Q: I just read the letter about the person with the shuddering Sable (Motoring, Sept. 26) and have an idea. A friend of mine had a Ford...

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Q: I just read the letter about the person with the shuddering Sable (Motoring, Sept. 26) and have an idea. A friend of mine had a Ford Thunderbird that acted similarly. He went through the transmission before discovering it was a bad throttle position sensor. The clue in the reader’s letter is that it acts up when running at part throttle.

A: You’ve brought up a fairly common fault. Modern vehicles employ a throttle position (TP) sensor, or a pair of them, as an input to the engine management system. This matchbox-sized device on the engine’s throttle body provides throttle opening and rate-of-change information.

TP information is used by the engine management system to anticipate acceleration/deceleration and control fuel injector operation, along with several other functions. Since the TP sensor is partially mechanical — as opposed to most other sensors that are purely electronic — it’s prone to wear and degradation. Once a vehicle hits perhaps 60,000 to 70,000 miles, it’s not uncommon for the TP to develop problems that result in a sudden yet brief power loss, a hiccup of sorts.

I didn’t mention the TP as a possibility in the shuddering Sable as the symptom seemed to be more continuous. In hindsight it would have been worthwhile.

A faulty TP can often elude detection, but testing it is fairly simple, using a graphing multimeter or oscilloscope.

E-mail Brad Bergholdt at under-the-hood@juno.com