Full Size Jeep Network

Tim, great info as always.

Took a good DVM, battery read 12.68v.... started battery was at 15.1 to 15.3v... turned on heater fan, 15.3, headlights 15.3... after 2 mins same readings. Turned off the engine.... and loaded the off-road motorcycle to the carrier... about 20min.

Restarted battery reading was 14.3v... I drove about 30min, popped the hood, battery was at 14.3v... same on the return trip, so the voltmeter is off on the low range, but about very right for anything above 13.5v. I have a secondary led voltmeter for giggles, and it is as good.

What can we ask to a 10 year old NAPA alternator... it is a 75 amp... not the 85 amp that is supposed to be stock when you have AC... well my AC is missing, it is in a box in the cellar, waiting for better days!

so after cleaning all the connectors with contact cleaner... same, 15.1 to 15.3v.

So went the reasonable option, replaced alt with the spare new one, that I have just in case, it is easier than messing around with resistors ... a bit better at 14.8v at idle. Of course in the process I realized that my alt bracket is in need of an upgrade, when I removed the AC compressor I lost one of the reinforcement... but it works! the sense wire looks a bit tattered...

Thank you for your readings...

I'd note that these SI alternators are easy to take apart and repair. Hardly anybody messes with them today, since the McParts store replacements are so cheap - possibly indicating the quality of the parts within. You can buy the regulator and replace it. SMP/Standard VR111 might be worth a try - https://www.rockauto.com/en/moreinfo.ph ... 84&jsn=396

I recall there is a zener diode inside the voltage regulator that is compared with the charging voltage to determine the alternator output current. The higher the output current, the more voltage across the battery due to its internal resistance (plus electrochemical potential), and the higher the charging voltage. Not much you can do to adjust this voltage set point. Likely there is a voltage divider inside the regulator that is compared to the zener, but that's not accessible.

To reduce the charge voltage at the battery, you'd need to increase the voltage that the regulator sees at its sense terminal. There's no reserve of positive voltage to draw from - the charge voltage is the highest voltage present in the vehicle. One could imagine adding an additional supply (switching? a battery?) to provide the extra positive voltage, but that's really out there compared to just making the system work as designed.

I'd note that the Jeep SI alternator with a voltmeter picks the sense voltage right at the alternator. The sense wire wraps back on a very short wire to the big wire to the battery. With an ammeter, the charge wire goes up through the dash and back to the battery (yellow 10 ga wire), where the sense wire connects. In both cases, I believe any resistance along the charge path (times the charge current) will be added to the sense voltage the alternator sees. The zener does not change, but the alternator has to work against the path resistance to bring the voltage up to the set point at the sense wire.

Normally the wire and connector resistance is effectively nothing, and you get whatever voltage the zener calls for. I could imagine some bad connections or burnt wire or something could make that resistance a factor. One way to test this is to take the red voltage sense wire and run a large new clean wire directly from the alternator sense terminal to the positive battery post. You should then measure exactly the same voltage at the alternator sense connection, the sense wire battery connection, and the alternator charge wire connection to rule out the wiring contributing.

D2 is the zener diode.


Note the inner dotted line is within the regulator. Connection 2 is the sense wire. The diagram shows R2 as adjustable, which would change the set voltage by the voltage divider between R2 and R3. Pretty sure there is no access to this divider on regular production SI regulators.

Excellent explanations, thank you.
I did not find any difference on voltage at the ALT, I also checked the exciter wire #1... with a jumper to the battery plus, no changes.

Cleaning and making sure that the #1 and #2 wires were tight, that molex is old, the spades were not as tight as should be. I got rid of the molex, the spades are covered with shrink tubing, not shrunk, slipped over to prevent shorts and are easy to slide out ... at least DVM voltage at the battery is 14.88 to 14.93v

So I went the initial route and use a step down voltage gizmo to 13.1 to 13.5v to the gauge, the same will be used for the robot display that is also the victim of overvoltage. so I will see what is happening... installed at bottom left!

Update, trying to beat the one year procrastination symptom... tested the sender, no resistance... so it failed. Ordered a different brand, but the display was not that good, black unless the lights are on, at least I tested their sender... so it was returned, and getting another set whose sender seems similar to the failed Max Tow.

Hoping you can get this working, as I am building my '84 GW for towing our camper, and want to put a trans temp gauge in it as well.....so following

Update to beat the one year procrastination, after testing of few chinese cheap s--t, cool because the width was about 9/16", but no working after 5 mins... Another set had a very large sender that did not fit, so I bought another maxtow but without the digital display at $50...
In the process I reinstalled the old sender with the new gauge, and after 10 min the hand was starting to flutter... at least I did not reinstall the carpetting... Now the new sender is steady after 10min, but still using the voltage reducer.

Keep us in the loop