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Speed / Distance Sensors

Dan Stern wrote (paraphrased): The speed sensor on post-redesign P-bodies tends to go bad, particularly those made before 1992. There is a TSB on this. Dan pointed out that you might be able to fix it by squeezing the
connector, which is the part that usually dies, with pliers so that the sockets will grip the terminals on the sensor. He wrote that if you take off the air cleaner and, from the driver's side, look down at the axle housing on the transaxle, you will see a two-wire plug connecting to an oval-shaped "thing" held onto the axle housing with a single 10mm bolt.

Ed Hennessy added:

I have replaced at least 3 of them and have never had any of these cars store a code 15. In at least one code list I saw, the 15 says "no speed-distance sensor signal," in which case the typical failure mode will likely NEVER set a code, since they don't usually stop sending a signal.

This sensor comes out with the removal of a 10mm bolt and by disconnecting a wire (and by removing the speedometer cable and maybe by separating the two halves, on older units). Takes about 5 minutes (more if you have to find it first).

I did the most recent of these on Saturday on my mom's '86 Reliant 2.5/A413. She had the same 'stall as you come to a stop' symptom, as well as the transaxle acting (in my mom's words) 'as if the transmission was 'slipping' on the highway. This is the other common symptom of a bad speed/distance sensor. The sensor gives the computer the speed info, and the computer uses it to lock and unlock the torque converter. When the sensor is bad, the transmission will often go into and out of lock as the erratic sensor 'says' the car is above or below the lockup speed (about 38 mph).
Speed/distance sensor replacement

(For 2.2 / 2.5 liter engines)

Park on a flat and level surface. On a TBI car, pull the air intake hose off the airbox and move it out of the way. Then reach down behind and under the airbox, following the wires that go down that way. In the end of the transaxle just behind the bellhousing, you'll find the speed sensor at the end of the wire. (This is easier to do than describe.) Disconnect the connector by pulling the tab out from the connector body and pull the connector out.

Feel around the connector for the bolt. This is a 10mm bolt. Remove the bolt, then gently pull up and/or twist the speed sensor and it will pop out. If the transaxle is not overfull and you parked on a flat surface, no transmission fluid will leak out.

The speedometer pinion will be snapped onto the end of the sensor. Remove it and reattach it to the new sensor. Then reinstall the sensor, twisting as necessary to mesh the speedometer pinion with the drive gear. Put the bolt in and tighten (doesn't have to be really tight), reconnect the wire, put the air hose back in place and go wash up.

If you're slow and it is the first time, it will take a half-hour. I can do this in less than 10 minutes, having done several now.

BTW, you can test the sensor with an ohmmeter. Remove the sensor as above, and connect the ohmmeter to the two terminals on the sensor. Rotate it. You should get 8 pulses of zero resistance, and infinite resistance in between. Anything else, and the sensor is bad. You may want to check this, and look for chafed wires, etc. before replacing it to be sure of the cause of the code 15.

Ed Treijis added: I suspect the most common way for the sensor to go is for the wire coming out of the sensor to fray and break. Depending on how the main harness is routed, the wire has to make a very sharp 90 degree bend right at the sensor. The junkyards are full of sensors with broken wires, but good ones can be found. This also saves you money and lets you experiment with pulling the sensors on a car you don't care about.

Ed Hennessy responded: I have done that with pressure regulators, injectors, and MAP sensors. Got a handful of MAP sensors for $5 once. As far as the speed sensors go, if the wire is fine (it was for the 3 cars I have replaced it on--an '86 Reliant, an '87 LeBaron, and a '91 Acclaim), the device simply stops sending the proper number or type of pulses. The sensor is designed to send 8 pulses of zero resistance, with infinite resistance in between, for each rotation of the sensor. If it doesn't test out this way with an ohmmeter, the sensor is bad. It will often not set a code when it fails this way (code means that the sent data is out of range, and this failure will often send in-range, but completely incorrect data). If the wire breaks, you may get a code, since NO data is certainly 'out of range.'

Airtex makes a replacement MAP sensor for Chrysler products. It is a very high quality unit, and it stopped the 'replace once every 12 to 18 months' cycle I'd been on with the MAP sensors in my '87 LeBaron 2.5.

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