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Old 10-19-2020, 09:26 PM   #1
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Jeep Won’t Start After Flat Tow

Help! I have flat towed a Jeep JLU behind the motorhome 9 times now. 4 of those times the Jeep failed to start after being towed. That is a 45% fail to go adventure ratio

Each time the Jeep was placed into and taken out of recreational towing mode I followed procedures described on pages 397–402 of the 2019 Owner's Manual.
At the end of the procedure to place the Jeep into tow mode, the START/STOP Ignition Button was left in the OFF position. It was not left in the ACC position while being towed. There was no rapid clicking associated with a dead battery when the Jeep would not start. The battery was checked with a voltmeter and found to be fully charged. I simply put my foot on the brake, punch the START Button and nothing happens.

Troubleshooting to isolate: without connecting the Jeep to the motorhome, I placed the Jeep into tow mode and then took it out of tow mode the next day. I must have tried this daily for over 8-10 days. The Jeep started every time. The problem only occurs after the Jeep has been towed behind the motorhome, the last time being after troubleshooting so it seems related to the motorhome.

The only electrical connection between the motorhome and the Jeep is a 7-wire power cord that operates the Jeep’s left/right turn signals, brake lights, and tail lights.
The charge line on the 7-wire power cord was checked and found to be providing proper voltage from the motorhome to the Jeep battery.

None of the 3 different Jeep dealerships that have investigated the problem have been able to replicate the problem or find anything wrong.

What could the motorhome possibly be doing to the Jeep? Maybe it just doesn't like the Jeep
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Old 10-19-2020, 09:54 PM   #2
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What do you do that eventually gets it started?
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Old 10-20-2020, 06:33 AM   #3
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Will the jeep start while it is attached and in tow mode before disconnecting?
Anything else not working besides the starter when this happens?
Is there a sensor that tells the starter that you have your foot on the brake being affected by auxiliary braking system?
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Old 10-20-2020, 07:51 AM   #4
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What do you do that eventually gets it started?
1st time - after about 10 minutes of attempting the “Shifting Out of Neutral” sequence it started. I have no idea why. I was a complete newbie at going in and out of tow mode so I chalked it up to not knowing what I was doing.

2nd time – it was towed to the nearest dealer. When they got it the next day, it started without any problem.

3rd time – it was towed to another dealer (different city). When they got it the next day, it started with any problem.

4th time – was at the RV dealer who installed the braking system. The tech pulled out and plugged back in the Jeep’s starter relay and it started!
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Old 10-20-2020, 08:18 AM   #5
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Originally Posted by Rocky Larson View Post
Will the jeep start while it is attached and in tow mode before disconnecting?
Anything else not working besides the starter when this happens?
Is there a sensor that tells the starter that you have your foot on the brake being affected by auxiliary braking system?
I haven’t tried to start the Jeep while attached and in tow mode before disconnecting so I don’t know if it would start under that condition. I’ll give it a try but it's weird that half the time the Jeep starts. The Owner’s Manual doesn’t address the sequence for attaching power cords or air hoses.

Everything else is working at the time other than the ignition which immediately goes from RUN to OFF.

About the sensor. The Jeep has a sensor that requires my foot to be on the brake when starting it. The Jeep’s supplemental braking system is an Air Force One so there’s no electrical connection to the Jeep other than turn signals, brake, and tail lights. The Jeep’s braking is driven by the motorhome’s air brake. If I misunderstood your question then please let me know.
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Old 10-20-2020, 08:33 AM   #6
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I would get a voltmeter across the battery when this condition exist. While the meter is across the center of the posts (not on the cables), turn on your headlights if possible.

The goal here is to get a load on the battery. Surface voltage is useless. Think of surface voltage as a layer of ice over a stream or lake that has now run dry. Your meter may read full voltage but once the ice breaks there is nothing there. A load will break the ice.

I can't remember how many batteries diagnosed over 50 years that could give you the craziest symptoms.

Of course you could have some goofy item like your pc not allowing some app to run but not another and you have to re-boot. Modern vehicles are full of computers and perhaps your jeep thinks the starter is engaged and will not allow it to start.

If this was my vehicle with a battery that has been run down ( at least it looks like it ), I would have a new battery in place immediately. This is one case where I DO recommend throwing parts at a problem. We don't ever buy batteries, we just rent them.

A new battery or even a known good used one will suffice.

Once the battery is replaced and the problem persists, disconnect the negative terminal if at all possible when towing. Or at least add a cutoff switch on either the positive terminal or negative terminal so you can re-boot the jeep upon arrival.

I suggest this because there is no way some dealership will catch this thing in the act and actually diagnose it unless this problem surfaces with more vehicles and becomes some sort of known issue with XYZ computer module or flash upgrade.

