Speedometer acting up

KRIS11222

MEMBER
1988 CHEVROLET BLAZER
4WD
AUTOMATIC
190,000 MILES

I have a 1988 s10 blazer 4x4 with about 190,000 miles on it. My question I have is about my speedometer, when it is cold outside my speedometer will move back and forth. When I'm driving sometimes where I cant even see the needle and while coming to a stop are starting to move forward it will bounce up and down. Also when I'm driving it may shoot over to about 70mph to 80mph for example when I'm going 50mph are so will hold at 80mph or so until I slow down are come to a stop and while it does this sometimes it will make a loud winning/vibrating loud sound, but when the engine heats up it will still wiggle but it really wont hold at 80mph. If I'm going 65mph it will wiggle in between 50mph and 70mph. The needle doesn't stick and in warm weather it reads just find any I go the same speed as everyone else. I have replaced the cable and just to assist the cables movement I but graphite in it also the old cable wasn't worn frayed or torn and I also have pulled the new cable and it was perfectly fine. I don't know what to do and like I said this happens in cold weather does anybody know why this is happening?

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

The Speedometer gear may have some issues( See TSB Below)

9216chevy04

Number: 91-98-8C

Section: 8C

Dates: October 1990

Corporate Bulletin No: 067104

Subject:
MALFUNCTION OF THE SPEEDOMETER
DRIVE GEAR, DRIVEN GEAR AND/OR SPEEDOMETER

Model and Year:
1987-91 S1 (LLV POST OFFICE VEHICLES)

TO: ALL CHEVROLET DEALERS

Some of the above postal vehicle chassis may experience a malfunction of the speedometer drive gear, driven gear, and/or speedometer adapter.

Effective January 24, 1990 with chassis' VIN 1GBCS10E2M2901183, a new driven gear was installed in production. The new driven gear can be recognized visually as the following table illustrates.

GM PART # COLOR # OF TEETH
NEW 3987920 BLUE 20
OLD 3860347 YELLOW 24

IMPORTANT: The new driven gear has eliminated the need for the speedometer adapter.

Recommended service procedures are outlined below:

Service Procedure # 1

1. Remove and discard speedometer adapter (GM Part # 368025).

2. Remove original speedometer driven gear and inspect for damaged teeth. If damaged, follow service procedure # 2. If it is not damaged, replace the (yellow) 24 tooth driven gear with the (blue) 20 tooth driven gear (GM Part # 3987920).

For vehicles repaired under warranty use:

Labor Operation: # T1611

Labor Time: .3 hr.

FAILURE CODE: 92

NOTE: The above procedures do not apply to 1991 Postal Vehicles (LLV) with L38 engines. Effective August 13, 1990 with chassis VIN 1GBCS10A0M2912017, a new engine (L38) and rear axle ratio (GU6) were installed in production. As a result of the axle ratio change, a new driven gear is required. Use (gray) 22 tooth driven gear (GM Part Number 3987922) if vehicle is equipped with L38 and GU6.

Service Procedure # 2

1. Remove and discard speedometer adapter.

2. Remove driveshaft and rear transmission extension housing.

3. Replace both speedometer drive and driven gear with:

Drive Gear (blue) GM Part 96040176
Driven Gear (blue) GM Part 3987920

NOTE: The above procedures do not apply to 1991 Postal Vehicles (LLV) with L38 engines. Effective August 13, 1990 with chassis VIN 1GBCS10A0M2912017, a new engine (L38) and rear axle ratio (GU6) were installed in production. As a result of the axle ratio change, a new driven gear is required. Use (gray) 22 tooth driven gear (GM Part Number 3987922) if vehicle is equipped with L38 and GU6.

Parts are currently available from GMSPO.

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Operation: # T1612

Labor Time: 1.0 hr.

FAILURE CODE: 92

Could also be a bad speedo cable