Speedo/tach needles bouncing

[Rex2nr]

New issue sprung up today- my tach and speedometer both started bouncing. My truck (99 xlt 4x4 w/4wabs) was down for the last week for a rear-most crossmember replacement. This is the one under the cab. I had the box off, and welded plates on the rear of the frame, and replaced the tire carrier with a spiderweb of tubing. I did disconnect the battery when welding.

My abs light has been on, but i havent been able to catch anyone at autozone who realizes one of their scanners does indeed read abs codes. I had been curious of the rear abs sensor, but i dont think that alone would cause the tach to bounce also.

Any hints on where to start? I already checked wiring from the cab back and it all looks to be undisturbed. I found a sensor on amazon that i will order tomorrow also, but i didnt have a chance tonight to test it. For 9 bucks, its not a big waste to just order....

 

[Rex2nr]

It has to be something with the wiring because it was fine this morning.

 

[RonD]

It would be a Ground issue to effect tach and speedo since that are unrelated except for using the Ground

 

[Rex2nr]

Firewall ground or would there be any other common? I think someone had said that is the cluster ground.

 

[RonD]

Yes, they would both use the instrument cluster Ground
And that would use the Cab ground, i.e. engine to firewall ground strap

Engine and trans are isolated from body and frame by rubber mounts
Cab and body are isolated from frame and engine by rubber mounts and the fact that body parts are fully painted before being assembled, so bolts have no bare metal to pass a ground.

So ground straps are very important
Battery to engine(the big one, for start motor and alternator)
Battery to rad support(head lights)
Engine to Cab(all cab electrics)
Engine to Frame(tail lights)

 

[Rex2nr]

Alright.. I'll check that out tomorrow when it's light out.

Is the tach signal translated by the pcm like the RABS signal is for the speedometer? And that signal is generated by the cam sensor? Now that I think about it I did have to tack weld a n onto one of my bed bolts because the Torx bit stripped it. I don't know how true it is that welding can cause shorts, but the caution had to come from somewhere. I have no CEL and the truck is running fine, so I would hope that wouldn't be the cause.

 

[RonD]

Yes, tach signal comes from the PCM, pin 48.
I thought it came from the Crank Position(CKP) sensor, but no reason it couldn't come from CPS, its just that all engines didn't have a CPS but all did have to have CKP sensors after distributor and single coils were eliminated