Brakes - Improved Loaded Vehicle Performance
88chevy29GMC NUMBER: 87-T-116
GROUP: 5 - Brakes
DATE: December, 1988
CORPORATE
NUMBER: 665005R
SUBJECT: IMPROVED BRAKE PERFORMANCE
MODELS: 1985-1987 M VANS
THIS BULLETIN CANCELS AND SUPERCEDES TRUCK SERVICE BULLETIN 86-T-136 DATED FEBRUARY, 1987, ALL COPIES OF WHICH SHOULD BE DISCARDED. THIS BULLETIN IS BEING REVISED TO INCLUDE SOME 1987 MODEL VEHICLES.
Safari/Astro vans are designed to carry varying loads, and therefore, they are equipped with brakes that are capable of good stopping performance under a variety of conditions. Some customers who operate their Safari/Astro vans heavily loaded most of the time may prefer additional rear braking. If so, the following procedure is an acceptable method of increasing rear braking on 1985, 1986 and some 1987 Safari/Astro vans. It will result in a higher rate of deceleration at wheel slide and shorter stopping distances in a heavily loaded condition. The customer should be informed that there will be an increase in rear brake performance and the potential for rear wheel skid in unladen applications. Safari/Astro vans, in both their original condition or as changed by this bulletin, meet all the stopping requirements of Federal Motor Vehicle Safety Standard 105 for Light Duty Trucks.
Service Procedure
Important: Care must be taken to prevent brake fluid from contacting any painted surface.
1985-1986 Models:
Always ensure that the brake system is operational after performing brake work and before returning the vehicle to the customer.
FIGURE 1 - COMBINATION VALVE:
Steps A, B, and D involve all subject vehicles. Step C is engine dependent.
A. Replacement of the combination valve, (Figure No. 1)
1. Disconnect negative battery cable.
2. Disconnect the four brake lines at the combination valve.
3. Disconnect electrical harness at the combination valve.
4. Disconnect combination valve at bracket on the brake booster and discard valve.
5. Install new combination valve (P/N 15571205).
6. Reverse above procedure for reinstallation and tighten bracket nuts to 50 N-m (37 ft. lbs.).
7. Reinstall brake lines and torque nuts to 25 N-m (18 ft.lbs.).
FIGURE 2 - DRUM BRAKE COMPONENTS1985 & 1986 SAFARI/ASTRO VAN:
B. Replacement of the brake shoes, wheel cylinders, and link pins (Figure No. 2).
1. Loosen lug nuts on rear wheels.
2. Hoist vehicle and remove lug nuts and rear wheels.
3. Remove brake drum retaining clips (2).
4. Remove brake drum.
5. Remove brake shoe hold down and retainer springs.
6. Remove brake shoes and discard.
7. Disconnect brake line at wheel cylinder
8. Remove wheel cylinder and discard.
9. Install new wheel cylinder (P/N 18004888) after inserting link pins (2) (P/N 5464748).
10. Tighten wheel cylinder bolts to 18 N-m (13 ft.lbs.).
11. Connect brake line to cylinder, torque nut to 20 N-m (14.5 ft.lbs.).
12. Install new brake shoes (P/N 12321427).
13. Adjust running clearance to .030 +/- .005" adjustment clearance, 14. Reinstall brake drum, retainer clips, and wheel.
15. Bleed brakes, (Refer to Safari/Astro Service Manual, pages 5-5 and 5-6).
16. Repeat on opposite wheel.
FIGURE 3 - TACHOMETER TERMINAL AT DISTRIBUTOR CAP:
1985 Model 4.3L Carbureted Engine
C. Reduce cold idle speed to 1600 rpm.
1. Remove dash extension.
2. Remove engine cover
3. Install tachometer. The tachometer (tach) terminal is next to the engine control switch (bat) connector on the distributor cap (See Figure No. 3).
Notice: The tachometer terminal must never be allowed to touch ground, because damage to the module inside the distributor and/or ignition coil can result.
