DYNOS - MOTOR

DYNOS:

MORE THAN A

SMOG CHECK

MACHINE

BY BOB CERULLO

If you¡¯re using your dyno only for smog

checks, you¡¯re missing out on some

great, profitable service opportunities.

D

ynos in auto repair

shops were pretty

much unheard of

prior to the introduction of stricter

government-mandated emissions standards resulting

from the passage of the Clean Air Act

amendments. It wasn¡¯t until various

state smog inspection programs went

from requiring static emissions tests

to loaded-mode testing that the term

dynamometer became a household

42

November 2000

word in shops located in areas of

heavy air pollution.

When this legislation was passed,

there wasn¡¯t any real choice given to

repair shops in some states, like New

York, that were already doing state

motor vehicle inspections. If they

wanted to stay in the inspection business, they had to invest in a dynamometer programmed to work

with a computerized emissions-testing machine.

However, having a dyno in your shop

November 2000

43

Photo courtesy Environmental Systems Products

Photos courtesy Snap-on Diagnostics

DYNOS: MORE THAN A SMOG CHECK MACHINE

The eddy-current brake is the heart of the modern dynamometer. It absorbs

horsepower without overheating to help simulate road conditions.

can be a huge blessing in disguise. It

can provide exciting opportunities for

those shop owners willing to explore

the diagnostic possibilities beyond inspections of even a basic unit.

Types of Dynos

Actually, there are all sorts of dynamometers, from those designed to

test automatic transmissions and engines on a test stand to units on

which anything from a motorcycle to

a giant multiaxle truck tractor can be

driven under simulated road conditions. Special dynos are available to

test braking and steering systems, as

well. But it¡¯s the chassis dynamometer that can now be found in thousands of repair shops across the country, thanks to the various state smog

programs currently in force.

Essentially, a chassis dynamometer

is designed to simulate the conditions

a car would encounter on the road.

The term indoor road is as good as

any to describe the basic concept.

The typical chassis dynamometer is a

two-roller system, comprising a drive

roller and an idler roller. The drive

roller drives a brake through a coupling. In years past, chassis dynamometers were designed with wa-

44

November 2000

ter brakes that created resistance by

the use of cumbersome pumps controlled by valves and attached to the

rollers to effectively absorb power.

Practical use has proven the modern eddy current dyno more reliable

and simpler to build and use. The eddy current dynamometer uses a magnetic brake that¡¯s particularly effective

because it¡¯s almost impossible to overheat. It resists heat so well because the

braking effect decreases as the temperature rises. And since it¡¯s basically

an electric magnet, it can be turned on

and off as needed and is readily adaptable to computerized controls.

The idea behind the eddy current

dyno is to simulate road-testing under a

variety of driving conditions¡ªuphill,

downhill, rapid acceleration, slow acceleration, etc. It¡¯s the answer to the mechanic¡¯s prayer of being able to quickly

simulate conditions that existed when a

problem occurred, which you¡¯re now

trying to duplicate in the shop.

Three Major Systems

Modern chassis dynos have three major systems¡ªpower absorption, vehicle support and rollers, and control

and measurement. Let¡¯s look at these

systems individually.

Power absorption. This is what

makes the difference between simply

running a car with the wheels off the

ground and running it on a dynamometer. Power absorption is accomplished using the eddy current

brake, which has the capability to absorb more power than any vehicle

can be expected to deliver.

Vehicle support and rollers. On

most two-roller systems, the tire sits

down on and between the drive roller,

which drives the eddy current brake,

and the idler roller, which supports

the vehicle. An elevator is positioned

between the rollers to make easy entry and exit from the dyno possible.

Control and measurement. These

functions are generally programmed

into a controlling computer. In the case

of state-mandated testing, the software

program is specified by the various

states for their required tests. However,

many basic motor vehicle inspection

dyno systems can be used in an operator training mode or in other modes,

depending on the make and model, for

testing outside the realm of a smog

check or for pinpointing problems that

may have caused an inspection failure.

In addition, there are optional programs and equipment now available as

add-ons, and others soon to come, that

offer additional capabilities. These features greatly enhance the diagnostic capability of the dyno for vehicle performance testing.

Big-Time Versatility

The dynamometer is a multidimensional machine that can perform a variety of functions apart from a standard emissions inspection. Some of

them are:

Preconditioning. Let¡¯s say a customer complains that his front-drive

car is running hot and there are no obvious coolant leaks. By running the car

on the dyno with a light load¡ªtypically 5 to 10 hp¡ªthe engine can be

brought up to operating temperature

quickly. This allows you to check the

operation of the electric cooling fan,

along with overall cooling efficiency,

without the risks to the transmission

inherent in power braking or a potentially abusive road test. This same pro-

cedure can be used to quickly get the

feedback system into closed-loop, for

analysis. You can save considerable

time getting the catalytic converter to

light off using the dyno, as well.

Speedometer check. Without

performing an actual state inspection,

a mechanic can run a car through a

full range of speeds while comparing

the reading on the car¡¯s speedometer

with the reading shown on the inspection machine¡¯s monitor. Thanks

to the demands of state emissions

programs, these readings are quite

accurate, and can pinpoint inaccuracies in the vehicle¡¯s speedometer.

Driveline vibration. Suppose a

customer with a rear-drive vehicle

complains that at 45 mph, he feels a

driveline vibration through the seat.

