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  #1  
Old December 5th, 2019, 20:51
ThinAir ThinAir is offline
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Question Random voltage spikes & limp mode.

Hello All,

I thought I had the problem solved two years ago when I replaced the battery temperature sensor, but the symptoms are back. I will be returning the sensor to Advance tomorrow for a replacement. Maybe the part was was faulty, maybe its something else....dirty grounds? Before I replaced battery temp sensor I had the alternator tested (good) and I replace the battery, so I though I had ruled those out. Symptoms still occurred with the new battery.

Stock 97 xj 4.0L, 1 owner

The intermittent symptoms are as follows:

Start car, put the car in gear (always reverse as far as I remember), accelerate a few yards, check gauges light comes on, voltage is pegged at 19v!, put car in drive, drive a few yards, stop, check gauges light goes off, voltage back to normal 14v. Try to accelerate again and it goes into what I believe to be limp mode, correct me if I'm wrong. The vehicle it super slow to accelerate, will get up to speed, but once stopped again will not accelerate normally. Dangerously slow acceleration, like the gas pedal does hardly anything. Turn the car off, start up, repeat.....or start up everything is fine and dandy and I drive off into the sunset.

This is a random occurrence and I cannot pin down anything typical accept for being in reverse when this occurs, though I think that is circumstantial. I will replace the bat temp sensor again and thoroughly clean all the grounds that I need to. Guidance on which grounds to pay close attention to will be really helpful for me. Your help and wisdom is greatly appreciated.


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  #2  
Old December 5th, 2019, 22:51
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xeroOTG xeroOTG is offline
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Random voltage spikes & limp mode.

There's not a limp mode as far as I know, which means you can add the loss of power to your symptoms list. I'd start going through sensors, ohming them out one at a time, starting with the TPS, then the MAP. There's some great info burried on here about the various impedances of the various sensors on your Jeep. You didn't mention, are there codes? Check engine light? That's a pretty big breadcrumb when it comes to troubleshooting these sorts of situations.

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  #3  
Old December 6th, 2019, 03:35
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Tim_MN Tim_MN is offline
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Re: Random voltage spikes & limp mode.

Is or was the Check Engine Light ON ? What is the condition of the cables and connectors ?

When the dashboard gauge shows 19 volts, there is almost always a connectivity issue such as loose or corroded wire connection or grounds, or internally corroded wires or cables.

The secondary symptoms are very likely caused directly by the high voltage. Cure the root cause and they should clear up as well.

Perform routine maintenance of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Battery terminals and battery wire connectors should bright silver, not dull gray to blackish and corroded. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, wire brush, and clean until shiny the cable/wire ends and whatever they bolt onto.
 
Jeeps do not tolerate low voltage, bad wire connections, or poor grounds.


I always suspect non Jeep engine sensors. Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a " Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.



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  #4  
Old December 6th, 2019, 05:19
RCP Phx RCP Phx is offline
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Re: Random voltage spikes & limp mode.

I would check everything from the battery to the PCM. I use De-ox on every connection I touch to prevent corrosion.
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Old December 7th, 2019, 08:23
lawsoncl lawsoncl is offline
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Re: Random voltage spikes & limp mode.

How are the engine mounts? I'm just wondering if you have excessive movement that is exacerbated in reverse. I don't recall if the 97 has the ground between the head and the firewall, but definitely check that you have a good ground to the engine block from the battery. Look at the wiring going down near the exhaust and make sure nothing is touching or melted. I've seen intermittent shorts there that only happened in reverse. If the codes are for CPS, check that the bell housing bolts are tight (that just recently came up again as a rare cause in another thread).
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Old December 7th, 2019, 11:19
ThinAir ThinAir is offline
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Re: Random voltage spikes & limp mode.

Quote:
Originally Posted by Tim_MN View Post
Is or was the Check Engine Light ON ? What is the condition of the cables and connectors ?
The check engine light never comes on. There is no physical sign of a voltage spike, like lights getting brighter, engine surge etc. Its very strange. The voltage meter goes back to normal and then, about half the time, it will do the slow acceleration thing.

