Speedometer wildly inaccurate

1997 Astro AWD 171K miles. Since I've owned this over the past couple of years, the speedo has been getting steadily more and more overly optimistic on the indicated speed. Lately, it reads 85mph when I'm doing about 35mph. As much as it's an ego boost to bury the needle past 100mph when tooling along the freeway at 60, it's a real pain trying not to get dinged for speeding in town as I have no idea how fast I'm really going. I first thought it might be the Vehicle Speed Sensor (VSS), but the transmission shift points are normal and the odometer is spot-on. Both of these are driven by the VSS so my guess is the VSS is fine. I think it's either the output signal from the computer to the speedometer, or the speedometer itself. The 97 uses an analog needle but there is no classic spinning speedo cable; it is an electrical signal from the computer. I sort of doubt it's the computer since everything else seems fine. I suspect something in the speedo head itself, like a bad ground? I've done some forum searches and come up with a few hits of others saying their speedo is also reading way high, but I haven't come up with any solutions. Any idea? Thanks

Bad grounds can make a car do all kinds of crazy sh*t including the problem you're describing. A good place to start is to check all of the ground wires for corrosion, broken/pinched wires, etc. and fix and wiring problems you find. If you do that and it doesn't work, post back and let us all know what you tried, the more details the better. Keep posting until the problem is resolved, because there's probably someone on this forum who has more expertise in your car and it's common quirks.

Thanks Mike, yeah, I was afraid it would come down to checking grounds. I'm 6'4" so the idea of crawling under the dash to pull out the instrument cluster is not appealing. But, I've done more awkward things on this van. When I replaced the heater core I first removed the seat; maybe I'll do the same to make some room on the driver side. It'll probably take me a couple of weeks or so to get to it but I'll post an update on how it goes. And if anyone else has solved a similar problem, please let us know.

Dale - Don't let that stop you. I'm also 6'4" and I have TWO Astro vans to keep running. Each one is fully optioned, too. It gets interesting at times, but is not impossible:

http://www.wwdsltd.com/files/AstroLowerIPWiring.jpg

The titanium shell in front of the instrument panel might give the van a ton of rigidity and strength, but it can also be a PITA to work around when getting into the dash and cluster.

Blue Bowtie
Well, now you're just scaring me with that picture! :smile: Yeah, I've looked up under there and said, "Wow, there's a lot of crap up in here...." It's a conversion van so the aftermarket company added a few extra things for the lighting, TV, etc. They pretty much tie-wrapped their mystery boxes to the GM gadgets, just to make it more interesting. Fortunately, this isn't my daily driver so I can afford to spread the pain out over a few days. Once it gets a bit drier. Thanks - Dale