Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
by Ellsworth Chou (1998)
The problem: your Chrysler vehicle runs perfectly, but the engine stalls when slowing to a stop.
Intermittently, the 2.2 liter injected engine in my 1986 Shelby GLHS died during coast-down when the clutch is disengaged. The starting, idle, acceleration and cruise are unaffected. Eventually, the condition was constant, and I developed a habit of roll-starting the car as I slowed to every traffic stop.
The suspect item is called the Distance/Speed Sensor. It's an electrical item, mechanically driven by the speedometer drive mechanism in the tailshaft of the transaxle. On our two L-bodied (Omni) Shelbys, it's mounted where the mechanical speedometer cable enters the transaxle. It caused such a spectacularly mysterious set of symptoms that I documented this repair in March of 1995.
The DSS apparently contains (I'm guessing from external experiments, I've never sawed open the faulty device) 15 or 16 reed switches wired in parallel. These reed switches are arranged radially around a shaft with an eccentric magnet. As the shaft turns, the DSS produces 15 (or 16) open-close cycles per rotation.
At least one of the many reed switches sticks closed intermittently, shorting the entire sensor closed; hence, the engine control computer thinks the vehicle is at rest (at least when Distance Sensor parameters are considered by the system.)
The Distance/Speed Sensor provides road distance telemetry to the engine management computer - this data is used for cruise control and engine control. My guess is that one of the engine control applications related to road speed is maintaining high idle during coast to provide additional vacuum for power brake boost. When the vehicle groundspeed drops below a certain speed (7mph on my car,) idle drops to normal. In the journal text below, I suggest that it's emissions-related, but now that I think about it, that really could be accomplished with just manifold vacuum, oxygen sensor and throttle-position data - groundspeed wouldn't matter.
My experience was with my 1986 Shelby GLHS. This is a modified Dodge GLH Turbo, and the engine is Chrysler's 2.2 liter inline four, but I suspect Chrysler uses the same part in many injected vehicles. The Shelby does NOT have a factory cruise control, but if your vehicles has one, you can perform a simple test with your factory cruise. First, make sure your cruise control works at all. If the cruise works: next time the vehicle stalls, try to use the cruise control (remember that you must exceed a certain minimum speed to engage the cruise, usually over 30-35mph.) Keep in mind that any mechanical shock or vibration could free up the stuck reed switch, so if the cruise WORKS, that's not conclusive. But if it DOESN'T work, and then DOES work later, I'd say the DSS is a good $50 gamble.
This may all sound like a condemnation of computer-controlled cars. On the contrary: this car's computer actually told me what its problem was - although I was hesitant to accept it. Thanks to digital controls, my car makes 200 bhp from a 2.2 liter four, gets 19-28mpg and is perpetually tuned. It's only had three sets of spark plugs (two, really - I just changed back to hotter plugs to improve driveability.) And it starts every time I turn the key. Remarkable, and for the average auto owner, a turn for the good. On the other hand, take a look under the hood of my friend's 1965 Plymouth Valiant. It's a backyard mechanic's dream. It's got maybe six things you could call a hose. Maybe less wires. My car's like a bucket of black linguine. And any of those noodles could ruin the whole meal.
All in all, both our Shelbys (we have a 1986 GLHS and a 1987 GLHS) have been pretty good cars. Mine's coming up on its 11th birthday, and just passed 90,000 miles. Considering how much time it's spent at double the design horsepower of the motor, boiling the rubber off the front tires, I'd say it's been worth it.
See also this other page on speed/distance sensors
All Mopar Car and Truck News