It is not uncommon for someone to claim that their vehicle experienced an "unanticipated acceleration" or "sudden acceleration" (S/A). Despite having looked at many cars thus accused, I have yet to personally find support for such claims. Though one German car maker (Audi) may come to mind as a result of much bad publicity about this issue in the 80's, an web search on the words "sudden" and "acceleration" will bring up pages alleging the problem has occurred in vehicles manufactured by virtually all the major car makers.
Brakes Not Strong Enough?
Passenger car brakes have 3 to 5 times more horsepower than the engines. This is why 100 to 0 km/h happens in much less time than 0 to 100 km/h. Personally testing numerous cars has shown that from 80 km/h, if one jams on both brake and throttle pedals, in most cases the vehicle stops. This includes one inadvertently conducted test when my winter boot caught the edge of the throttle pedal as I braked my fairly small car approaching a stop sign. In that case, as in all but two of the tests I've intentionally conducted, the car came to a dead stop despite protest from the engine. The stopping distance was sometimes increased, but the vehicles stopped.
One exception was an import sedan I tested which experienced severe front brake fade. When the transmission down-shifted to first at 40 km/h or so, the engine revved near to redline, and was able to hold the speed steady even with the brakes applied. This test was stopped due to excessive heat build-up in the pads, but the conclusion was that the driver had most probably double pedalled, the front brakes had faded rapidly, the engine had continued to pull hard but the rear (dead axle) had locked up. This explained the set of (rear) skidmarks overlapping the evidence that the front wheels were pulling as the car crossed a grassy median.
Another exception involved a large SUV which had reportedly "revved up a little bit" partway through a stop as the woman braked while approaching a stop light. This increased her stopping distance enough that she gently rear-ended the car ahead of her. I tested the vehicle's response to double-pedalling on a secluded roadway, and found that the big V8 coupled with the automatic transmission was able to drag the vehicle along at 40 km/h despite my greatest efforts to depress the brake pedal. The arrangement of the pedals was such that the brake pedal was higher than the throttle when not applied, but could easily be depressed enough to allow a misplaced shoe to contact both pedals. The driver was adamant that the engine had not roared to life, but rather seemed to rev slightly, which was consistent with her having had a foot slightly overlapping the throttle pedal as she braked.
It Just Roared Off!
Most S/A cases I've had were "from a stop" in which "the car just roared to life and took off." Usually, the driver is sure they were on the brake, but the car just kept going. "The harder I pressed on the brake pedal, the faster it went." Yes, I have actually had people say that to me! Sometimes they say the gear lever jumped into gear, as well. I've never found a problem with the brakes, the gear lever, or the throttle in any of these cars, but not for lack of trying.
Though I generally cannot dispute the possibility of one or more transient electrical gremlins having engaged the cruise unexpectedly (on vehicles so equipped), a firmly planted left foot should be able to stop pretty well any car, unless there is a brake problem. The most credible reports of this nature that I have seen to date come from some recent news reports. The cruise control servos in some pre-1995 US Fords have the potential to pull the throttle wide open as a result of one loose wire and one shorted wire. If the brake-pedal travel switch (or dump valve) is not functional, then brake application won't release the cruise. So far, I haven't seen this problem, but there are reports that Ford has told service technicians who call in with a sudden acceleration problem to check these wires for nicks or damage.
A Different Problem: Stuck Throttles
A stuck throttle event may be termed a "sudden acceleration" by someone who experiences it, but it is, in fact, a very different problem. This phenomenon arises after the operator depresses the throttle pedal and releases it, but the throttle remains engaged to some degree. At one time or another I have found throttles which failed to release fully for the following reasons:
- Deep-pile floor mats that hooked the pedal;
- worn carb shot-pump barrel;
- chafed housing that caused the inner cable to rust/stop moving smoothly;
- debris that jammed the throttle plate open;
- crud and swollen polymers in the cable fittings of a Bronco II I owned briefly;
- corrosion between the aluminium and steel in the bell-crank ball-joint of a full-sized van.
The Interlock
Another issue to consider in cases where the automatic transmission vehicle was initially at a stop in Park, is the brake-interlock switch. This feature is becoming more and more common on passenger cars, but is by no means ubiquitous at this point. This switch prevents a vehicle from being shifted into gear unless the brake (or the clutch in manual-transmission cars) is depressed.
When these switches are working they prevent simple pedal error, which is certainly the most common cause of "unanticipated acceleration". If the vehicle is equipped with such an interlock, an additional simultaneous malfunction is required to allow the vehicle to take off after the engine roars to life.
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Complaints before shift interlocks* |
Complaints after shift interlocks* |
| Thunderbird/Cougar |
17.3 |
2.9 |
| Aerostar |
16.6 |
1.7 |
| Town Car |
14.8 |
4.1 |
*Sudden-acceleration complaints per 100,000 vehicles. Vehicles studied were 1985-93 Ford Motor Co. models. Source: National Highway Traffic Safety Administration
Audi effectively solved their S/A problem many years ago by installing brake-interlocks on all their vehicles. Jeep now offers to retrofit interlocks on vehicles they built if the owner wants one, as an answer to the recent spate of S/A problems reported about their trucks. And as the accompanying table shows, reports of S/A in these three Ford vehicles exhibit a definite trend after installation of interlocks.
The brake/shifter interlocks generally only work if the transmission is placed in Park. In one case I handled, the owner left the car in neutral, with the keys in the ignition. A child, who was too short to depress the brake pedal, started the car and was able to move the selector from Neutral to Drive, hitting someone working in front of the car.
The Aftermarket
There are a variety of aftermarket components, including emergency vehicle light systems, remote car starters, and alarms, which can intrude on the operation of these interlocks, even if installed as directed. One such highly publicized case, in which a police van brake-light-flasher system effectively bypassed the brake interlock system, resulted in two deaths in Minnesota in 1999.
In one case I handled, a remote starter system was installed in such a way that it bypassed the park-interlock switch altogether. When the operator left it in gear one day and subsequently started the car with the key fob, the vehicle took off across the parking lot. She was hurt attempting to stop the vehicle.
Conclusion
Though there are some questions about a vehicle's condition and performance which cannot be answered by a post-accident inspection, many potential hypotheses for what went wrong can be either supported or refuted. An inspection of a vehicle after a claim of "sudden unanticipated acceleration" is made should include:
- an examination of the condition of the entire throttle system, including cables, cable ends, linkages, pedal pivots, throttle plate, and return springs for signs of poor operation;
- an examination of the carpets and other interior items adjacent to the pedal for possible interferences to pedal motion;
- a test of the operability of the brake system;
- a test of the brake-shifter interlocks;
- inspection of wires to cruise control servo;
- a test of the cruise control system, if possible.
As cars are fitted with more and more electrical componentry, the possibility for transient electrical problems that may go unidentified despite extensive post-accident testing looms larger. The recent inclusion of "black-box" data recorders in virtually all US General Motors vehicles may go a long way towards answering the question of what went wrong after the fact. GM recently took the bold step of allowing anyone to purchase the tool necessary to collect this information. Hopefully, "black box" information will become more accessible to the aftermarket as time goes on, and perhaps they will record even more information than the vehicle speed, throttle position, brake light condition, and seatbelt condition now commonly available.
*Wade Bartlett, BS, MS, MSAE, is the principal of Mechanical Forensics Engineering Services, a US company specialising in accident reconstruction. This story is current as at September, 2000.
http://mfes.com/index.html
Footnote: the vehicle accidents shown here are for illustrative purposes only; they did not necessarily result from unanticipated acceleration.
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