Marlin Crawler: Cheap Tricks

Here are some cheap tricks and tips I've found with my Marlin Crawler transfer case:

• The facts
• Driving Experiences
• Caveats
• Gear oil change for a Marlin Dual transfer case
  
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The Facts:

A Marlin Ultimate Crawler (#263) consists of a dual transfer case in which a 4.70:1 gear is installed in the rear transfer case. The front transfer case reduction unit (or crawl box) retains the stock 2.28:1 low ratio. The front crawl box is sandwiched between the transmission and the actual transfer case and it is 6.5" long. Only the high-low gear range is retained in the crawl box, the front drive part is removed. With the dual case, the front drive shaft is lengthened 6.5", which allows for longer wheel travel. In the rear, the shaft is shortened by the same amount. With a dual transfer case, you effectively have a 4-speed transfer case (see table below) and you gain a 2-Lo range (without having to unlock the front hubs) by putting the crawler box into low and shifting the transfer case between 4H and 2H.

With a Marlin Ultimate Crawler (dual transfer case), 5.29:1 R&P gears, overall crawl ratios work out to:

To put this in perspective, take a typical engine idle speed of 800 RPM and using 33" tires with the above gearing, your speed is about 29 feet/minute, or less than 1/3 MPH!

Driving Experiences:

A few things I've noticed about driving with a Marlin Crawler:

• With the 4.70 gears engaged, there is much more play in the drive line. I find it most noticeable in rocky, flat trails where the engine alternately switches from driving to braking the wheels. I imagine some of this comes from the Detroit lockers I have, a 200K+ driveshaft, but it is *worse* in 4.70 than in 2.28 range.
  • In any situation where you are going down drop-offs, I find riding the brake as the wheels go over drop off is crucial. Otherwise, there seems to be a moment of free fall as the driveline unloads then re-tensions on a new tire. This too, is partly due to free-play in my lockers, but it is worse in 4.70 than in 2.28 gear.
    • Nice thing now is that in low-low-1st, no amount of braking can kill the engine!
  • A stock dual transfer case would be an alternative to this problem as you would get a low-low range of 5.19:1 and a 108 crawl ratio (w/ 5.29s).
  • I imagine my engine and transmission mounts are a bit sloppy (200K+ on the originals), so I'll be replacing those shortly and probably will use mounts on both the crawl box and transfer case like Jay Kopycinski did.
• The 4.70 gears are noisier than the 2.28 gears. There is a distinct whine that sounds kind of like a turbo.
• With my 22RE engine, I find it will stall in low-low-2nd gear at idle (121:1) but will not stall in low-low-1st, even up a steep hill w/ brakes applied (why is this important? - see below).
• I can now get the front TrueTrac differential to lock up using brake biasing.
  • The TrueTrac is a gear driven limited slip differential, not a true locker. It functions by the internal worm gears binding up and transferring torque between the wheels before spinning begins. The TT has a transfer ratio of ~3:1, meaning up to 3x the torque on the slipping wheel can be transferred to the gripping wheel. This is fine until a wheel is in the air (when 3x nothing is still nothing!). To compensate for this, brake drag can be applied to mimic grip, thereby transferring torque to the wheel with grip. In practice with my 22RE and stock gears, I found adding brake drag while crawling up a steep hill would simply kill the engine.
  • Now its all different. On the Toy4x4 Rubicon '98 trip at the top of Cadillac Hill, I was crossed up with the rear diff. hung up on a rock. I had three wheels spinning (including the front wheel in the air). Letting the engine idle in low-low-1st gear, I began applying brake pressure and the vehicle slowly started inching forward, pulled along by the front wheel that was on the ground. It is a weird sensation to start moving by hitting the brakes! Granted, if I had a true front locker, I wouldn't have been hung up in that situation, but the TrueTrac is *so* nice in the snow, that this quirk is worth living with.
  • I find that I can only get this technique to work if I'm below about 75:1 crawl ratio, at least when climbing steep hills. What I find is that I end up killing the engine with the extra brake drag, when it is nearly bogged down due to the steepness of the hill. As I begin applying brake drag, once the front wheel with traction begins taking torque, that will stall the engine. At 75:1, I've got barely enough torque to handle the vehicle load plus the brake drag.
• I find that the Marlin gears have helped my driving in stock low range as well. I find I have a much better feel of the throttle and engine response and can now idle/crawl through stuff I once would have had trouble with. In fact on a recent trail run, I used 4.70 gears going in and 2.28 coming out. Granted I did stall a few times coming out, but it otherwise felt comfortable and in control.
• I've learned that the 22RE engine speed is fairly stable below aboout 1200 and above 2500 RPM. In between those speeds, I find it tends to surge when the load is decreased, such as when you drive up and over a rock (high load up, low load down). Therefore, try to pick a gear to stay in one of those RPM ranges. With 20 forward gears, there is always one or two to choose from.

