Jump starting vehicle, do in low range?

[88mjblue]

If I can't get vehicle to move fast enough to jump start in first gear, would going into low range help it start easier?

What about jump starting in reverse?

 

I ask because I have a small "runway" to attempt this, and I'll only get one shot at it...

[ParadiseMJ]

I've always done it in 2nd.

 

So, what's the deal, starter motor, battery?

[gogmorgo]

Doing it in low range will also add drivetrain drag to the system which could impede the start, at least in my mind. Lower gears will turn the motor more quickly at a given wheel speed but will also increase the force required to make it turn. Higher gears do the opposite, slower turning speeds but more resistance.

I still haven't managed to bump start my '92 4.0 XJ, although I've only tried twice. The battery in it is either worthless or there's a huge drain, charging system seems to work (voltmeter sits at 14 the whole time) but it was on a charger overnight when I picked it up, and it started great twice there and didn't have enough juice to crank it over after I stalled in the driveway (oops) after the 40-mile drive home. But otherwise it runs and drives great. The hill I tried on is maybe 10% grade and 100ft long, so should be a no-brainer.

First time I let in the clutch in 3rd about half-way down the hill, but no bueno. I then reattempted at the bottom in second, also no good.

The second time I attempted to start it down the hill, I went for 1st gear about 1/3 of the way down the hill, and left skid marks when I let the clutch back in. Figuring I let the engaged the clutch too quickly, I let it out more slowly towards the bottom of the hill, still in first, and it spun the engine over a couple times but didn't catch before it slid (again leaving skid marks) to a stop. If it happens again, I think I'll try for second about half-way.

When boosting it, it always requires a couple revolutions before it fires, and I've read it usually takes a couple to "wake up" the ECU after a completely dead battery, and also that the Renix's are worse than the HO's for that. But in my mind it makes more sense there simply isn't enough juice to prime the fuel pump. My take-home is there may not be enough distance in your "small runway" if it's a dead battery.

[Eagle]

If I can't get vehicle to move fast enough to jump start in first gear, would going into low range help it start easier?

What about jump starting in reverse?

 

I ask because I have a small "runway" to attempt this, and I'll only get one shot at it...

 

You're doing it backward -- you want to be in 2nd or 3rd gear.

 

1st gear gives the engine mechanical advantage over the wheels. To push start, you want the wheels to have mechanical advantage over the engine. Using low range will not help, it will make things 2.47 times worse.

[88mjblue]

Ok thanks guys, that makes sense, I will try 2nd gear.

[MJanky]

Ok thanks guys, that makes sense, I will try 2nd gear.

Please let us know if it worked. I've always had an interest in bump starting vehicles.

 

A Renix ignition Jeep poses additional challenges in this regard because of the way it works. It is unlikely to start unless moving at a higher and sustained rate of speed compared to more traditional ignitions. I've been collecting the parts needed to change my MJ to a carb if for no other reason than to confirm this for my own information.

[Marine1Texas]

If you have a HEI dizzy it does not need even a full rotation before it will start. The HEI is self contained and does not use signal CPS.

 

I have one truck that runs HEI and a carb. starts up on less then a rotation, normally on 1st or 5th order.

 

On my truck that has HEI and fuel injected it is the same way at starting. After a few years of living in Texas with all the mud that can get caked on the cps, I switch and have had no problems. Also I can adjust the timing and then go much farther advance when needed.

 

I am not putting down the computer run dizzy works great!

 

I have bump started it a few times, just with a little slop and pop clutch, right off less then 1 mph....

[MJanky]

If you have a HEI dizzy it does not need even a full rotation before it will start. The HEI is self contained and does not use signal CPS.

 

I have one truck that runs HEI and a carb. starts up on less then a rotation, normally on 1st or 5th order.

 

On my truck that has HEI and fuel injected it is the same way at starting. After a few years of living in Texas with all the mud that can get caked on the cps, I switch and have had no problems. Also I can adjust the timing and then go much farther advance when needed.

 

I am not putting down the computer run dizzy works great!

 

I have bump started it a few times, just with a little slop and pop clutch, right off less then 1 mph....

Thanks for that info, that's the sort of thing I like to know about.

 

I have the 2.5 so no HEI swap for me, but I prefer carbs over fuel injection for other reasons anyway so doing away with the Renix *for my purposes* is a win-win.

[88mjblue]

I saw a video of somebody jump starting a vehicle by jacking up the rear and then using a rope or strap to spin the wheel and then pop the clutch.

[MJanky]

I saw a video of somebody jump starting a vehicle by jacking up the rear and then using a rope or strap to spin the wheel and then pop the clutch.

I've tried that on several vehicles and it did not work, not even close. I'm not sure what is different about the vehicles in the vids (I've probably seen the same ones as you) but at least one was a European vehicle and likely had a low compression engine which makes a huge difference. If someone was to attempt this on an MJ/XJ what you'd find is how hard it is to get the wheel to turn in low gear (practically impossible) and how slowly the engine would turn in high gear (too slow to start.) Stick to roll starting it. I agree with everything that has been said in this thread...2nd or 3rd gear is best, don't use low range (too much drag) reverse can be used if necessary (rolling backwards) and the Renix is evil when it comes to this.

 

BTW, jump starting is when you apply an external power source to the vehicle's battery to give it a boost. What you're asking about is usually called a push start, bump start, roll start, clutch start or pop start.

[Eagle]

 

I have the 2.5 so no HEI swap for me, but I prefer carbs over fuel injection for other reasons anyway so doing away with the Renix *for my purposes* is a win-win.

 

 

Don't forget that the 2.5L came out in 1984 in the XJ. The '84 and '85 models had carburetors and HEI distributors, no CPS or injection.

[MJanky]

 

 

I have the 2.5 so no HEI swap for me, but I prefer carbs over fuel injection for other reasons anyway so doing away with the Renix *for my purposes* is a win-win.

 

Don't forget that the 2.5L came out in 1984 in the XJ. The '84 and '85 models had carburetors and HEI distributors, no CPS or injection.

Don't remind me! I sold off an 84 XJ with the carbed 2.5 for less than I'll end up spending for all the parts I need to convert my MJ (It wasn't running)...ugh!

 

Of course I never thought I was going to come across a deal on an MJ, like ever. Or I would have kept it! Now I have a jones for a 2 door XJ! Curse the Jeep gods!

 

Edit:

The only part I have so far is the dizzy from a carbed XJ. I guess it is HEI but I meant I can't just change that out and leave it fuel injected like can be done with the inline 6's. Or can I? I haven't seen anything about it but it should work the same as the 6 conversion, no? Idk I don't know enough about fuel injection to be sure.

[FrankTheDog]

Always bump in 3rd high range.

First you'll just skid.

I have a 92 MJ 4.0, the speed sensor is intermittent so when I slow down for a stop quite often it stalls. I usually catch it and bump start it as I'm coming to a stop. Third works best.

I have lots of practice bump starting.