4. Reduce cold idle speed to 1600 rpm by adjusting the fast idle screw. Refer to tune up label for procedure.
5. Remove tachometer and reinstall engine cover and dash extension.
6. On a clean dry surface, next to the existing tune up label located on the fan shroud, place label P/N 10066488.
FIGURE 4 - PROM 4.3L 1986 M VAN:
1986 Model 4.3L TBI Engines
C. Replacement of the PROM (Figure No. 4).
1986 model vehicles built before VINS listed below require PROM replacement.
GMC GB524481 Federal Emissions
GB524599 California Emissions
Chevrolet GB203188 Federal Emissions
GB203623 California Emissions
Vehicles built after these VIN breakpoints have PROM factory installed.
Notice: Anytime the PROM is installed backwards and the ignition switch turned on, the PROM is destroyed and must be replaced.
1. Disconnect negative battery cable.
2. Remove kick panel from right side of the vehicle (1 screw) interior.
3. Remove ECM (2 screws).
4. Disconnect electrical harnesses from ECM module.
5. Remove ECM cover (2 screws).
6. Remove PROM from ECM and replace with new PROM, P/N 16068877 (Federal), P/N 16068926 (California).
7. Replace ECM cover.
8. Reconnect electrical harness.
9. Secure ECM module to its mounting location.
10. Replace kick panel.
1985 - 1986 2.5L TBI Engine
C. Vehicles equipped with this engine do not require PROM replacements.
D. Brake Burnish Procedure
On a dry clean level roadway, make twelve stops using the following procedure:
First Stop: Stop vehicle from 40 mph by applying the brakes to achieve a deceleration rate of 10 ft/sec/sec. (A 10 ft/sec/sec deceleration is equivalent to stopping the vehicle from 40 mph in 6 seconds.)
Second through Eleventh Stops: Stop vehicle from 50 mph by applying the brakes to achieve a deceleration rate of 15 ft/sec/sec. (A 15 ft/sec/sec deceleration is equivalent to stopping the vehicle from 50 mph in 5 seconds.)
Twelfth Stop: Stop vehicle from 40 mph by applying the brakes to achieve a deceleration rate of 10 ft/sec/sec.
In order to allow brake temperatures to stabilize, it will be necessary to drive 1/2 mile between stops without applying brakes. Following
completion of burnish procedure (twelfth stop), drive vehicle 3 miles keeping brake usage to a minimum in order to achieve sufficient brake cool down.
1987 MODELS
Effective 1987 start of production, all Safari/Astro models were manufactured with new wheel cylinders described earlier in this bulletin, however the new combination valve was not installed in the first several months of production. The new combination valve was installed effective with the following Vehicle Identification Numbers:
GMC: HB509335
Chevrolet: HB137507
1987 vehicles produced prior to these VIN breakpoints can be modified by following the service procedure listed below:
FIGURE 1 - COMBINATION VALVE:
SERVICE PROCEDURE:
1987 Model 2.5L TBI and 4.3L TBI engines
Important: Care must be taken to prevent brake fluid from contacting any painted surface.
Always ensure that the brake system is operational after performing brake work and before returning the vehicle to the customer Replacement of the combination valve (Figure No. 1).
1. Disconnect negative battery cable.
2. Disconnect the four brake lines at the combination valve.
3. Disconnect electrical harness at the combination valve.
4. Disconnect combination valve at bracket on the brake booster and discard valve.
5. Install new combination valve (P/N 15571205).
6. Reverse above procedure for reinstallation and tighten bracket nuts to 50 N-m (37 ft.lbs.).
7. Reinstall brake lines and torque nuts to 25 N-m (18 ft.lbs.).
8. Bleed brakes (Refer to Safari/Astro Service Manual, Pages 5-5 and 5-6).
Parts are currently available from GMSPO.
PARTS LIST
1. Combination Valve (1) P/N 15571205
2. Wheel Cylinders (2) P/N 18004888
3. Brake Shoe Kit (1) P/N 12321427
4. Link Pin Kit (2) P/N 5464748
5. Brake Fluid DOT 3 P/N 1052535
(as required) or equivalent
6. Label (1985 4.3L Engine) (1) P/N 10066488
7. PROM for 1986 4.3L P/N 16068877 Federal engine
8. PROM for 1986 4.3L P/N 16068926 California engine