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September 2013 Your Jeep

1978 Jeep J10
Verne Simons
| Senior Editor, Jp
Posted August 8, 2013
Photographers: JP Staff

Your Jeep Tech Questions Answered

We Feel Your Pain
I have a ’79 J10 with a 360 engine, T-18 tranny, and a Dana 20 T-case. Recently, my work transferred me to another plant 15 miles away. The J10 is my daily driver. I was wondering if anyone here has done or has a link to an article about swapping in a five-speed tranny. I just want to lower my rpms while doing 65 mph. Hopefully this will improve fuel economy as well.

Yep, that sounds familiar. Our ’78 J10, Piggy, has the same drivetrain and came from the factory with 4.10 axle gears. Somehow someone was patient enough to put about 85,000 miles on it with stock-size tires. That’s a lot of high rpms from a V-8 that does not like to rev or some really slow highway driving. Now we have lifted Piggy and are running some 36-inch tires, so you’d think it would be a highway cruiser, but it still approaches 2,700-3,000 rpm on the highway. Well, like us, you have a few options. One option you could pursue is to track down an NV4500 from a ’93-’94 Chevy truck. There are other transmissions with an Overdrive, but they lack the granny First gear that your T-18 has. An NV4500 is not gonna be the cheapest or easiest option, but it is an option. Another way to lower highway rpms is to switch to higher (numerically) axle gears or go to larger diameter tires. Unfortunately the AMC V-8s don’t seem to be at all efficient. They are the heavy drinkers of the V-8 world. On top of that, the factory did not skimp on the steel when building our trucks. The sheetmetal, when not rusted away, is thick and heavy. Some people will advise an entire drivetrain swap like a diesel and a tranny with an overdrive. How ‘bout a more-modern V-8 and an auto tranny with an overdrive? That has given many FSJ owners a mileage increase. Having said that, the cost of any of these conversions probably won’t ever pay for themselves with fuel savings. That does not mean you should not do it, it just means your FSJ and mine can’t easily or cheaply get decent gas mileage. Another option, and again, it’s not gonna be cheap or easy, is to try to track down a rare Borg Warner/Rancho Overdrive for a Dana 20. They are heavy, only work in 2WD, and are very rare, but they do exist, and if you can get one that works or can be fixed, you can bolt it to your J10, have a shorter rear driveshaft built, and drop the rpms on the highway. One last option would be to swap to an NV3550. You’d lose the granny gear, but you’d pick up an Overdrive.

The Pressure Is Killing Him
I own an ’87 jeep YJ with the 4.2L engine, and I keep having trouble with my oil pressure. I’ve already replaced the oil sending unit twice, and the pressure on the gauge keeps dropping. Could this be the oil pump going out on it, or could it be another PCV valve, or what? I’m stumped and have no idea. I do know for a fact that it is a mechanical gauge, but should the pressure really read the line before zero at interstate speed?
Derrick Knight
Plant City, FL

Hmm. It sounds like you may need a new oil pump. But to check the first thing, I’d toss a new PCV valve at it (cause that ain’t gonna hurt) and hook up a mechanical aftermarket oil pressure gauge. That is a new one with a capillary tube. If your current gauge has a sending unit that is replaceable, then it is probably not truly a mechanical gauge. A true mechanical gauge will tell you if your gauge is malfunctioning or if you need a new oil pump. Sure, you’ll have to buy a gauge that may tell you what you already know, but it’s easier than tossing a new oil pump in the Jeep first thing, and heck, who doesn’t want a nice aftermarket oil pressure gauge?

Right From the Horse’s Mouth
I have a question about a project you guys did a while back. You guys did a 5.9L Magnum swap on a Jeep TJ (“More Mopar,” Parts 1 and 2, July ’11 and Aug. ’11). I am doing the swap as well, and with California’s smog stuff I have to run a mechanical fan. With that said, have you guys had any cooling problems? I will be blasting the A/C in 115 degree heat in the California summer in the desert. Please let me know how it is going! Everyone online has gone with electric fans, and that will not work for me!
Mike Zeko
Brea, CA

To answer your question we went right to the source and talked to Ali Mansour, Four Wheeler magazine Technical Editor, who wrote the articles covering the V-8 conversion in his very own TJ. Here is what he said: “The TJ does not have A/C, but does in fact still run the mechanical fan (which I prefer over electric). I have never had any heating issues, even in weather over 100 degrees. I don’t have much of an inner fenderwell, which helps dissipate some of the heat. Prior to swapping in the engine, I removed all of the core plugs and cleaned out the water jackets (it was pretty gunk-filled). I also put in a new thermostat and water pump to be on the safe side. To date, I never have heating issues, but I wouldn’t mind putting back the A/C one day!”

Attack of the Flasher
I don’t know if this is the right section or not, but I was trying to contact Randy’s Electrical. I have been a subscriber for a few years now and finally ran across a question I could ask him. I have a ’98 TJ, and recently changed the taillights out for some flush-mount LED oval-shaped trailer lights. I have them wired up and everything is working. My question is, when I turn the turn signals on, it flashes fast as if the bulbs are blown, but none of them are blown. Is there a diode or anything I can put in line to stop the fast flashing?
Benjamin Tugwell
Lucama, NC

Yep, that will happen with a Jeep with an older style flasher installed. There are a couple ways to deal with this, but the easiest is just to go down to the parts store and get a new flasher that has provisions for LED lights. One that will work is the Trico EP-26 flasher with the “LED” notation on it. Then when you get home, stick your head under the dash, pull the old flasher, and install the new one.

More Mysterious Clunks
I have installed an ’01 Super Duty Dana 60 front and Sterling 10.5-inch rear in my ’01 TJ. After much reading on the subject, I decided on Detroit Lockers for both. They were installed by a national 4x4 company. There are the occasional clunks that I expected, but overall I’m happy very with the lockers. The only noise that I can’t seem to verify as “normal” from any source is a regular knocking from the rear when coasting that is about in time with the tire rotation. I expected and accept noises, but haven’t been able to find out if this one is normal. Thanks for any input.
Via email

Well, clunks and drivetrain noise are probably one of the hardest things to diagnose without getting some seat time in a moving vehicle. On top of this, Detroits, while being brick-shithouse-solid and reliable, are also noisy. Having said that, your description of the noise being in time with the tire rotation probably means it’s coming from a wheel bearing or carrier bearing. Any pinion bearing noises will be faster than tire rotation because the pinion is before the reduction from the ring-and-pinion. You could try jacking the axle that you think is making the noise off the ground, put the Jeep in Neutral, and rotate the tires by hand to see if you can hear the noise. This will also help verify if the noise is in time with tire rotation or not. A carrier bearing could have been damaged during installation, or just from mileage and age if you reused the wheel bearings in these axles (you should have bought new carrier bearings for the differential install). If you can’t figure out what is causing the noise and the place that installed the Detroits is not worried about it, there is not much you can do but drive it. If there is a problem, it will develop into something that the 4x4 shop that did the work will be able to identify. Hopefully they will stand behind their work. Just make sure they know that you think something is off and that you contacted them about it after the modification and they gave you the all-clear. Let’s hope they have some warranty on their work that they will stand behind.


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