Doing a sustained 45 mph usually requires the car to be driven on a highway. But, not if you put the car on the

dyno and run it up to 45 mph. Doing

this on a rear-drive car can rule out

the front suspension as the cause of

the problem and narrow your search

to the drivetrain.

Intermittent driveability problems. Technicians are often called

upon to diagnose conditions that simply can¡¯t be duplicated in the shop.

Part of effectively using a scan tool

during a road test is to try to catch a

snapshot recording of key data as a

problem occurs, then play it back at

the shop to determine the likely

cause. This requires driving while attempting to push a button on the

scan tool to begin a ¡°movie.¡± On a

dyno, a mechanic can concentrate on

the scanner without worrying about

running into a tree. He can also accelerate or decelerate at will, then

trigger the scanner at just the right

moment to get the before-and-after

recording of what¡¯s happening when

the hiccup occurs.

Quality control. One great use

for an inspection dyno is as a general

check of how well a vehicle is performing. This can be a prevacation

check, a prior-to-purchase used car

check or simply a quality control

check to be sure that once a vehicle is

repaired, all systems are go. This can

be used, for example, after a major

engine overhaul or transmission rebuild. Before-and-after testing for

driveability problems, meanwhile,

provides an assurance previously possible only while road-testing.

Setting up and running a car on the

dyno through a full emissions inspection in the ¡°operator training mode¡±

assures the tech and the owner that

the feedback system is operating correctly and that a MIL won¡¯t illuminate the first time the customer takes

the car out on the road after a repair.

This type of check can be performed

in less than 15 minutes, as opposed to

the time it would take to actually drive

to the nearest highway and perform

the same test, without having the

November 2000

45

DYNOS: MORE THAN A SMOG CHECK MACHINE

Dynos have often been called an indoor road for their ability to simulate road conditions without a mechanic ever having to

take the vehicle out of the shop. Hooked to a good emissions machine or engine analyzer, its capabilities are virtually endless.

benefit of an actual emissions report

in hand.

In the case of a prior-to-purchase

used car check, doing an emissions

test on the dyno ensures that the

owner won¡¯t come back to complain

that a car that ¡°looked good¡± failed its

smog inspection. Being able to advise

the customer beforehand that a particular vehicle will not pass a smog

test provides good information for

the prospective buyer, and an opportunity for your shop to get the job of

correcting the emissions problem.

Isolating driveline-related vs. engine-related vibrations can be accomplished easily on a dyno by first running the vehicle up to the speed at

which the vibration is noticed. Note

the mph shown on the computer monitor and the engine rpm. Then shift to

another gear and bring the speed up

to the same shown on the monitor

during the previous test. If the problem occurs at the same engine rpm,

it¡¯s likely to be engine-related. If it occurs at the same dyno mph, the problem is more likely driveline-related.

Transmission shift points, kickdown

and slippage often become obvious

during the acceleration and deceleration cycles required by some states¡¯

drive cycle. Retesting after a TV cable

adjustment or major repair can be

done quickly and safely on a dyno.

Tires that have broken belts or an

out-of-round condition will show up

46

November 2000

immediately on the dyno as they roll

against the dyno¡¯s smooth rollers.

You¡¯ll hear the noise and feel the vibration in the steering wheel. Distorted tires will cause a vehicle to

move from side to side on the dyno,

sometimes violently, as the tires fight

the restraining belts.

Transient test results, when printed

out, provide a graphic depiction of

where emissions should be at specific

rpm and mph readings during an actual or simulated smog inspection. By

printing out the results, your tech can

get a good idea of when the failurecausing problem occurs.

Other Diagnostic

Capabilities

Typically, basic state inspection dyno

systems are designed to do what each

state requires for its test. In some

cases, trying to do too much outside

of a dedicated system procedure may

result in a ¡°tampering¡± condition.

But with a little know-how, you can

use most machines for diagnostics

outside of the realm of an official

state smog program. For example,

with the ESP SystemOne unit, after

you enter engine size, number of

cylinders, vehicle weight and emissions limits, you can enter into an

ASM 5015, ASM 2525 or RG240

mode to obtain emissions levels that

will provide a baseline. Then use the

dyno to perform diagnostics without

going through an official smog inspection. Once the repairs are done,

this same method can be used to verify that they¡¯ve been successful prior

to actually running an official emissions test.

Snap-on Diagnostics¡¯ SIS 900 Service Inspection System is fully capable of state inspection dyno testing,

while also being able to accomplish

general repair diagnostics, including

gas analysis, compression testing and

ignition and charging system diagnosis. On the machine¡¯s control panel,

various dyno modes can also be entered, including Road Simulation, Incline, Power, Speed Control, Constant Torque or None, which is essentially freewheeling.

Bear/Allen Emissary Series machines from SPX Corp. also offer

choices beyond the basic requirements of state emissions tests. Coupled with the company¡¯s Exxam engine analyzer, they provide a wide variety of diagnostic capabilities right in

the shop.

It¡¯s important to read the operator¡¯s

manual carefully to understand what

your dynamometer can and cannot

do. Better yet, call the manufacturer

of the dyno you presently own or plan

to own to discuss capabilities and upgrades. Some makers we talked with

have plans in the works for additional

software that will provide fuel

mileage checks so that actual fuel

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