Voltage jumping, louder than usual

[Clay-Goose-Davis]

A few weeks ago, my recently acquired 91 (57k miles) died in the rain. Alternator was dead and 2 year old battery wouldn’t hold a charge. I replaced the battery, alternator, spark plugs, plug wires, did a coolant flush, oil change, and did the JB Weld Magnet job while I had it all out of the way. It ran great after this and got rid of my “electrical problem” and “service engine soon” lights.

Previous owner replaced alternator, coil pack, and battery about two years ago. 
Recently my voltage has been jumping to about the 3/4 mark. I’ve been keeping my headlights on while driving for peace of mind, cause it keeps the voltage stable. But the other day it started jumping again with the lights on. Car sounds like it’s running rough and right as I got home, temp gauge went up 1 notch from center on a 60 degree day avg 45mph drive (has never moved up past center). Coolant is brown again too. Oil also smells of gas and I’m getting 20mpg. WHATS GOING ON? Also feels like there’s still a slight misfire

[2seater]

May need to break up the problems into smaller bites to get a better focus on each one.

 

Two batteries and alternators in two years is not normal. I know parts quality is getting to be hit or miss proposition at times but??? I am always a little hesitant as the 91 is rarer and we find differences we didn't know about, but I believe the alternator is not self-regulating and is controlled by the BCM. The problem may lie there. That said, I would go over all the ground points as poor grounds can cause all sorts of weird behavior. Are you familiar with the Reatta systems?

 

Brown coolant this soon sounds like the initial flush may not have been as thorough as needed. The temperature gauge could be part of the electrical issues or a coincidence if it works normally otherwise. 

 

20mpg for an overall average wouldn't be terribly out of line, especially not knowing what the conditions are based on. This car may be compact but it is not lightweight, and weighs as much or more than a LeSabre of similar vintage. Also, the 91 has a numerically higher gear ratio. It does have slightly taller tires but the net result is 9% higher rpm at the same speed. If the O2 sensor age is unknown, it should be changed. It is what sends the signal for the ECM to control fuel mixture.

[Clay-Goose-Davis]

Thank you 2seater. I am barely familiar with the systems. Where should I start with checking grounds, are there any good diagrams you could point me toward?

Is there any way, using the on board diagnostics, to see if the BCM is acting up/ the cause of the high voltage output. I don’t have a multimeter so testing electrical components has been more of an intuitive process lol.

I’m planning on changing the fuel filter and O2 sensor this weekend. I knew gas in oil could be from faulty injectors, bad piston seals, ect.. but I didn’t think of rich fuel mixture caused from the O2 sensor. Could explain the soft misfire too right?

 

Thanks again, I’ve been on this forum about every day for the last 3 months and have read a lot of your posts. Great to chat with ya!

[Ronnie]

25 minutes ago, Clay-Goose-Davis said:

Where should I start with checking grounds, are there any good diagrams you could point me toward?

This shows where to start checking the grounds.  Ground Terminals & Junction Box Location

[Ronnie]

Also check the battery cable connections as described here: Battery Cable Checks & Repair  This happened to me and caused the electrical system to go crazy.

[2seater]

Ronnie has pointed to a very good and important resource, the How To and Diagnostic guides that are at the top of the page. There is a wealth of data and real world examples from the Reatta community. Also, use those sections to learn to use the onboard diagnostics and controls. 

 

I believe the 91 may have more items available in diagnostics but this is what is available on my 90, which should be similar. Under BCM Data, BD50 is live battery voltage and BD51 is Generator Field Strength percentage. These can be read even while driving once you get used to the system. It may help to see if the voltage or field is erratic. Under the BCM Inputs section, there is B151 which is Generator Feedback. I am not 100% certain on that function but I believe it indicates if the generator is following commanded operation?

 

There could be several reasons for rich running, possible misfire, poor mileage, but the O2 sensor is inexpensive and a good place to start. It is the only actual fueling feedback on these cars. The O2 operation and resultant fueling indicators can also be read in the ECM Data section of diagnostics while sitting still or on the road. The system is relatively basic compared to modern vehicles but it can tell you a lot and it is already built in. It can do pretty much anything a scanner can do. Good luck

[Clay-Goose-Davis]

On 9/13/2023 at 7:50 PM, Ronnie said:

Also check the battery cable connections as described here: Battery Cable Checks & Repair  This happened to me and caused the electrical system to go crazy.

Thanks Ronnie, my positive terminal bolt was stripped and wouldn’t get tight enough. JUST picked up a new set before heading to the laundry mat, so I’ll change em out and see how she runs when I get home.

[Clay-Goose-Davis]

22 hours ago, 2seater said:Under BCM Data, BD50 is live battery voltage and BD51 is Generator Field Strength percentage. These can be read even while driving once you get used to the system. It may help to see if the voltage or field is erratic. Under the BCM Inputs section, there is B151 which is Generator Feedback. I am not 100% certain on that function but I believe it indicates if the generator is following commanded operation?

Hey 2seater, thank you!

 Do you know what percentage range is acceptable for the Bd51 test while car is idling? 

[2seater]

53 minutes ago, Clay-Goose-Davis said:

Hey 2seater, thank you!

 Do you know what percentage range is acceptable for the Bd51 test while car is idling? 

Not a clue🙂 I have never had the need to observe it. My assumption is there are multi factors involved such as state of charge in the battery, or condition, accessory load etc... It just adjusts the field strength in the alternator to reach whatever target output it is programmed for. I suspect a poor battery connection may explain a lot of bad behavior.

[Clay-Goose-Davis]

After swapping out the terminals, the BD50 went from jumping up and down quickly between 13.9 - 15 (while idling pre terminal swap) to a steady 14.1-14.3. 
The BD51 went from jumping up and down quickly between 14 -56 (idling without headlights pre swap) to up and down between 21-61 and mostly in the 40s.
And between 29-73 with headlights lights on but mostly in the 50s.

 

If I find the expected BD51 range, I’ll post it here.

[Ronnie]

I think the battery cable connection was most likely your problem.