Speed Sensor Problem Again

[HIZNHRZ]

I had a problem about a month ago where, among other things, my speedometer and odometer stopped functioning. The resulting error code was #24 which indicted a bad speed sensor. I found a Napa part locally and changed out the old speed sensor for the new Napa part. A test drive indicated that all was well. The speedo and odometer and all functions driven by the speedometer were again functioning normally.

On the last two pleasure drives I’m now noticing that the speedometer is intermittent. I’ll be driving and the speedo will suddenly zero out. Sometimes it comes back on its own. If it stays out for more than a few minutes it throws a code and the check engine light comes on. If I shut the car off and immediately turn it back on, the check engine light is off and the speedometer works again until it intermittingly drops out again.

Anyone experience a similar problem or have any suggestions?

[VetteMed]

There's either a bad connection at the connector from the dash to the IP, or bad connection from the IP to the LCD display, OR your sensor is faulty. What I'd probably do is use the HVAC display to show vehicle speed (parameter #7), to see if the ECM is receiving VSS input when the LCD goes out.

If the HVAC displays the correct speed when the LCD goes out, then you can pretty much isolate the problem to the IP, at least in my mind.

[VetteMed]

For reference, here's the master list of parameters on the HVAC display:

http://i680.photobucket.com/albums/vv163/agchoset/1991%20ZR-1/img105.jpg

[scottfab]

If you had not had a past issue with the speed sensor I'd take you down a different path. However you have and it seemed to fix the problem.

I'd start right at the speed sensor connector. Check the mounting hardware on the sensor itself to make sure it's snug.
Disconnect the connector, clean contacts and move the wiring harness around and look for where it maybe rubbing on something.
Physically bend and move the harness around. Even if you don't find the root cause this movement can serve to helpful.
Reassemble with liberal amounts of dielectric grease on the contacts.
If the problem seems to go away you may not be out of the woods if the harness is rubbing somewhere and you did not find it.
The fact that you can shut the car off and restart it tells me you have a mechanical intermittent. (wire open/short)

[HIZNHRZ]

There's either a bad connection at the connector from the dash to the IP, or bad connection from the IP to the LCD display, OR your sensor is faulty. What I'd probably do is use the HVAC display to show vehicle speed (parameter #7), to see if the ECM is receiving VSS input when the LCD goes out.

If the HVAC displays the correct speed when the LCD goes out, then you can pretty much isolate the problem to the IP, at least in my mind.


When the Check Engine light goes on, the car idles like crap, the adjustable suspension is no longer adjustable, the radio volume that increases with speed no longer works, etc seems to indicate the ECM is not happy (i.e. seeing no data input). Your suggestion is a logical check and it's also easy - certainly worth checking. Thanks!

[HIZNHRZ]

If you had not had a past issue with the speed sensor I'd take you down a different path. However you have and it seemed to fix the problem.

I'd start right at the speed sensor connector. Check the mounting hardware on the sensor itself to make sure it's snug.
Disconnect the connector, clean contacts and move the wiring harness around and look for where it maybe rubbing on something.
Physically bend and move the harness around. Even if you don't find the root cause this movement can serve to helpful.
Reassemble with liberal amounts of dielectric grease on the contacts.
If the problem seems to go away you may not be out of the woods if the harness is rubbing somewhere and you did not find it.
The fact that you can shut the car off and restart it tells me you have a mechanical intermittent. (wire open/short)

Before I replaced the OE sensor with the Napa replacement the speedo was as dead as it could be. The replacement sensor definitely addressed "a problem". The intermittent issue is a real brain tease. I think you're right as far as it likely being some type of mechanical issue. If I get another new sensor, replace the one currently in the car, it may help pinpoint if the problem is the current sensors "internals" or an electromechanical problem external to the sensor (I.e. car wiring, ground, etc). I'm counting on the likelihood of getting two new sensors that both have the same internal intermittent problem is not real high.
Thanks for the input. I'll let you know what I find...

[VetteMed]

When the Check Engine light goes on, the car idles like crap, the adjustable suspension is no longer adjustable, the radio volume that increases with speed no longer works, etc seems to indicate the ECM is not happy (i.e. seeing no data input). Your suggestion is a logical check and it's also easy - certainly worth checking. Thanks!

VSS is used for the SRC (ride control), and the radio volume auto-adjust, so it seems likely that the ECM and/or CCM (more likely to be influencing SRC and radio), is in fact not receiving the VSS signal.

[scottfab]

When the Check Engine light goes on, the car idles like crap, the adjustable suspension is no longer adjustable, the radio volume that increases with speed no longer works, etc seems to indicate the ECM is not happy (i.e. seeing no data input). Your suggestion is a logical check and it's also easy - certainly worth checking. Thanks!


Something to do as a quick test. Unplug the sensor and just plug it back in and check the looseness of the sensor. What you have to hope for is that this fails all the time and not intermittently. Wishful thinking I know.

[Marc Haibeck]

The Selective Ride Control can help diagnose this. The signal from the sensor goes to the SRC and the ECM. If the sensor has no output the SRC error lamp and the check engine lamp will both display on the drivers information panel.

If you don't have a SRC error 23 the sensor and the wiring to the SRC controller are okay.

The ECM receives the signal, squares it up and re-sends the signal to the rest of the car. If you have only an ECM error 24 the ECM is suspect. I recommend swapping the ECM with another ZR-1 and see if the problem goes away.

Note that the ECM and the SRC controller require that the problem persists for a few minutes before setting an error code.

Marc