The cables and connectors are original but I keep them clean and they don't look corroded. I could stand to replace them due to their age.
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Old December 7th, 2019, 11:34
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Re: Random voltage spikes & limp mode.

Have you checked for stored or pending OBD-II Trouble Codes ?

19 volts on the gauge means the alternator is maxed out, typically from poor condition wires or loose/dirty/corroded connections. A faulty battery temp sensor might cause the symptoms, I would get a genuine Jeep part.

Testing the Throttle Position Sensor would be a good idea.
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Old December 17th, 2019, 18:10
ThinAir ThinAir is offline
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Re: Random voltage spikes & limp mode.

Quote:
Originally Posted by Tim_MN View Post
Have you checked for stored or pending OBD-II Trouble Codes ?

19 volts on the gauge means the alternator is maxed out, typically from poor condition wires or loose/dirty/corroded connections. A faulty battery temp sensor might cause the symptoms, I would get a genuine Jeep part.

Testing the Throttle Position Sensor would be a good idea.
No Codes. Replaced the bat temp sensor with new OEM sensor. Still having the problems. I'm pretty convinced this is only happening when put into reverse btw. Next, I will replace the battery cables and clean all the grounds. Ill keep y'all posted as I go.
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Old December 17th, 2019, 19:07
dktool dktool is offline
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Re: Random voltage spikes & limp mode.

The TCM has a limp mode of it's own, it believe it commands a 2nd gear start and what else I don't know. It should throw a P0700'ish code though.

Just keep checking / cleaning connections you may get lucky.
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Old December 17th, 2019, 22:04
lawsoncl lawsoncl is offline
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Re: Random voltage spikes & limp mode.

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Originally Posted by dktool View Post
The TCM has a limp mode of it's own, it believe it commands a 2nd gear start and what else I don't know. It should throw a P0700'ish code though.

Just keep checking / cleaning connections you may get lucky.

TCM doesn't really have a limp-home mode per-se. If the TCM has no power, then it will start in 4th gear with the shifter in (D) just because no of the solenoids will get power. You can manually shift put the shift in (1) and get 1st gear, and third in (3).


Check the grounds, particularly the ground strap to the body. Also look at the wiring around the exhaust as things shift in reverse, something might be touching.
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Old December 18th, 2019, 03:54
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Re: Random voltage spikes & limp mode.

Why does the Check Engine Light never come ON ? It should at least self test when starting.

Chafed/melted wire insulation from contact with sharp metal or hot parts ? Damaged wire plugs or corroded/bent wire plug pins ? Worn out motor mounts could allow excessive engine movement and cause wire harness contact with sharp metal or hot parts, or simply cause the wires to fatigue and become an open short.
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  #12  
Old December 18th, 2019, 18:12
ThinAir ThinAir is offline
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Re: Random voltage spikes & limp mode.

Quote:
Originally Posted by Tim_MN View Post
Why does the Check Engine Light never come ON ? It should at least self test when starting.

Chafed/melted wire insulation from contact with sharp metal or hot parts ? Damaged wire plugs or corroded/bent wire plug pins ? Worn out motor mounts could allow excessive engine movement and cause wire harness contact with sharp metal or hot parts, or simply cause the wires to fatigue and become an open short.
I does self test when I start up.
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Old December 19th, 2019, 10:35
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Re: Random voltage spikes & limp mode.

Quote:
Originally Posted by Tim_MN View Post
Why does the Check Engine Light never come ON ? It should at least self test when starting.
Maybe the bulb is burned out?
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  #14  
Old December 19th, 2019, 13:15
ThinAir ThinAir is offline
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Re: Random voltage spikes & limp mode.

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Maybe the bulb is burned out?
Bulb is not burnt out. It comes on at startup with all the other gauge lights and goes off with all the others.
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Old December 19th, 2019, 15:23
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Re: Random voltage spikes & limp mode.

Solved the 19 volts symptoms yet ? Physically and visually inspected the wire harnesses and tested the wire continuity ?
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