Caveats:

With such extreme low gearing, a few cautions must be exercised:

• Engine torque can be multiplied nearly 5 times over stock. Combined with traction-aiding differentials, this can result in all this torque ending up on one axle. In low range I try to stay off the gas and let the engine idle whenever possible. Afterall, the reason you are in the low gear is to go slow, SO GO SLOW!
• Combine a lower than stock differential ratio with much lower than stock transfer case gearing and you have a potential problem when out of gear. If you try to coast with the clutch disengaged, it can be exposed to extremely high RPMs at even slow speeds. Even at just 3 MPH in low range, the clutch will be spinning at around 8000 RPM, which is enough to cause it to disintegrate.
• I found my crawler gears to be much tougher to shift than my old gears.
  • After a few thousand mile breakin and changing to some nice synthetic gear oil, made them shift much smoother.
  • Note, that shifting in and out of the 4.70 : 1 gear requires a full stop in most cases.
  • The 2.28 : 1 gear can actually be used with judicious double-clutching.
  • Also, the 4WD shift-on-the-fly feature only works if you have the front hubs engaged. I thought I had lost that capability after the Marlin gears went in, but I had the front hubs unlocked.
• I notice the 4.70 : 1 gears have a lot of slop. This is most apparent when you are on undulating terrain and alternating between driving and braking. For level and downhills, I try to stay out of the lowest ranges for better control.
• I found another Marlin low use, that is crawling in deep snow. It seems that if you turn the wheels slow enough to allow the snow to compact, you can go in deeper, softer snow than you can by churning through whith faster wheel speeds. At 98:1 I was getting farther than I could at 48:1, but at 224:1, I just floated over the powder.

Gear Oil Change:

After about a 5000 mile break-in period, I decided it was time to change the lube to the Marlin cases. I opted to use 75-90W Mobil-1 synthetic gear oil. Those dual cases seems to really heat up with the mineral oil, the slicker oil seems to help out some. Also, with the added friction of the second case, I found that I picked up 1 MPG or so after changing to the synthetic oil.

So, upon close examination of a Marlin Crawler dual transfer case, I noticed there are two drain plugs but only one fill plug, (on the stock transfer case). It turns out the crawl box shares the same gear oil with the transfer case. To change the gear oil in this setup involves the following steps:

• Warm up the transfer cases;
  • A 10-20 mile drive should do it.
• Remove the fill plug and both drain plugs (one at a time unless you have a large drain pan).
  • If the plug on the crawl box is not drained, about a cup of old gear oil will remain there.
• Replace both drain plugs and fill both gear boxes through the rear fill plug (GL/5 grade oil).
  • Total gear oil required is approx. 1.7 qts. for the transfer case and 0.7 qts. for the crawl box.
  • Wait 5-10 minutes after the initial fill and top off the oil as needed.
  • For a proper fill, this should be done on level ground.

Rating:

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