: Erratic speedo



protodelorean
07-21-12, 01:05 PM
Hey everyone. First post. I just picked up an 06 V in black. The speedo has been acting up, erratically jumping up. I'll be cruising at say 40 and the speedo bounces up to 70+ and comes back down. It affects both the needle and the digital readout. It never drops below the actual speed and seems to do it mostly when I under light to moderate throttle. Coasting usually makes it come back to normal. Sound like a VSS? I'm used to bad VSS causing speedo to drop to zero completely. Any input would be appreciated.

Thanks, Steve

alradco
07-21-12, 03:41 PM
I think typically that's caused by the speed sensor.

protodelorean
07-22-12, 12:12 AM
Thanks! I'll be replacing the VSS tomorrow. I'll report back results.

Cadillac Cust Svc
07-23-12, 01:43 PM
Hey everyone. First post. I just picked up an 06 V in black. The speedo has been acting up, erratically jumping up. I'll be cruising at say 40 and the speedo bounces up to 70+ and comes back down. It affects both the needle and the digital readout. It never drops below the actual speed and seems to do it mostly when I under light to moderate throttle. Coasting usually makes it come back to normal. Sound like a VSS? I'm used to bad VSS causing speedo to drop to zero completely. Any input would be appreciated.

Thanks, Steve

Welcome to the forum, Steve! Hope you're finding it helpful so far, and if you'd ever like further assistance please send me an email at Katie_Lucille@gmexpert.com.

Best,

Katie
Cadillac Customer Service

protodelorean
09-09-12, 06:23 PM
So I swapped out the VSS sensor today and the it's still doing the same thing. Here's another symptom. I noticed the speedo was jumping up just when I was on the throttle. So it seemed it was either related to engine/tranny movement or the throttle itself. To rule out the throttle, I can sit still, idling, put it in gear, one foot on brake and one on clutch, let the clutch out just until the engine starts to load, and the speedo will start to jump. (No throttle) That says it's somethng to do with movement of the drivetrain. Any thoughts? My next step is to replace the connector/pigtail of the vss harness.

darkman
09-09-12, 07:02 PM
According to the Service Manual:

SPEEDOMETER

The instrument panel cluster (IPC) displays the vehicle speed based on the information from the engine control module (ECM). The ECM sends a 4,000 pulse per mile signal to the cluster discretely. The IPC calculates the vehicle speed using the time difference between consecutive pulses. The speedometer will default to zero if the time duration since the last speed pulse is greater than or equal to one second.

So the circuit involves a connection between the vehicle speed sensor (IPC) and the ECM and then a connection between the ECM and the instrument panel cluster (IPC). If the problem does not originate with the VSS it is either wiring or the ECM or the IPC.

See attached.