Multiple problems in my 2006 v6 4.0L

[BlueHeisenberg]

Hi guys!

I have a Mustang 2006 v6 4.0L convertible that I have had for almost 3 years now.

A little bit before I had had it for a year, the transmission started to have some issues: the shifts were hard and you could hear a "clok" noise when that happened.

I decided to take it to a specialized American transmission mechanic (I live in Spain, and these cars are pretty rare here, so finding a mechanic who knows them is difficult). They rebuilt the transmission (3000€), but even then the "clok" sounds and hard shifts were still happening. I took the car to them many times and they adjusted the servos or something in the transmission that made the car work wonderfully with nice shifts for a while, but after driving it a bit, it just continued with the hard shifts. I told them about the servo bore wear that I found online and they just said "yeah yeah, we know what we are doing"... so after almost 2 years with the transmission not working right and with COVID in between (the "transmission guy" had a pretty bad COVID recovery), they just decided to just rebuild the whole transmission again, changing all the pads and everything (without me paying, obviously), but the problem persisted, so I'm inclined to just buy the servo bore wear kit and tell them to change it.

That's one of the issues, but that per se didn't fully affect my ability to drive the car. The problem I'm having now is an electrical issue.

I think I could have some grounding problems as first of all, at idle the car had fluctuating rpm and the lights also fluctuated, I checked the alternator and it seemed to work only sometimes, so I changed it and now the rpm is more stable, but still all the lights fluctuate when the car is on. And even though the car "works", if I step on the gas pedal more than 1/4 (or less) the car starts to hiccup and lose power.

I tried to connect a couple of AWG9 cables (the thickest I had lying around) from the casing of the alternator to the ground of the battery and the chassis ground next to the battery, but nothing...

Another thing to note that might also indicate what's wrong is that I have a bluetooth OBDII reader. If I disconnect it once I turn the contact on, the odometer changes from the actual miles value to "---------" and if I try to crank it does nothing, but as soon as I connect the OBDII it reads back the miles and I can crank the car, so maybe the OBDII dongle is doing some grounding and that's why it works?

Sorry for the long post, but I'm a little bit lost on what to do

 

[DieHarder]

Some basic troubleshooting I would do first. Get a VOM (Volt Ohm Meter) and check output across the battery. If 14.5v or higher your engine grounds are likely good so look at grounding inside the car. Still, the lights flickering indicate grounds or wiring are an issue that need to be checked out.

If you do verify low voltage out of the alternator find the main body/engine grounds in the engine bay (strut tower, main battery cables, starter, other body grounds, etc) check them for oxidation/corrosion; check continuity; if okay, clean them and reattach. Also, I've found it may be necessary to run a new ground from one of the alternator mounting bolts back to the passenger strut tower to get a good ground for the alternator but only do this if the alternator output is low (< 13.5v).

Look up the charging/starter circuits in the manual and check those grounds to ensure they're good. You should have less than 1/10 ohm from the farthest end of any ground line back to where it connects to the body (or point near that connection point). Regarding your gauge I've seen the same problem with the miles in my car and resolved it after I checked/cleaned/reattached the body grounds for the associated circuit. Like you said likely that your OBDII is making a connection to one of the grounds it connects to.

Here's the starter/charging circuit for your car:
Charging - https://iihs.net/fsm/?d=40&f=Charging System.pdf
Starter - https://iihs.net/fsm/?d=40&f=Starting System.pdf
Ground locations - https://iihs.net/fsm/?d=40&f=Component Location Views.pdf&p=1
(anything with a "G" is a ground; main grounds in engine bay are G106/107). Primary ground for lights is G100 - https://iihs.net/fsm/?d=40&f=Headlamps. Autolamps.pdf and at the other end (under/inside dash (G201) - https://iihs.net/fsm/?d=40&f=Component Location Views.pdf&p=6

If none of this resolves your problem I'd disconnect/clean/reconnect any grounds you can find in the engine bay and inside the car. You should not need the OBDII to provide a ground to crank the car.