The Chrysler Lean Burn engine control system: first onboard auto computer

The Lean Burn system was an early spark advance control system (introduced in 1976 and continuing to the 1980s) that was troublesome due to the nature of the contemporary electronic components, as well as the yards of vacuum hose and competent mechanics needed to keep it working. Materials technology eventually caught up, but many mechanics did not, and many owners simply switched to conventional carburetors.

Chrysler boasted, “Electronic Lean-Burn System, with its electronic spark control, is one of the most important developments in engine control since the 1920s and early 30s, when manual spark control was replaced by the automatic spark advance. Eight sensors and an electronic computer combine to adjust spark advance correctly for all driving conditions.” (This turned out to be true in the long term; every car currently made uses electronic spark and fuel control.)

The second generation system, described below, came out a single year later, in 1977. It was similar but integrated electronic ignition and eliminated the centrifugal advance flyweights and one of the two pick-up coils from the distributor. In 1978 it was expanded to be used on most Chrysler V8s. By this time, Chrysler’s related inventions - electronic ignition and the electronic voltage regulator - were already standard.

The 1978 description of the second generation notes, “This new computer consists of one integrated circuit board which computes all vital information for controlling the ignition system. It eliminates the starting pick-up coil and the centrifugal advance flyweights from the distributor and performs these functions electronically. The spark-control computer also includes the electronic circuitry for the electronic ignition-thereby eliminating the separate control box for the electronic ignition.”

Chrysler's Electronic Lean-Burn System, with its electronic spark control, was cited as providing:

• Smooth engine warm-ups
• Improved engine performance
• Improved smoothness of engine operation
• Controlled emissions-even before they reach the exhaust system
• Less emission-control "hardware": several responsive sensors feed information to an electronic computer to keep the lean-burn engine performing efficiently.

The system measured:

• 1. Engine speed (r.p.m.). As engine speed increases, spark plugs must be fired sooner for the engine power and efficiency needed. This sensor reads engine speed and signals the computer to adjust spark timing accordingly.
• 2. Engine load. This sensor signals the computer to advance the spark for optimum fuel economy at part throttle.
• 3. Throttle position. This electronic sensor reads accelerator pedal position and signals the computer to advance the spark under conditions when speed and load alone do not give the correct signal.
• 4. Speed of throttle movement. This sensor signals the computer to advance the timing briefly in anticipation of the desired acceleration. The faster the acceleration, the longer the duration of extra advance. There is no lag.
• 5. Air temperature entering the engine. As outside air enters the engine, its temperature is monitored by a sensor which signals the computer to advance the timing at cooler temperatures to enhance fuel economy and retard it at warmer ones to protect against destructive detonation or pre-ignition.
• 6. Engine coolant temperature. This sensor tells the computer to limit spark advance during engine warm-up while the choke is on. This helps control emissions.
• 7. Carburetor throttle open' or closed. This sensor operates a timing counter which remembers how long and how often the throttle is open or closed. It allows full spark advance at highway speeds but in city driving spark advance is modulated to control exhaust emissions.
• 8. Engine starting. This sensor tells the computer when the engine has started and advances the timing for about one minute to cut down on engine stalls.
  

The electronic spark-control computer responds to data from all sensors and instantly adjusts the timing of spark­plug firings. Since conditions are constantly changing, the computer is constantly making instantaneous adjustments in spark-plug firings.

This second-generation spark-control computer consists of one integrated circuit board which computes all vital information for controlling the .ignition system. It eliminates the starting pick-up coil and the centrifugal advance flyweights from the distributor. Information for these functions is programmed into the spark-control computer and is performed electronically. The electronic circuitry for electronic ignition is also contained in the spark-control computer.

Examples of the system in operation:

1. The engine is cold. The coolant sensor tells the computer, which retards the spark so the engine isn't putting out a lot of pollution while the choke is on.
2. The engine coolant warms up. The sensor reports to the computer and the spark is advanced.
3. The engine is started in the morning and the car is driven away. Sensors tell the computer to provide spark advance to get you going.
4. Driving up a hill or pulling a trailer. The engine load sensor reads manifold vacuum and tells the computer whether the engine needs less spark advance to prevent detonation when pulling up the hill-or more spark advance going down the other side. Responsive spark timing does the job.
5. Driving on an interstate highway. The spark is advanced.
6. Rapidly depressing the accelerator. The sensor signals the computer to instantly advance the spark for more power to accelerate the car. There's no lag in spark advance like you get in ordinary engines.

