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Chevrolet Repair/1987 Chevy Truck Speedometer problem

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Question
I bought this 1/2 ton pickup used. It has a 305 TBI and TH400. The speedo was always off. It works, but it would read (approx )15 mph under actual speed. I replaced speedo cable (new) and speedometer (new), but it still does the same thing. I noticed that there is a sensor that plugs into the back of the speedo and have been told that it is a Vehicle Speed Sensor. Problem is, nobody can tell me what it does. Some say it's for Cruise Control. The cruise control works fine. I've asked if it controls the speed that the speedo shows, but no one really knows. Some mechanics, I've had look at it, say the speedo (reduction) gears in the trans are the wrong ones, but I tell them the transmission is the original one that came with the truck. I realize the gears could be worn, but I'm tired of replacing parts and would like to just fix the problem. What do you think?

Answer
HI RANDY,THE SESNOR ON THE BACK OF THE SPEEDO IS A PROTECTOR THE SPEEDO GEAR HEAD FROM BOUNCE..THIS IS WHAT I WAS TOLD BY GM...I DO NOT THINK THIS IS YOUR PROBLEM..DO YOU KNOW IF ANYONE HAS CHANGED THE REAR GEEAR RATIO IN YOUR TRUCK,TRNASMISSION EVER BEEN REBUILT ?? ARE THE TIRES THE SAME SIZE FROM OEM SIZE..YOU MECHANICS ARE CORRECT ABOUT CHANGING THE TRANSMISSION TAIL SHAFT GEAR IF NEED BE.IF IT IS WORN THIS WILL NOT CAUSE YOUR PROBLEM. BUT IF THE GEAR RATIO HAS CHANGED YOU WILL NEED TO REPLACE THIS GEAR. NOT THAT HARD TO DO,JUST REMOVE YOUR TAIL SHAFT COVER AND REMOVE THE CLIP THAT HOLDS THE GEAR TO THE TAIL SHAFT AND SLIDE IT OFF THE SHAFT..TAKE THIS GEAR TO YOUR CHEVY STORE AND THEY CAN CALCULATE WHICH NEW GEAR YOU WILL NEED....IT GOES BY THE NUMBER OF TEETH ON THE GEAR AND COLOR. I HOPE THIS HELPS YOU AND GOOD LUCK,,,TOM

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TOM SHEETS

Expertise

PLEASE READ ALL OF THIS BEFORE ASKING ANY QUESTIONS. IF THIS CHEVY SECTION IS MAXED OUT.YOU CAN REACH ME IN THE OLDS/BUICK SECTION. Will answer questions from 6:00am to 10:00pm all days [EST][[[ NO DIESEL OR PAINT QUESTIONS]]] will answer all 1965 to present QUESTIONS ON ALL GM'S on mechanics of these cars. Keep in mind i am not there in person i am going on what you are telling me !! PLEASE IF ASKING A QUESTION GIVE ME AS MUCH INFO AS POSSIBLE. THE MORE THE BETTER. SUCH AS YEAR,MAKE,MODEL,ENGINE SIZE. THIS WILL HELP ME HELP YOU. I will not be able to answer all questions because of some situations that i would really need to be there and see things for myself,such as electrical issues. In many cases i would need wiring diagrams. I can also give you an estimate on what the cost would be at our repair shop as an idea to help you. Keep in mind that my answer is ONLY MY ADVICE,ck with your mechanic as well.. >> THANK YOU, TOM..

Experience

I have been a ASE certified master mechanic for over 30 years and certified IN ALL 8 CATEGORIES,[engine repair,engine performance,auto trans,manual trans,clutches,steering and suspension,brakes,electrical systems,heating and air/cond. I am also a state CERTIFIED inspector for over 30 years. I do computer work [pcm]fault codes,pcm flashing,state inspection monitor readiness,repairs on exhaust,fuel service,[injection and carburetor,] charging system problems, running issues,alignments,frame problems,tire balance,struts and of course standard and anti-lock brakes.

Organizations
nys state auto repair industry and ase group.

Publications
N/A

Education/Credentials
high school,4 years of tech school,and many many hours of auto training with many gm classes and aftermarket training classes..

Awards and Honors
ASE MASTER TECH CERTIFIED IN ALL 8 CATEGORIES.NYS INSP LICENSE AND A/C CERTIFIED

Past/Present Clients
MANY CUSTOMERS IN OUR BUSY SHOP.

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