Weird problem with speedo/shifting

Ralo · January 10, 2012

I have a 94 Blazer (5.7) that I'm having a issue with. I had the transmission rebuilt not long ago and everything was fine for a limit time. I started noticing that my speedo would be jumping around but just a few mph (not bad enough to throw the shifting off) and the ABS light came on as time went on that problem got worse. First thing I tried was changing the speed sensor on the transmission, nothing changed. So then I unplugged the ABS wheel sensors up front and the truck drove perfectly normal for a couple weeks. Then my radiator developed a small leak and it started again but from the looks of it this was only because it was dripping onto the disconnected plugs. So I blew the plugs out and taped them up good and got the radiator to stop leaking but my problem is still coming and going. I can't drive the Blazer because the speedo is off so bad it won't get out of 1st. If anyone has had or known of this issue before any advise would be greatly appreciated.

Ralo · January 10, 2012

http://www.autozone.com/autozone/parts/Duralast-Vehicle-Speed-Sensor/1994-Chevrolet-Blazer-4WD/_/N-j0k4xZ9gsum?counter=0&filterByKeyWord=speed+sensor&fromString=search&itemIdentifier=190516_0_0_1277

That is the sensor I changed and it had no affect.

I see this sensor too on autozone's site. Does my truck have both?

http://www.autozone....562_0_2026_1277

Carder09 · January 11, 2012

Hmm, so fluid leaked in the connectors/components? And you didn't use connector cleaner on them then let them dry out? Could have corroded comp or possible break or short in a wire so look everything over, try using electrical connector cleaner, let dry out then plug everything back up. Also try unhooking the battery for a bit then give everything a shot again.

Ralo · January 11, 2012

Just the plug on the driver side front ABS wheel sensor. Pulled the gauge and brake fuse earlier nether looked burned in the center and both had black marks on the outer sides. Replaced both of those and took the truck around the block and it was fine but could just be a coincidence with my luck.

Ralo · January 12, 2012

Alright I drove it around some yesterday and never had a issue at all with shifting of the speedometer but I got in it this morning and the speedo stayed at 0 for a little bit then made it's way 5-10mph of the speed I was doing.

Ralo · January 13, 2012

This may be the most helpful forum I've been a part of

jwilley · January 16, 2012

Ralo, what actually fixed your truck? I changed the VSS on the transfer case. The sppedometer still bounces around and causes shifting issues.

Ralo · January 17, 2012

Nothing it's right back to doing it again. I'll be looming for a broken wire tonight.

Carder09 · January 17, 2012

You need to break out the wiring diagram, start shooting the plugs and make sure you isolate other plugs or components if need be. It sounds as though its a wiring issue, and this forum can be helpful but if no one has really ever had the same problem with a fix it makes it harder to figure out.

thx1138v2 · January 19, 2012

I had something similar on my 4.3L Sierra with a 4L60e transmission. I had the transmission rebuilt and soon after started getting codes that didn't make any sense. It was shifting through all the gears but the timing of the shifts seemed off. It just didn't feel right. I started tracing through the speedometer sensor wiring and that wire goes through the brake light switch at the pedal to a jumper block (I forget what it is called) that sets the shift points according to the rear end gear ratio. Since the transmission was just rebuilt and a really good shop did the work I didn't think it was the sensor. The other stuff I was looking at replacing was a couple of different parts at ~$200 each. The brake light switch was $12. So I replaced the cheapest part first and that was the problem. ~200K miles with maybe half of that in city driving so I can understand the brake light switch wearing out. It seemed weird that the speed sensor was routed through the stop light switch but that's the way it is.

I know it's a different truck but it might be worth looking into just to see if your speed sensor circuit goes through the brake light switch.