Speedo drops when braking???

[brant89]

I have a 2000 Ranger and the speedo will drop when I apply or let off the brakes (sometimes stays all the way down the entire time I'm braking). It also drops each time the right turn signal blinks. I do have both check engine and ABS lights on. The check engine light has been on since I have owned the truck but the ABS light is a more recent development. I know I need to check the speed sensor in the differential but I don't see how the lights could be contributing. All my fuses seem to be fine.

 

[yyc_tbird_sc]

Get the codes pulled...you'll probably have to go to a NAPA, or other place that has a professional code scanner as the ones you can buy at most places now don't have the ability to read ABS or Airbag codes. My money is that there is going to be a fault somewhere in the wiring, which would throw a code.

Pull your dash apart and check for loose, corroded, or broken wires. A loose ground will cause exactly what you're describing. Check the connectors on the back of the cluster too. If you feel up to it, disassemble the cluster and check out the speedo to see if there is anything obviously wrong.

While you're in there, replace the bulbs in your cluster while you have the chance.

 

[brant89]

It isn't behaving anything like a loose wire in the dash because bumps, rapid accel/decel, etc don't change anything. It's only when the brakes are applied or released and the rest of the time it is accurate to like 2mph (I have checked it against my gps) so I seriosuly doubt the problem is inside the speedo. I do wonder if a have a short in the right rear brake light wiring somewhere but it would only be to the bulb for braking/turn signal because it doesn't do it with the running lights on. But I haven't blown any fuses so idk. It just seems like something is drawing too much power and idk what.

 

[martin]

did this problem start when the abs light came on as the abs controller seems like the common factor here?

 

[mattpresley]

When I apply my brakes my speedo drops too. Its because im slowing down...

 

[BRUTUS_T_HOG]

It isn't behaving anything like a loose wire in the dash because bumps, rapid accel/decel, etc don't change anything. It's only when the brakes are applied or released and the rest of the time it is accurate to like 2mph (I have checked it against my gps) so I seriosuly doubt the problem is inside the speedo. I do wonder if a have a short in the right rear brake light wiring somewhere but it would only be to the bulb for braking/turn signal because it doesn't do it with the running lights on. But I haven't blown any fuses so idk. It just seems like something is drawing too much power and idk what.

sounds like shorted wiring. check the wiring harness on the left frame rail (or at least thats where they most likely are) where the brake lights and speed sensor wiring run together (its on the tail of the transmission)

get a decent multimeter with some long wires and check the resistance between the right brake light circuit and the wires of the speed sensor. it should say OL or otherwise indicate NO connection. if it says anything other than that the wires are shorted together.