My gut feeling is the battery has an internal issue and a combination of some aux brake load and the charge line not actually refilling the battery do to weak connections and long runs of wimpy wires are giving you a dead battery upon arrival.

I'm not looking over you shoulder so I can only go on hunches and 50 years of diagnosing (bad batteries).
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Old 10-20-2020, 11:33 AM   #7
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Originally Posted by YC1 View Post
I would get a voltmeter across the battery when this condition exist. While the meter is across the center of the posts (not on the cables), turn on your headlights if possible.

The goal here is to get a load on the battery. Surface voltage is useless. Think of surface voltage as a layer of ice over a stream or lake that has now run dry. Your meter may read full voltage but once the ice breaks there is nothing there. A load will break the ice.

I can't remember how many batteries diagnosed over 50 years that could give you the craziest symptoms.

Of course you could have some goofy item like your pc not allowing some app to run but not another and you have to re-boot. Modern vehicles are full of computers and perhaps your jeep thinks the starter is engaged and will not allow it to start.

If this was my vehicle with a battery that has been run down ( at least it looks like it ), I would have a new battery in place immediately. This is one case where I DO recommend throwing parts at a problem. We don't ever buy batteries, we just rent them.

A new battery or even a known good used one will suffice.

Once the battery is replaced and the problem persists, disconnect the negative terminal if at all possible when towing. Or at least add a cutoff switch on either the positive terminal or negative terminal so you can re-boot the jeep upon arrival.

I suggest this because there is no way some dealership will catch this thing in the act and actually diagnose it unless this problem surfaces with more vehicles and becomes some sort of known issue with XYZ computer module or flash upgrade.

My gut feeling is the battery has an internal issue and a combination of some aux brake load and the charge line not actually refilling the battery do to weak connections and long runs of wimpy wires are giving you a dead battery upon arrival.

I'm not looking over you shoulder so I can only go on hunches and 50 years of diagnosing (bad batteries).
Thanks, YC1, I’ll give your load test a try but I’m starting to think the problem is computer-related as you mentioned. JL Jeeps have 2 batteries managed by an IBS (Intelligent Battery System). This video goes over starting the Jeep (or not) using one or the other of these batteries.



One thing I failed to mention was that the Jeep supplemental braking system uses a proportional air brake system so no need for battery power.
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Old 10-20-2020, 12:32 PM   #8
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Stunned by this but sounds like a fun one to chase down.

I would get a voltmeter onto the posts where the aux battery connects. They showed that removing that battery from the system makes it look like things should work but apparently do not.

However, the aux battery alone is enough to start the thing and also supplies enough power to the fuse block that everything works, including the charging with the alternator.

If the aux battery is sick or just gets run down while towing it is easy to see why it won't start based on this video. Your charge line may NOT be charging the aux battery.

I tried to zoom in on the schematic in the video but still can't get a clear picture.

Any chance that battery is some sort of Lithium?? Just curious.

With the keyless start systems there can be a huge draw while towing and a small battery could be run down easily.

I would love to see the Aux battery checked and even before doing that I would connect the charge line wiring to the toad and measure the voltage on the posts where they took the battery cable off on the fuse panel. This would be to verify the aux battery is getting charged enough.

If you can come up with a schematic for the jeep system and post it there are lots of folks here that can sort it out.
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Old 10-20-2020, 06:14 PM   #9
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Besides the batteries, I would look at whether the vehicle has any safety lockouts for the starter, like a shift position sensor that would prevent starting if the transmission selector is is in drive.
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Old 10-20-2020, 06:23 PM   #10
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The only electrical connection between the motorhome and the Jeep is a 7-wire power cord.......charge line on the 7-wire power cord was checked and found to be providing proper voltage from the motorhome to the Jeep battery.
To the battery or to a battery charger (as mine is wired)?


A very convenient tool is a cig lighter(?) plug-in device that indicates battery voltage. Many also have USB sockets. No need to reach the battery with DVOM cables.
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Old 10-20-2020, 07:28 PM   #11
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What brand wiring harness did they install?
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Old 10-20-2020, 08:00 PM   #12
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What brand wiring harness did they install?
Roadmaster Univeral Wiring Kit 4-wire 4D mfg part # 17-0361
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Old 10-20-2020, 08:08 PM   #13
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I have had the same problem intermittently. Don’t ask me how or why, but if I push the brake pedal really hard, it will start every time.
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Old 10-20-2020, 08:26 PM   #14
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To the battery or to a battery charger (as mine is wired)?


A very convenient tool is a cig lighter(?) plug-in device that indicates battery voltage. Many also have USB sockets. No need to reach the battery with DVOM cables.
Not sure about the connection to the battery but will find out.
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