Prevent amped-up air repair costs
Routine voltage checks ensure good health in clutches and compressors
By Steve Montgomery
Clutch circuit low voltage is a common cause of clutch and possibly compressor failure, but it is preventable. The higher the voltage the greater the strength of the magnetic field strength. The magnetic field pulls the iron drive plate into contact with the belt driven hub to drive the A/C compressor. Performing a routine voltage check as part of the routine maintenance procedures could save operators and fleets untold dollars in repair cost and vehicle downtime.
In any case, routinely check the air-gap between the drive plate and hub. Where there is too much gap — more than the spec ranges from .025-inches – .035-inches — the magnetic field will have to pull the plate further to engage. Too little gap may result in rubbing when operated at elevated temperatures in the engine compartment. A fairly consistent gap is required around the full circle.
If that is not happening first try to carefully pry a little at the spring straps as shown in the compressor manuals, and adjust the amount of shaft shim to obtain the correct air-gap. Neither the spring steel nor the drive surfaces can be nicked or scarred. Ultimately, technicians should replace a warped or leaning drive plate before it ruins the bearings and possibly the entire compressor.
Even with the proper voltage and air-gap, approximately a half-turn of slip occurs during clutch engagement. With low voltage, several turns of slippage are not unusual. In combining low voltage, high head pressure and increased slippage, the vehicle will generate enough heat to melt the bearing seals and cook out the grease. At that point, the compressor main shaft seal will also fail.
If the vehicle clutch is slipping rust will often appear around the clutch area. The excessive wear creates fine iron particles that accumulate on nearby surfaces as well as the clutch itself. The telltale red of rusted dust is evidence of slippage and may be a visual cue to check the voltage.
Ideally the supply voltage is the same as the alternator output voltage (13.6-14.2). A typical 12-volt clutch will usually draw 2.5 amps to three amps of current and cause minimal reduction of voltage (less than one volt drop maximum). If the contacts in the A/C thermostat or any component in the supply circuit were to corrode (oxidize), the added resistance may reduce the supply voltage. This could be a significant problem for long-life service vehicles.
If the A/C were added in the typical aftermarket, then the clutch ‘request’ that starts at the control head may run the full length and height of the bus to the rear evaporator (the thermostat) and then back again to the engine compartment. This extended wire length will create a significant voltage drop even with only a 2.5-amp draw on an 18-gauge wire. Using larger 16-gauge wire would be better and 14 better still, however the correct solution is to add a clutch relay up front.
If using the old run to the back circuit to trigger a relay, the relay coil will only draw one tenth of an amp. This may allow that old circuit to remain as is, that is un-repaired for the time being, without impact, as the added relay carries the current and provides the full voltage to the clutch. The original A/C kit may have included a clutch relay and the installer may have overlooked its importance. This becomes more critical as the vehicle ages and oxidation occurs. Service technicians should be encouraged to check their older hard-working vehicles in the fleet as well as any new installs for proper clutch voltage.
Because they age it is necessary to periodically inspect and repair clutch circuits that incorporate a relay with a short 16-gauge wire to a 10-amp fuse connecting directly to the alternator stud. Dielectric grease at the relay and fuse terminals will slow down the corrosion.
To observe the voltage drop it is imperative to test with the clutch connected. The ampere current draw is what reduces the voltage potential.
Routine checks of the clutch supply voltage will identify the potential problem before excessive engagement slippage causes your repair costs to get “amped-up.” BRM
Steve Montgomery is director of engineering for
Omega Environmental Technologies, Dallas, TX.