American RV Motorhome Gearbox (GM 4L85-E)

The Chevrolet Motorhome chassis has been around for a long time now. In the early 1980s, it has powered by the 454 cubic inch (7400cc) engine, which was fitted with a carburettor as none of the fuel injection systems had been invented.

In the early days behind the engine was a three-speed gearbox known as the TH400. This gearbox proved to be very reliable and remained unchanged until the 454 engine was fitted with fuel injection. With the need for electronics to work the engine, the need for fuel economy arose.

In the late 1990s, the GM 4L85-E gearbox was born. This gearbox had four gears, the fourth being an overdrive. Instead of relying on a cable and engine vacuum to govern the gearbox changes, electronic speed sensors were used which also lead to a whole list of additional information being incorporated. By doing this, a smoother gear change and better fuel economy was provided.

The gearbox was designed to handle vehicles from 3265kg up to 7500kg, which was ideal for the American Motorhome chassis.

Its gear ratios are:
1st 2.48:1
2nd 1.48:1
3rd 1.00:1
4th 0.75:1
Reverse 2.07:1

The options come with the axle ratios which are: 342:1, 373:1 and 4.10:1, with the last one being the more common.

In this country, the 4L85-E gearbox has not had that many uses, although its smaller brother the 4L80-E has some of its applications that might surprise you.

1992 – 1998 Rolls Royce Silver Sprinter
1993 – 1996 Jaguar x J5
1996 – 1999 Aston Martin DB7

The difference between the two gearboxes is the amount of torque you can power through the gearbox with the 4L85E allowing 624NM.

Both the 4L80E and the 4L85E have been a very successful gearbox, even though some major changes have taken place throughout its reign. One of the main advances came around 1995 when General Motors changed the way it operated the torque converter lock-up clutch by using a pulse width modulator (PWM). This enabled them to control the aggressiveness of the apply and release of the torque converter clutch and then allowed for a bit of slippage for a smoother feel.

By now there was now a need for better fuel economy. General Motors responded to this in 1998 with a different strategy for controlling the lock-up clutch called “Ec3”, or electronically controlled capacity clutch. This strategy made a huge difference in fuel economy; this was done by starting to apply the torque converter clutch at a much lower speed, which then continually slips until it reaches the highway speeds. In order for this process to work, a new converter clutch lining had to be developed to stand for the slippage and heat that was generated.

General motors came up with a woven carbon fibre material that was very porous and allowed fluid to flow through it for improved heat transfer. This lining is practically indestructible. As General Motors hold the patent, if a fault occurs with the torque converter then the best course of action would be to replace it with a genuine new General Motors one.

Please contact LAS Motorhomes on 01604 861999 if you require any more information on gearboxes.