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
A Simple Test
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.
- About a week ago, I started experiencing a very specific
and annoying symptom in my 1986 Shelby GLHS. At nearly every
traffic stop, the engine would stall. Every other operational
state was apparently unaffected. Idle, cruise, acceleration,
deceleration all seemed fine. Only in the transition from closed-throttle
coast-down to clutch disengagement did the symptom appear. The
smallest application of throttle would keep the engine alive.
If the engine did die, a brief re-engagement of the clutch was
sufficient to roll-start the car, and became a habit over the
past few days which I'm now having to unlearn. Once roll-started,
the car idled flawlessly.
- I should mention that the problem was intermittent - a potentially
frustrating diagnostic factor. Both our '86 and '87 GLHS' have
multi-point fuel-injected engines, controlled by a Chrysler microcomputer
engine management system. This "logic module" controls
ignition timing, fuel injector delivery rate, turbocharger wastegate,
electric radiator fan, and electrical charging systems. Engine
management is based upon information collected from a number
of sensors, including:
- brake switch
- distance/speed sensor
- neutral safety switch
- oxygen sensor
- temperature (water) sensor
- charge (manifold air) sensor
- detonation sensor
- throttle position sensor
- distributor fuel sync pickup
- distributor ignition reference pickup
- MAP (Manifold Absolute Pressure) sensor (also samples barometric)
- Upon assessing these sensor states, the logic module picks
the appropriate ignition timing, injection timing and turbo boost
settings from a "map" of factory programmed data. If
some of these sensors fail, the logic module lights a dashboard
"Power Loss" light - in some of those cases,
the module goes into "limp home" mode, allowing the
vehicle to operate, albeit less efficiently, to get the driver
to his/her destination.
- Or so the theory goes. In reality, there are many electrical
connections which the logic module is not designed to
monitor, or can't monitor. Should any of these fail, the
engine could falter or fail to run. Moreover, there are probably
some twenty vacuum hose circuits, all prone to leakage. Some
of these vacuum systems affect engine performance, such as the
turbocharger wastegate solenoid and the EGR valve. An electrically
controlled vacuum valve is used to switch the MAP sensor from
measuring pressure in the manifold to reading ambient barometric
pressure - but there's no way for the vehicle to know whether
the solenoid opened or not - and it may assume that a near-atmospheric
reading from the manifold is a barometric reading.
- The factory Chrysler manual describes a troubleshooting process
to diagnose engine control problems with the multi-point EFI
system. The first step of that process is to check all
the electrical connections and vacuum connections. A daunting
task, at best. At worst, a nightmare of uncertainties. Especially
with an intermittent problem. Often, the process of inspecting
electrical connections changes their state. So the culprit connection
may be overlooked in the first phase of diagnostics - and can
mislead future diagnostics terminally.
- So, all of the above is running through my head, and I'm
trying to figure out what should be happening during the
de-clutching period, but isn't. I knew that there was some system
for letting the car control throttle-closing conditions. When
you lift the throttle to decelerate, there is an immediate rich-mixture
condition. The fuel just leaving the jets or injectors is provided
in anticipation of the air denied it by slamming the throttle
butterfly shut. This unburned fuel is an emissions no-no and
tends to get catalytic converters a bit frisky for their own
good, unnecessarily depleting the catalyst and heating up the
converter more that necessary. Most modern vehicles have a servomotor
which meters additional air around the closed throttle
plate for a sufficient period to burn the unused fuel. Chrysler
calls this an Automatic Idle Speed motor (AIS.) Just for fun,
I visited a couple of auto parts stores looking for the AIS on
Saturday. I wanted to know how expensive a gamble it was, and
whether it was available in the aftermarket. My first stop was
a NAPA store, because they tend to carry more obscure (read:
"dealer") parts, although at a premium. I was still
surprised that the counterman actually found a listing: $69 and
he'd have to call "the warehouse" on Monday. If they
didn't have one, 8 to 10 working days. Good intel, but I'd keep
- Now, as I'm visiting auto parts stores, I'm also looking
for this "thing." The Chrysler manuals always list
"special factory tools" and their part numbers. Which
is pretty aggravating, since you can't get those tools,
unless you're a dealer. I've tried. The heck of it is, those
factory tools exist because it's otherwise a Cast Iron Bitch
(pardon the language - it's an automotive expression) to do whatever
it is that those tools facilitate. So the "thing" I'm
looking to find is a little box with two 8-segment LEDs on it
and a slide switch and a push button. Chrysler calls it a C-4805
Diagnostic Read-Out Tool. Much of what it does, many Chryslers
do already. If you turn the ignitions on our Shelbys on-off-on-off-on
within 5 seconds (without starting the engine,) they'll "report"
any fault codes the logic module has stored. The aforementioned "Power Loss" indicator lamp on the instrument cluster displays these codes as sequences of flashes. For instance, the sequence 2-3-5-5 indicates that the Air Charge Temperature Sensor has as some point, while the engine was running, sent a voltage to the logic module which was out of range; the 23 is the fault code, 55 is "end of message." (I have posted fault code charts for our 1986-7 Shelby GLHS' and our 1988 Caravan 3.0 liter V-6.)
