Resolved Gauges and lights pulsing

[sav22rem22]

So for some background, my 89 GT has a 3G alternator from LMR and I’ve done the 4 gauge wire upgrade and am not using the factory orange and black wires. All pulleys are stock and all grounds are in their factory locations and are clean. I’ve also added a ground as per a thread that I’ve seen about adding an additional 4 gauge wire from the empty hole on the front of the drivers head to the sway bar bracket which definitely helped speed up my already tired starter but nothing else gained from it.

My problem is that my interior lights which includes the gauge cluster lights, hvac control lights, dome light, and possibly the headlights and tail lights but to a lesser degree, all are pulsing which by that I mean rapidly getting dimmer and brighter. It happens at idle and less so when driving. Voltage at the battery when idling is around 13.9-14.3 volts depending on load. The battery gauge also rapidly bounces but I don’t see that at the battery when measuring with a volt meter.

This problem existed with the stock alternator as well but I partially blame that on the underdrive pulleys that I had on ant the time and with the factory alternator being so weak.

Does anyone have any idea what my next steps should be? Im hoping this brand new alternator isn’t bad internally as I’ve already replaced the external regulator that the D shaped plug plugs into due to an overcharging issue caused by me running the factory orange and black wires along with the upgraded 4 gauge wire. The new regulator fixed the overcharging issue but the pulsing/ flickering of the lights persists.

 

[Mustang5L5]

1 or more bad diodes in the alternator's rectifier bridge can cause ripple voltage which would cause this. I'd have a local auto parts store test the alternator to see if this is the case as their machines should be able to detect this.

If the alternator is bad, just spec a 94-95 GT alternator from local parts store.

 

[sav22rem22]

1 or more bad diodes in the alternator's rectifier bridge can cause ripple voltage which would cause this. I'd have a local auto parts store test the alternator to see if this is the case as their machines should be able to detect this.

If the alternator is bad, just spec a 94-95 GT alternator from local parts store.

Would I need to pull the alternator off and bring it to them or would they be able to do it while it’s still in the car? I’ve never actually had anything tested at a parts store besides my battery at one point a long time ago

 

[AeroCoupe]

I have always removed them.

 

[Mustang5L5]

Yeah, i always remove the alternator and bring it in. Easy to do on a fox as it's 2 bolts and a plug harness.

With it being present on 2 different alternators, it may not be the cause but it ultimately needs to be ruled out first.

 

[General karthief]

Is the ground wire present from the drivers side head to the firewall?

 

[sav22rem22]

1 or more bad diodes in the alternator's rectifier bridge can cause ripple voltage which would cause this. I'd have a local auto parts store test the alternator to see if this is the case as their machines should be able to detect this.

If the alternator is bad, just spec a 94-95 GT alternator from local parts store.

Well I just took it to an alternator and starter rebuild shop and they tested it. They showed me the machine and the alternator is dead steady at no load and steady even at full load. Guess it’s time to look elsewhere

 

[sav22rem22]

Is the ground wire present from the drivers side head to the firewall?

Yes and it’s also brand new with both the head and the firewall down to shiny bare metal. The main ground is in its factory location and the two grounds on the shock tower I re did and have the metal nice and shiny and in a thicker part of the shock tower

 

[AeroCoupe]

I would look at voltage drop on all the cables next.

https://www.fluke.com/en-us/learn/blog/automotive/electrical-automotive-troubleshooting

There are a slew of YouTube videos on this so lots of examples on how to do it.

 

[sav22rem22]

I would look at voltage drop on all the cables next.

https://www.fluke.com/en-us/learn/blog/automotive/electrical-automotive-troubleshooting

There are a slew of YouTube videos on this so lots of examples on how to do it.

I haven’t had a chance to do it but when I was poking around the solenoid and looking for the other end of the alt harness, I found some nasty cuts and splices that looks like someone bypassed the fusible links. The harness is also super tight where there was so much wire cut out. I’m going to check out voltage drop on that

 

[sav22rem22]

It’s finally fixed. Took a closer look at the alternator/light harness and it was pretty bad so I ordered a ron Francis front light harness from LMR and put it in along with a new solenoid and now the voltage gauge is dead steady and so are all the lights. Only problem I’m having now is that my fog lights aren’t working. Always something.

Thanks to all those who helped point me in the right direction

 

[Mindseye007]

It’s finally fixed. Took a closer look at the alternator/light harness and it was pretty bad so I ordered a ron Francis front light harness from LMR and put it in along with a new solenoid and now the voltage gauge is dead steady and so are all the lights. Only problem I’m having now is that my fog lights aren’t working. Always something.

Thanks to all those who helped point me in the right direction

This is a reply on LMR..
Great quality, appreciate the RIGHT sized gauge wire for the lights, nice wrap/tape, and quality connectors. The only modification I had to make was splicing in the ford OEM fog light connectors. They give you new ones, but I wanted to keep it factory correct for future foglight

 

[sav22rem22]

This is a reply on LMR..
Great quality, appreciate the RIGHT sized gauge wire for the lights, nice wrap/tape, and quality connectors. The only modification I had to make was splicing in the ford OEM fog light connectors. They give you new ones, but I wanted to keep it factory correct for future foglight

I did splice in my factory connectors as well but they don’t light up. They did on the old harness. I’m sure it’ll be something simple

 

[sav22rem22]

If anyone ever has the same issue, simply checking the resistance between the yellow wire at the D plug and the solenoid isn’t enough. As stated above you’ll need to do voltage drop tests. Here’s what my harness looked like at the solenoid:

The yellow reference wire ohmed out just fine but the questionable splices and sections of wire were corroded and barely making a connection.