Causes of Improper Vacuum at Modulator

1. Engine.
- Tune-up.
- Loose vacuum fittings or improperly routed hoses/lines.
- Vacuum-operated accessory leak (hoses, vacuum valve, etc.).
- Engine exhaust system restricted.
2. Vacuum line to modulator.
- Leaks.
- Loose fitting.
- Restricted or incorrect size orifice.
- Carbon build-up at modulator vacuum fitting.
- Pinched line.
- Grease in pipe (delayed or no upshift-cold).