- So even without a C-4805 we can benefit from the logic module's
built-in fault detection. And we have. Including this
problem. More on that later. But the really cool thing
that the 4805 does is ATM. Not a cash machine (although that'd
be a cool tool as well,) but Actuator Test Mode. Oh, this is
neat. Push that one button on the C-4805 and watch the display
increment from "01" to "11". Depending on
which of the eleven codes you choose, the system will now:
- 01 - Spark Activation - Once every 2 seconds
- 02 - Injector Activation - Once every 2 seconds
- 03 - AIS Activation - On pulse open/one pulse closed every
- 04 - Radiator Fan Activation - One pulse every 2 seconds
- 05 - A/C Cutout Relay Activation - One pulse every 2 seconds
- 06 - ASD (Automatic Shut Down) Relay Activation - One toggle
every 2 seconds
- 07 - Purge Solenoid Activation - One toggle every 2 seconds
- 08 - EGR Solenoid Activation - One toggle every 2 seconds
- 09 - Wastegate Solenoid Activation - One toggle every 2 seconds
- 10 - Barometric Read Solenoid - One toggle every 2 seconds
- 11 - Alternator Full Field Activation - One toggle every
- This is great stuff. You can work on a cold, non-running
engine, and (attempt to) operate devices that normally work only
when the engine is running. And it'd eliminate several possible
component failures from the diagnostic process. The thing is,
the Diagnostic Connector on our cars is just six pins. One's
ground, one' s 12VDC. The rest go to the logic module or the
power module (electronic power supply.) The data's probably moving
serially. I'd bet the ATM button is just jumping one of those
four pins to hot or ground. Unfortunately, I might blow up a
$300 logic module if I was wrong.
- I'd noticed in the past year that Sunpro (automotive diagnostic
devices) was selling diagnostic boxes. I wasn't sure if they
made a Chrysler box. Turns out they don't. GM and Ford, around
$40 each - cheap. One of their products tests ignition modules
only, including Chrysler, but it's no help. I'm still looking for this device. Or a few minutes with a C-4805 and a VOM. If you know anything about C-4805s or the pinouts of the Diagnostic Connector, please drop me a line at .
- An absolutely invaluable resource that I do have at
my disposal is an identical engine and control system, parked
just shy of 2 feet from my car: my wife Joni's 1987 Shelby GLHS.
As long as it doesn't involve serious surgery, or risk to the
healthy system, I can swap parts. One caveat: when the problem
is intermittent, you might end up with a very long waiting game.
- So I swapped AIS motors, rather than gamble 70 bucks. Good
thing. It wasn't the problem. In fact, I haven't swapped them
back. As a matter of fact, I think the Air Charge Temp Sensors
have been switched in our Shelbys since a diagnostic swap in
'87. That turned out to be a faulty Shelby Automobiles splice
in the harness to said Charge Sensor, which had been relocated
into a custom intake manifold.
- First time out with the exchanged AIS motor, my car died
a the first stop sign, a half block from home. Before this point,
I'd ruled out any temperature sensors, reasoning that if the
sensor failed only enough to mislead the logic module, but not
enough to generate an out-of-range fault code, then at some point
in its temperature range, the engine would run better. But when
it had the problem, it was at all temps.
- I was still convinced that the AIS motor, although functional,
was not being controlled appropriately. If it never opened, then
the car might tend to be over-rich as the engine fell toward
idle speed, flooding the engine at its weakest point. I'd read
in the Chrysler books that AIS operation depended on the state
of the Throttle Position Sensor (TPS,) Distance Sensor, Coolant
Temperature Sensor and Neutral and Brake switches. I'd ruled
out temp sensors (and the temp gauge seemed OK, which gets its
data from the logic module.) I'd wondered about the TPS, but
for some reason decided it was an unlikely culprit. Ditto the
Clutch and Brake switches.
- Monday we were in my car, and Joni suggested I check the
fault codes. I'd done it a couple of days before with no result,
but tried it anyway. There was a new code, indicating
a fault in the "vehicle speed/distance sensor circuit."
A chart Joni'd gotten from the Shelby Dodge Automobile Club (we've
been SDAC members since purchasing the cars) said that this fault
code was "put into memory . . . If the speed sensor signal
indicates less than 2mph when the vehicle is moving." I've
wondered for some time how the system knows the car is moving
when the Speed/Distance sensor doesn't. The chart also
said that this circuit is monitored "Over a 7 second period
during deceleration from highway speeds when the throttle is
closed." So it apparently looks for closed throttle and
higher than idle vacuum and rpm for a 7 second period, and assumes
you must be moving.
