Another Electrical question

[Laga]

2005 Mustang GT. I’m still chasing electrical gremlins and I need some conformation. CEL has never come on.
I fixed the bad grounds and checking connections.
When I first had problems with a stubble under load. I found a stored P0340. I replaced sensor and it ran great for one day. Then stubble under load returned.
Car has been sitting for a couple of weeks because I didn’t have time to work on it.
Today I went out and disconnected all electrical connections to the 5 year old PA Performance alternator and the battery terminals. The alt is still mounted in car because with the E-Force SC it’s a pain to remove. I did the diode test with MM and got a reading of .48 with black lead on pos post of alt, and red on case. When I reversed them, I got a reading of 2.45 I’m guessing there is a bad diode. Will the bad diode cause the stubble under load? Thanks.

I tried a different MM and got the same .48 with black lead on positive post. But got zero, the other way around

 

[Autokyrios]

Maybe. Diode should prevent electrical feedback which would cause injector and spark problems.

P0340 is a camshaft sensor issue, but you said you replaced that, so my guess is the sensor is fine but it's reporting odd behavior.

How old is the battery?
Also, for the alternator, are you getting a solid 13+ volts all the time?

Two other things I'd check as well: spark plugs, intercooler. Spark plugs to see if they're fouled, and intercooler to see if it's got sludge in it (sludge buildup can happen over time and can foul up the plugs...least that's an issue I've seen on turbo cars/trucks). I don't know how much this affects the Edlebrock SC as the intercooler is in the valley so probably stable.

 

[TexasBlownV8]

A P0340 has often been an alternator issue. If you measure voltage when running, when voltage drops down to around 12.5 volts more or less, it'll trigger a p0340.
The key here is to measure and monitor the voltage when running. It should always be in the upper 13 volts to low 14's. If voltage is hanging around there, when there is a stumble, then the issue is not the alternator. At least you'd be able to rule that out.

Just a couple of other ground things to check, not saying there is a ground issue, but since you mentioned it:
Is the small ground strap on back of driver-side head connected to the body/to ground?
Did you check the large ground wire on the passenger side shock tower?
If the motor mount bracket on passenger side was removed, was the ground cable properly refastenned?

 

[GlassTop09]

2005 Mustang GT. I’m still chasing electrical gremlins and I need some conformation. CEL has never come on.
I fixed the bad grounds and checking connections.
When I first had problems with a stubble under load. I found a stored P0340. I replaced sensor and it ran great for one day. Then stubble under load returned.
Car has been sitting for a couple of weeks because I didn’t have time to work on it.
Today I went out and disconnected all electrical connections to the 5 year old PA Performance alternator and the battery terminals. The alt is still mounted in car because with the E-Force SC it’s a pain to remove. I did the diode test with MM and got a reading of .48 with black lead on pos post of alt, and red on case. When I reversed them, I got a reading of 2.45 I’m guessing there is a bad diode. Will the bad diode cause the stubble under load? Thanks.

I tried a different MM and got the same .48 with black lead on positive post. But got zero, the other way around

Hey Laga, couple of questions......................
1. Did you experience any interior lights flickering while engine running?
2. What is the brand\make of the 2 DMM\VDOM you were using?

From reading your posting on running the diode check, you did the procedure correctly if you had the DMM\VDOM selector set on the diode symbol.......but both meters should have given the same type of results (show VDOM voltage running BLK-ALT POS\RED-ALT CASE GRD but no voltage running RED-ALT POS\BLK-ALT CASE GRD = Good diodes......diodes are 1-way electrical check valves that convert AC voltage\current into DC voltage\current output in an alternator......voltage reading in both directions indicates a bad diode) so you might test this again just to verify.

But you should also run the 2nd test to check for AC ripple (1st sign of a failing diode........essentially diode is starting to "leak" AC voltage\current into the car's DC voltage\current system........thus called "ripple") as AC ripple will certainly affect any 2-wire self-generating sensor (S197 cam & crank sensors----these sensors generate very low levels of AC voltage\current thru magnet polarization\reverse polarization when metal trigger wheel teeth pass thru the sensor's magnetic field) when AC ripple gets into the adjacent sensor wiring (will throw off the sensor's AC voltage\current output which can fool the ECU into thinking that the sensor has a circuit issue---most of these sensors output around .5v to 1v AC voltage at best).

To do this AC ripple test the engine will need to be running & batt needs to be fully charged. DMM\VDOM needs to be set to AC volts then test meter to see if meter will block out DC volts (some meters will not block out DC volts when set for reading AC volts.......meter shouldn't read DC volts when set to read AC volts to be able to read the AC ripple voltage output from alternator).

Then put red lead on batt + & blk lead on batt -............acceptable AC ripple voltage should be 50mV or less........higher than this is indication of a diode failing.......another way to test alternator diodes.

Hope this helps.

 

[Laga]

These are the DMMs I used. They were set on diode mode. First time I did test they were set to voltage. Rookie mistake. Both got sub .5 with black probe on pos post. Supposed to be 5-8 MV. One unit read 2.4, the other OL when switched.
Cannot tell about interior lights because it has only been driven in the day time lately.
The problem started after I changed 2 year old Brisk plugs. I change plugs every 1-2 years depending on use. All looked good. Running 11# boost. Gap is set at .032 per Brisk recommendations. I have pulled them and rechecked gap. Battery is less than year old and is always at least 12.6V
According to Interceptor gauge, alternator is always 14-14.2V.
When stumble happens there is no change in voltage or fuel pressure according to gauge.
I tried an experiment and put the AT into 2nd gear and I can run it over 5000 rpm with no problems, no stumble.
Only ground not checked is one on firewall. IIRC, I did check it about a year ago. Will check tomorrow.
Car never displayed CEL. Found P0340 code in stored codes. Read links on that code which pointed me to alternator and diodes.
With E-Force SC, alt is under throttle body and is mounted backwards. Have to remove TB to access alt wires. So car is not running right now.
New alternator will be here this weekend. If it tests like existing, I can return it. If not, I will install it. Will post results of rear ground wire check tomorrow.

 

[Laga]

Ground behind engine is good. Plus, I have added a 2GA neg cable from battery to alternator mounting bolt 7 years ago when SC was installed. Battery is less than year old and fully charged. SC has water to air intercooler. Just replaced plugs.

 

[Laga]

New alternator came in. Higher value in normal range with black probe on positive stud. 0 value when reversed. As it’s supposed to be. Going to install new alt.