Strange Voltage Drop

I have a 1990 Jeep Cherokee Laredo.

Inline 6 242ci 4.0L

AW4 Automatic

I have a situation when at idle I show 12 volts at the dash gauge. As soon as i begin moving forward the voltage drops to 9 (low end of the gauge).

If I sit in park at idle and rev the engine to 3K i still show steady voltage.

If I idle in drive at 3-5 mph i show the drop.

When at idle witht he lights on the voltage drops, when the blower is on the voltage drops, when the radio turn on the voltage drops. If i am in that state when I begin to move forward the voltage drops out completely and the car dies.

Auto store testing equip and one famous muffler shop all show the alternator and the regulator to be working perfect. (14.2-14.3v at alternator under load)


Time for a new belt or tensioner is my guess.Both of these options could describe your situation.

You must have those stupid clamp on battery terminals? If so, replace them.

The wife’s 90 Jeep Cherokee had charging system issues when these clamp on terminals corroded where they clamp onto the battery cable. Voltage drop.


Thank you. I thought the same. I replaced both of the battery wires. (to the block and to the starter) when I replaced the battery.

Thanks, I will adjust the belt tonight and see what happens.

FYI - Note that the voltage drop DOES NOT occur when the RPM’s are held above normal cruising idle as long as I am not in MOTION and the voltage drop DOES occur when I am in motion at idle RPM…

Adjusted the belt. It had just about 1" of drop in the belt between idler pulley and power steering pump. Now it has 1/2-5/8". NO joy…

SO to clarify:
Sitting at idle in Park - Gauge shows 12.5v
Sitting at 2500 RPM in Park - Gauge shows 12.5v
Rolling at idle in Drive - Gauge shows 9.5v
Driving at 2500 RPM in Drive - Gauge shows 9.0v

Putting on Lights, or Heater Blower, or Radio will cause voltage drop. (cumulative effect for using more than one)

Alternator tests perfect 14.3v Output No load, 14.2v Loaded
Regulator test functional
New Battery (tested 95% full charge)
New Positive battery Cable
New Negative Battery Cable

If the alternator voltage is always staying above say 13 volts but the voltmeter shows 9 volts then the trouble is in the power bus that feeds the accessories of the vehicle and not the charging system. Check the main wire that ties to the fuse panel under the hood. Also see if the voltage is low at the fuses there. If they are ok then check the voltage at fuses in the dash. The trouble could be with the ignition switch or a connection to it.

More than likely this vehicle has a computer in the air intake usually located on the left front fender behind the battery. It will have a rectangular multi-pin plug held in place by capscrew with an 8mm hex head in the center of the connector. This computer varies the resistance on the ground going to the field windings of the alternator which varies the voltage output, there is no separate voltage regulator on these systems and when the computer fails completely, the voltage output of the alternator can exceed 20 volts effectively frying all of the electronics on the vehicle if not causing it to burn to the ground. More than likely the computer is going bad or you have damage to the wiring harness between the computer and alternator. I strongly suspect the computer because the voltage drop is coming into play when any secondary load is put on the system. A typical radio draws less than 1/3 amp of current, not enough to register on the dash gauge. The problem can aslo be coming from the body module computer located under the dash, this monitors/controls electrical functions providing feedback to the computer in the air intake - either or both can cause very stange things to happen such as the headlamps coming on when the vehicle is parked and turned off.

Avoid the discount or chain stores, take it to a professional shop or the Dodge dealer and have it checked properly. You can also read the codes via flashes of the “check engine” lamp by cycling the key off and on three times then leaving it in the “on” position the fourth time but often the failure codes displayed via the check engine lamp will not disclose anything about the gripe you are experiencing, it must be tested dynamically using an external computer testing system. Be prepared to pay too, Dodge is proud of their computers and all three will run between $700 and $1200 each.

Thanks. I will check these tonight.

Thanks. Interesting… I am not sure how this could be the case with the conditions I am experiencing. You refer to “secondary load” being the indicator, but this effect occurs when I IDLE in gear (only when rolling not when held in place by the brakes) Hardly more “load” than in gear brakes on and RPM’s pushed up to 2500 and held (which does NOT cause the voltage drop)

So I was thinking on my way into work today (yes, in this Jeep)…

If the following is true
"this effect occurs when I IDLE in gear (only when rolling not when held in place by the brakes) while being in gear, brakes on, and RPM’s pushed up to 2500, and held (does NOT cause the voltage drop"

I wonder if the issue could be a short in “some” transmission wiring. Or perhaps a Cruise Control sensor that is effected by actual vehicle motion…

just thinking.

Any such short would have to consume quite a bit of power, and so it is unlikely since you haven’t blown a fuse or burned the Jeep up. The thought about a sensor sensing motion and causing the drop is fantasy.

You need to get a cheap DMM and measure the voltage at the battery, alternator, and power port in the car. If your dash gauge is really reading 12V, then the alternator isn’t working, even when you think it is. Just because it works correctly in the store, out of your jeep, does not mean that it is working correctly in the jeep. The power wiring to it may be damaged, or the one of the control wires.

I really think that this thing about in gear in motion is a wild goose chase.

Keep in mind that the alternator output is affected by RPM. It has a minimum rpm to work but it’s typically below idle speed. When you go to 2500 rpm, even a marginal alternator can look OK or overcome other obstacles impeding the output. Idle is the toughest condition for alternator output IMO.

I’d buy a cheap ANALOG multimeter and some small electronic jumper cables designed for electronics work. Radio Shack probably still carries them. Measure the voltage wherever you can get to it – positive battery terminal, accessory terminal(aka cigar lighter), starter solenoid, etc,etc, etc and see if it matches what you see on the dash meter both in neutral and in drive. Hopefully that will help you isolate what parts of the car electronics have low voltage and hopefully will give you an idea where to look for the reason.

Don’t forget to measure the voltage between the chassis and the battery ground terminal. It should never rise off zero except possibly just a smidge when the starter is running.

You can do all this with a digital meter, but it will be about ten times as hard.

Thank you.

The testing was done with the unit in the vehicle. Both times. Both showed same results. Alternator is putting out 14+

Testing the power ports at the fuse panel is next on the list.

Again the issue is CLEARLY associated with movement of the vehicle. the condition does not exist under any conditions that do not involve movement and always exists when movement is involved…

I already own both a DMM and an Analog meter. I have the Factory Service Manuals with schematics but was really HOPING to avoid the hunt and test…

Additional clue.

I drove home today for lunch in the rain. That means headlights on, wipers as well. Defrosters would have been nice, but not gonna go that low a voltage reading while driving. Each and every stop the voltage quickly climbs back up to 12+ (lights on, etc)

The additional clue was while thinking about comments above about dash wiring I started looking at the other gauges in regard to the stopping issue. Of course, the oil pressure gauge drops when it idles and increases when it drives, but that was it… Until I stopped at the house.

That is when I noticed that when I parked the voltage ran back up to 11, then turned off the lights and it went up to 13+ but I also noticed that the oil pressure qauge showed and increase in pressure when the lights got turned off… I flipped them on and off a couple times and saw that the movement in the reading for the voltage gauge and the movement in the oil pressure gauge was about the same… like maybe a short in there somewhere…

Have you checked the ground strap between the engine and the body?

I don’t think it’s a short. A short would consume a lot of power and get very hot. It’s an open (well partial open). You have a loose connection somewhere. It could be a power connection or a ground connection. Basically, one or more connection(s) has a high resistance.

thanks again. I’ll look with that in mind.