Isuzu 4x4 on 4x4Wire

Isuzu on 4x4Wire - featuring Isuzu 4x4 Truck, SUV, Amigo, Trooper, Vehicross

By: Fernando Rivero - 3/2001

Isuzu Tera-low Gears

Testing at Little Moab.

Through the years, Isuzu sport utility vehicles have built a solid reputation as excellent off-highway vehicles. They have proven to be very capable with few modifications and have conquered many difficult trails such as the Rubicon trail in California's Sierra Nevada mountains, and the Golden Spike trail in Moab, Utah. Even so, one of the most limiting factors for these trucks is their relatively high low-range ratio, only a 2.28:1 reduction. When Tera Manufacturing, of Sandy, Utah, announced their intentions to develop a lower ratio transfer case gear kit for Isuzus, it created great excitement in the Isuzu community. All through the year 2000, the growing Isuzu off-roading community anxiously awaited the production and release of Tera-low's Isuzu transfer case gears. Throughout this time, I stayed in contact with Mark Falkner, Tera Manufacturing's owner, to keep updated on this project's progress. In December 2000, after many design changes and tests, Tera installed the first prototype gears into my 1990 Isuzu Amigo.

The stock transfer case, with the rear cover and output shaft assembly removed.
Removing the stock low range.
The stock low range removed.

The Installation

On December 7, I met with Mark and his two research and development engineers, Rand Ridges and Curtis McNeil, at Tera's installation facility. Mark showed me the gear set, which just the week before, Tera showed at the SEMA 2000 show in Las Vegas. This new gear set reduces the low range ratio of first-generation Isuzus from 2.28:1 to 3.07:1. Second-generation vehicles receive an even greater improvement from the stock low-range of 2.05:1. The Tera kit includes a new idler gear, low speed clutch drive, and detailed instructions for the installation. Tera also includes a new shift rail that provides more clearance inside the transfer case for the larger gear.

As mentioned, the kit includes a complete set of instructions so this article details only a brief overview of the installation. Self-installers are in for a relatively involved project -- nonetheless, with a little patience and careful review of the detailed instructions, a capable do-it-yourself mechanic should be able to accomplish the project without much difficulty. Installation involves removal of the rear half of the transfer case, followed by removal of the stock idler gear, low speed clutch drive, and stock shift rail, which are all replaced by the Tera components. The specialized tools mentioned in the instructions will facilitate installation, but if you find that you cannot do the job without these, isn't that a perfect excuse to expand your tool collection? The instructions recommend removal of the entire transfer case from the vehicle, as opposed to just removing the rear half. This allows installation on a workbench, rather than under the vehicle, with the front half of the case still attached to the transmission. To be safe, set aside a full day for the installation to allow for any unforeseen problems.

The new Tera-low gear set.
Tera-low idler gear installed.
Tera-low low speed clutch drive.

Since the gear set used for my Amigo was still a prototype, we discovered during the installation that the older (pre-93) first generation Isuzus used a synchronizer gear tooth count different than the newer (post-93) first generation vehicles and second generation vehicles. The prototype gears used the post-94 Isuzus synchro-gear tooth which did not initially fit my vehicle. Rand gave me the option of reassembling the transfer case back to stock and waiting for the new gears to be cut, or leaving the vehicle with Tera, to see if they could solve the problem. I gladly left the Amigo with them to further research the problem. Rand and Curtis' ingenuity, persistence, and ability impressed me as they spent the next week mating the back half of a transfer case from a newer Rodeo with the front half of the transfer case from my 1990 Amigo, allowing the Tera-low to fit. The later model transfer case eliminates the rear slip yoke output present on the older vehicles, so I needed a new rear driveshaft made with the slip yoke in the driveshaft. Tera's dedication to product quality and customer service continued; Rand and Curtis worked for days to ensure that the transfer case and driveshaft worked as well as the factory drivetrain.

The Test

Rand called me on the phone after I suffered a week without the Amigo -- anticipating. I rushed over to Tera and crawled under the vehicle to look at the new setup. I was extremely impressed by the absolute factory appearance of the hybrid transfer case and stout new driveshaft. I couldn't wait to test the Tera-low gears, so Curtis and I took it out to the parking lot, shifted into low range and crawled. The difference from stock was remarkable -- the Amigo just crept along slowly at idle speed. I expected a little more noise from the transfer case when in low range due to the faster spinning gears, but this was only minimally apparent -- I quickly tuned it out. On the road, in high range, there is no difference in transfer case noise. Anxious to try the new gears off-highway, I thanked Rand and Curtis for their persistence and great work and left with my Tera-low Isuzu.

Testing at Little Moab.

I drove straight from their shop to the snowy foothills of Bountiful, Utah. This area has many steep trails, made more difficult by the deep snow. Once on the trail, I shifted the Amigo into four low to attempt a steep, snow-packed hill. The Amigo idled up the hill effortlessly and only on the steepest parts did I have to accelerate to keep the momentum. Coming down the same hill, the compression braking was so improved, that I never touched the brakes to slow the vehicle. On another uphill slope, I actually stepped out and walked alongside my truck as it idled slowly up the slope. I was very impressed by the low range reduction from the stock ratio.

For the next trial, I drove to an area southwest of Salt Lake City, called Little Moab. As the name implies, this area contains difficult slick rock terrain similar to that found in Moab. The first obstacle I attempted was a series of three steep ledges, followed by a steep climb over a crest, and then a steep descent. Again, the Amigo climbed with very little use of the accelerator and without the clutch feathering I had to do before adding the Tera gear set. On the descent, the low range held the Amigo back without having to use the brake at all. Again, I was very impressed at the improved rock-crawling ability from the Tera-low.

The Tera-low Isuzu gears are an affordable way to greatly improve their Isuzu's trail and rock crawling ability, downhill compression braking, and overall off-highway performance, all without spending thousands of dollars to replace entire drivetrain components. The result is a very respectable overall crawl ratio of 52.6:1 (4.55 axle x 3.07 Tera transfer case x 3.77 transmission), more than adequate for most difficult terrain. The lower crawl ratio greatly reduces wear and tear on the clutch and also provides increased safety through improved engine compression braking when negotiating steep downhill terrain.


Tera currently produces gear sets for 1997-and-newer Isuzu trucks with automatic transmissions. 1997-and-newer manual transmission kits will be available by the end of April. For the pre-1994 first generation owners, the good news is that Tera has decided to produce the gear sets and they should be available some time after April. For more information, contact Tera Manufacturing at (801) 256-9897 or visit their web-site at See you on the rocks!

Contacts Related Links

Tera Mfg. Inc.
8385 S. Allen St., #115
Sandy, UT 84070
Phone (801) 256-9897