white wire just ties the power plug with the voltage regulator plug.

If I did this wrong, please correct me—car seems to run fine for now. J

You could have simply plugged in your D shaped regulator connector into the 3G but if it is working fine just leave it. You did not mention any inline fuse on the 4G power cable- that is a must. Finally, get a cheap DVOM and test the voltage output at the battery +, the solenoid, and at the alternator + post at idle and around 1500 rpm to ensure the atlernator is working. The factory gauges are about as useful as an idiot light.
 

JulianR

Member
Jun 13, 2016
11
1
13
40
You could have simply plugged in your D shaped regulator connector into the 3G but if it is working fine just leave it. You did not mention any inline fuse on the 4G power cable- that is a must. Finally, get a cheap DVOM and test the voltage output at the battery +, the solenoid, and at the alternator + post at idle and around 1500 rpm to ensure the atlernator is working. The factory gauges are about as useful as an idiot light.


Yes, the 4 ga power cable came with a fuse. At idle, I was at about 14.3v no accessories at the battery. Didn't test at 1500 rpm yet or other posts. However, I'm sure the stock gauge is not too accurate-- but it's moving correctly it seems when I drive so I think I'm good..
 

Brando30

Member
Jan 4, 2019
34
2
8
39
Ashland ky
let me guess- you rehooked up the stock power plug and wiring to the 3g per their instructions.

The only wire to reuse off the stock 2G power plug is the one for the stator. Use a dedicated 4 gauge power wire from the starter solenoid to the + post on the back of the alternator with an inline 150 amp fuse. Tape off the old power plug and tuck it away. Hopefully you did not cut any wires yet. If you did. reuse the stock power wires from the 2G, disconnect them and tape them up/cap them off. You also MUST install another dedicated 4g ground from the bock to the chassis or 2ndary ground post.

Image courtesy of jrircker

3g-rear-view-75-gif.52292.gif



That being said, stock mustang gauges are notorious for jumping and being inaccurate. Check all the grounds in the engime and also for the gauge panel

Mike,

First off, thanks. I made the same mistake. My question is regarding the dedicated 4g ground. I replaced my factory negative battery cable to the battery, do I need to install a 2nd 4g neg cable? You mentioned from block to (any specific spot?) Or am it ok with the one?

Thanks in advance!
 

Blown88GT

Founding Member
Nov 13, 1999
2,144
505
164
Palm Beach Gardens, FL
Mike,

First off, thanks. I made the same mistake. My question is regarding the dedicated 4g ground. I replaced my factory negative battery cable to the battery, do I need to install a 2nd 4g neg cable? You mentioned from block to (any specific spot?) Or am it ok with the one?

Thanks in advance!
I wrote the "Bible" on this because I am an electrical engineer. My website is lost to history, but the remnants are here (minus the pics):
https://web.archive.org/web/2001020...ge.com:80/Auto/3G_130A_Alternator_Upgrade.htm

There is no dedicated 4 gauge ground at the alternator. Ground is the alternator case to the engine which grounds to the chassis. The negative battery cable attaches to the engine block.
 
  • Like
Reactions: Brando30

mikestang63

SN Certified Technician
Aug 27, 2012
9,892
6,675
204
In the garage
Mike,

First off, thanks. I made the same mistake. My question is regarding the dedicated 4g ground. I replaced my factory negative battery cable to the battery, do I need to install a 2nd 4g neg cable? You mentioned from block to (any specific spot?) Or am it ok with the one?

Thanks in advance!
to answer your question- yes- add a 2nd dedicated ground from the chassis to the block.