- Late Monday, we drove Joni's car to a nearby Dodge dealer
to buy a $50 Distance Sensor, and I paid close attention to what
her healthy 1987 GLHS did as I approached a traffic stop.
Experimentally, I disengaged the clutch early, at around 35mph,
and coasted to a stop. The RPM would hold at 1400 until the speedo
dropped below 7mph, and then dropped to a normal 1000rpm idle.
Subsequent experiments with longer and shorter coasts suggested
that the rpm change was speed sensitive, and not time-from-throttle.
- Even with this evidence, I chickened out of buying the sensor
at the Parts Counter, deciding that I'd swap between the cars
again to be sure. Upon returning home, I prepared for underbelly
surgery. It turned out to be a pretty tough operation. The sensor
is actually an adapter in-line between the speedo cable and the
transaxle. Poor accessibility necessitated removal of turbo air
plumbing (a familiar operation, at least) just to unplug
the sensor from the harness. It was enough to discourage me from
doing the swap, and I decided I'd go ahead and buy the part the
next morning. After an hour and a half or more, I had the old
sensor in hand. Optimistically, I stuck a VOM across the two
pins. The Dodge parts man had suggested that the sensor was a
sort of "generator," which seemed reasonable. I set
the meter to check continuity, and it beeped, suggesting that
there was, at least, some circuit within the mysterious shot-glass
sized cylinder. With the meter set for AC volts, I spun the speedometer
pinion (attached to the sensor.) Zero. DC volts. Nada. Hmm. Figuring
that I might not be making enough revs, I chucked a piece of
vacuum hose into the old Makita and made myself a flexible shaft
drive. 1800rpm later. Zilch. Zip. 0VDC. 0VAC. 0Ohms dead short.
Huh. In the past, I'd fiddled with some other spinning thing
that I never figured out, so I was resigned to playing $50 auto
- Joni came down to the garage to see how I was doing, and
I showed her the part. I'd been digging around for an old magnetic
compass that's been floating around the workbench for a couple
of years. I found the compass, and a quick experiment suggested
that something magnetic was spinning around inside the case.
- I still wanted some concrete evidence of malfunction.
But I didn't know what I was looking for. In desperation, I switched
the VOM back to continuity check and picked up the part while
the meter screamed away. I twisted and bent the connecting cable,
hoping to find an open short. Ah! A break in continuity. Which
didn't return. Fiddling with the part, I discovered that when
I spun the shaft, the continuity tone was evenly interrupted,
what turned out to be 15 or 16 times per revolution. Eureka!
Solid evidence. Suddenly, I understood what had happened. Holding
the part up to my ear (without the VOM,) I spun the shaft, and
heard regular little metallic tinkling sounds. Reed switches.
Like the burglar alarm sensors on doors and windows, a magnetically-operated
switch. Inside this sensor, 15 or 16 of them, arranged radially
about a shaft with a magnet mounted eccentrically. All of the
switches are wired in parallel, so every switch makes
and breaks the circuit through the sensor. However, if one of
those reed switches should stick, it would dead short
the entire device, regardless of the state of its peer
switches. Reed switches rely on spring properties to pull against
the magnet and return to their "rest" state. A quick
calculation suggests that my wheels have made some 7 million
revolutions to date. Depending on the gear ratio, these switches
might've cycled from half a million to many millions of times.
- I bought the new part at a Dodge dealer. The Chrysler part
number is 4740088. In March of 1995 it was $46.20 (tax
not included) in Van Nuys, California. Interestingly, the replacement
part is radically different looking from the original. Included
documentation explaining how to "convert" to the new
part is dated 1988, so it might have been a fix for a poorly
designed original. Indeed, around 6 or 7 parts failures that
have occurred on the 1986 GLHS have appeared, with nearly identical
modes of failure, and usually around 6 months later (the difference
in production dates) on Joni's '87 GLHS. The good thing
about that (or at least, a consolation) is that I'll know how
to approach the 2nd repair. If parts are inexpensive, I'll typically
buy a spare, knowing it'll probably go in the Charger.
- So I put the new Distance Sensor in this afternoon, and no
problems. I'm not absolutely convinced, but I'd say I'm
95% confident. Dunno why the doubt, really. Guess I just considered
so many other possibilities, and can't forget them.
- (NOTE 4/29/97: To date, the problem has NEVER reappeared.)
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 200bhp 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 '86 and '87 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.
Ellsworth's Other Pages
- Problems common to both our 1986 and 1987 Shelby GLHS'.
- Macintosh troubleshooting.
If you're interested in any of the above proposed pages and can't wait, drop me a line at .
I'll email you the files or try to answer your questions.
Email comments to (Please: no general automotive questions - I'm not an expert, this was just a solution I deduced to one problem.)
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