The invention

The patent was granted in August 8, 1978, listing Gordon Fenn as the inventor. Marc Rozman told us that Gordon Fenn had come up with the system very quickly, and had also worked on substantial improvements to the 1980-81 Imperial electronic fuel injection system (after its introduction, to make it workable on vehicles in the field).

According to the patent, signals used to control spark timing included “a throttle advance signal, a programmed vacuum advance signal, a break-idle advance signal, an engine start advance signal, an engine speed advance signal and a fixed timing signal. These signals are algebraically summed together to develop a resultant spark timing signal representing desired spark timing. Individual signals are derived from transducer circuits which are responsive to various engine operating and/or ambient conditions.”

The throttle advance included both throttle position and rate, increasing advance as the throttle opened; it was attenuated as the ambient air got hotter.

“The break-idle advance signal provides momentary advance when the engine throttle is displaced from the idle position. The engine start advance signal provides advance for a short interval after the engine has been started. The engine speed advance signal provides increasing advance as the engine speed increases. The fixed timing signal provides fixed timing which is used to adjust the basic timing for a given set of engine operating conditions. The control system is particularly advantageous when used in an engine which uses a leaner fuel-air mixture (17-18:1, for example) in comparison to presently used mixtures (15-16:1). Improvement accrues by way of reduction in exhaust emission products and better fuel economy.”

Bill Wetherholt wrote:

Lean Burn was the start of the computer age. We had a 1977 Chrysler New Yorker, the first year for Lean Burn. It was a great car for the first year. But she was on the way to work one morning, and it just quit, the Lean Burn module had burned out. They replaced it with a rebuilt one, and the car never ran right after that. Once, I was trying to pass this guy in a Cadillac, he was going about sixty miles-an-hour, and I wanted to drive at seventy miles, so I go to pass him, and he would speed up a little bit. But my car wouldn’t go any faster than his car. It just bogged down. Then it started running like it only had two spark plug wires hooked up, jerking and shaking, and it just quit on me.

I went back to standard ignition, the standard distributor and all that, and it still ran, but it didn’t run well. It was beautiful, in fact that New Yorker in 1976 was an Imperial, then they dropped the Imperial name and upgraded all the other cars. What was an Imperial, they made it a New Yorker, because I said I’m not the kind of guy that would drive an Imperial, but I am the kind of a guy that would drive a New Yorker.

Tannon Weber wrote:

My 1978 Chrysler Cordoba came with a Lean Burn-equipped 360/2bbl. At the time I got it (December 2001, with 107,000 miles), the system wasn't working very well, and didn't have any kind of sensor failure identification system that I could find. Given the lack of performance options with the system as well as the general lack of availability of parts and diagnostic equipment (I went to the oldest Mopar dealer in town, which still has their Plymouth and Pentastar signs up, and even they laughed at my suggestion of putting it on the scope) I retrofitted to a standard carburetor system with conventional electronic ignition, splicing into the wiring harness, replacing the distributor, and jury-rigging an electronic ignition module to the firewall. I saw about a five mile-per-gallon fuel economy improvement with the change, and I still pass emissions tests.

A friend of mine has a 1978 Magnum with a 400/towing package setup that also came Lean Burn-equipped. When the car was new he removed the Lean Burn, fixed the wiring harness, and converted to standard. In his estimation the conversion to a stock electronic ignition made no appreciable difference in power compared to Lean Burn.

It would appear that when Lean Burn was new, if it worked properly from the start it was fine. In my experience it ages quite poorly though, and due to a combination of a lack of available replacement parts and a lack of performance upgrades it's not worth trying to maintain.

(We’ve never heard anyone speak up for actually retaining Lean Burn, and support Tannon’s estimation, while noting the historical significance of the system. — ed.)

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