2nd Gen Non-Engine/Transmission Replaced heater core, speedo goes crazy

[dirty-miner]

I'm certain that I stressed one of the harnesses that the speed signal runs through somewhere when I replaced my heater core. Really dont want to pull the dash back again and start testing continuity. Just want to see if anyone on here made the same mistake when they replaced their heater core or had their dash back for any reason.

The problem: As I said, I recently replaced my heater core. Once I got the truck back on the road, the speedometer started to jump around especially on rough patches of the road.

A couple notes on the problem: seems to only occur at higher speeds 45+ mph; the speedometer will only jump around lower than the actual speed; seems to get worse the longer I drive it (not sure if this is legit, or just me).

 

[Road Dog]

What year of truck? 2WD or 4WD? What transmission? After '98 the dash is completely electronic and the speedo is controlled by the PCM.

 

[dirty-miner]

Sorry thought I had my signature set up.

 

[Road Dog]

With a 98.5 the speedo should driven by the ABS sensor at the top of the rear diff (through the PCM) .The connector could be dirty or the sensor is failing or it's not seated properly. Search this forum for "abs sensor" for lots of threads on the subject.

 

[klcammie]

After I replaced my heater core had same problem, pull bezel then pull the instrument panel out there should be two screws. On mine it just pulls out. I sprayed electrical cleaner on connection put it back together problem solved.