42 GEARS July 2003

Determining when to replacesolenoids seems like a topicmost transmission technicians

are concerned with. Many techs wouldprefer not to replace them duringrebuilds. This is most likely a cost-driv-en decision. And it’s understandable,because depending on the unit, replac-ing the solenoids can increase the costof a rebuild considerably. But there’s noclearly defined line as to when youneed to replace solenoids and when youdon’t.

One thing you need to consider iswhether you’re dealing with an on/offsolenoid or a PWM solenoid. Here’swhy: An on/off probably isn’t going tosee more than about a million cycles in70-80 thousand miles, while the PWMmay easily see 500 times that. Imaginebeating a hammer against your bench,

and the difference between what itwould look like after a million impactsversus 500 million impacts.

Ignoring other factors such as con-tamination, and recognizing the addi-tional wear they receive, how comfort-able would you be with reusing a PWMsolenoid that’s been in service for50,000 miles? That’s not to say you onlyneed to replace PWM solenoids, butrather to point out that solenoids arewear items, and they have a finite life.

Consider, too, the length of thewarranty you’re offering, and the costof having to rebuild a transmission forfree. How does that affect your deci-sion-making process? Or how muchtime will you spend diagnosing andreplacing a solenoid once the transmis-sion is rebuilt and back in the car?What’s the cost associated with that?

Developing a definitive list ofwhich solenoids must be replaced andwhich ones should be replaced could bedifficult. In general though, you shouldprobably plan to replace the solenoidswhenever:

• the transmission has a hard parts failure.

• the transmission has overheated• there’s obvious damage to the

solenoid, or even if you strongly suspect it’s damaged.

• the solenoid fails a resistance or operational test.

• the transmission has high mileage;over 70,000 miles or so.Other considerations, such as duty

cycle versus on/off solenoids, can bemisleading if you don’t understand thesolenoid’s relationship with the entiresystem.

A NOTE FROM ATRA’S TECHNICAL EDITOR, LANCE WIGGINS

REGARDING “SOLENOIDS: TEST OR REPLACE”Whether you choose to replace a solenoid during a rebuild or to simply test

the existing ones is undoubtedly based on a number of factors:

The original complaint, type of failure, length of warranty and cost are onlya few of the things we need to think about when it comes to replacing or test-ing solenoids.

In the following article, Scott Kirkendall and Will Goodman of Rostra Precision Controls (amanufacturer of solenoids), have been kind enough to share their experience on this subject.

Parts of the following article first appeared as a part of a discussion on the SLTG e-mailforum where it was suggested that this would make a great article in GEARS.

We agreed and are presenting it to the industry for evaluation and information.

Keep in mind as you read this article that, like all subjects, there are often times differingviewpoints. With that in mind, GEARS has made space available for those whose primary busi-ness is that of testing solenoids and the manufacturing of the equipment needed to do so(namely the good folks at Zoom Technology), and you’ll read their thoughts in next month’sissue.

We’re confident you’ll enjoy both articles on this subject.Lance Wiggins

by Scott Kirkendall and Will Goodmanof Rostra Precision Controls

Solenoids: Test or Replace

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GEARS July 2003 43

43 Superior 7-03.qxd 6/16/03 1:52 PM Page 43

44 GEARS July 2003

The key is educating yourself onhow solenoids work and on how they fitinto the whole system. Then you factorin the costs associated with warrantiesand the value of your time. Somewhere

in that mix you’ll find the decision thatbest suits your needs, your customer’sneeds, and the needs of your business.

Solenoid Testing and FlushingAny dis-

cussion aboutreusing sole-noids invari-ably leads tothe topic ofsolenoid testersand flushers.

Testing asolenoid beforeyou install itonly makessense, becauseit’s much easierand less timeconsuming totest the sole-noids while theunit’s on thebench. At aminimum, youshould checkthe resistance,

even on brand new solenoids. But whatabout dynamic testing and cleaning?

No matter what you’ve heard aboutsolenoid flushing, there’s no way to geta solenoid clean simply by flushing acleaning fluid through it, or evencycling it on and off while flushing. Allcontaminants collect where there’s littleor no fluid flow through the solenoid. Iffluid did flow through that location ofthe solenoid, chances are contaminantswouldn’t collect there.

What’s more, any metallic particleswill slowly become magnetized andwill start to stick to the sides of thesolenoid. All the flushing in the worldwon’t get rid of those particles. Andlet’s not forget, flushing still has noeffect on the wear in the solenoid.

So, in general, while flushing maybe better than nothing, it’s not a wholelot better.

Does Solenoid Testing Work?Solenoid testers are a great idea,

but finding one that can accurately testthe performance or function of the sole-noid is next to impossible. It’s not sim-

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TCC solenoid cut open to show valve wear. This type of wear could be difficult to find by using air or oil testing procedures.

Solenoids: Test or Replace

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GEARS July 2003 45

ply a matter of whether the solenoidworks or not, because the solenoid ispart of a hydraulic circuit in the trans-mission. It’s more a matter of how thatsolenoid’s performance affects the othercomponents on the same hydraulic cir-cuit. And the duplication of that circuitis critical if you want to get reliableresults.

Many repair shops use a bench toptester that just clicks the solenoid onand off; it doesn’t really test much. Andthere are some applications where thesolenoid doesn’t click, simply becauseof how it’s designed. That doesn’t meanthe solenoid doesn’t work; it just does-n’t click.

In general, hydraulic testers simu-late the normal operating conditions ofthe solenoid more accurately, so theresults they provide seem to be moreaccurate than pneumatic (air) testers.On the down side, hydraulic testers tendto be more expensive than pneumatictesters.

Regardless of which type of testeryou’re using, it’s important to be carefulwhen testing solenoids. Many techni-

cians have damaged perfectly goodsolenoids simply by testing themimproperly. The most common mistakeseems to be trying to make a PWMsolenoid click like an on/off solenoid. Ifyou leave a PWM solenoid energizedtoo long, you’ll melt the coil windinginsulation. And it doesn’t take long:maybe 20-30 seconds is enough to raisethe solenoid’s internal temperature towell over 400º F or 500º F.

That’s not to suggest that solenoidtesters aren’t a good idea; it simplymeans that you need to take test resultswith a grain of salt. Just because a sole-noid checks out okay doesn’t guaranteeit’ll work in the transmission. Solenoidtesting is a great idea, as long as youunderstand the tester’s limitations.

The real question is: How willsolenoid life affect your bottom line?Many repair shops are offering war-ranties for 3 year/36,000 miles. Withthat long of a warranty, it’s important tomake sure the transmission is going towork right… and keep on working.With the improved quality of compo-nents available from the aftermarket, a

rebuilt transmission really should bebetter than the original, so a 3/36 war-ranty isn’t unreasonable. A properlyrebuilt transmission should last wellbeyond the warranty period. This is oneof those variables you need to considerwhen deciding whether to replace thesolenoid or not.

Air vs. Oil There’s virtually no correlation

between testing with air and testingwith oil. While both media are consid-ered fluids, big problems come intoview when trying to draw any compar-isons between the two. Air is a com-pressible fluid, so the physical proper-ties of density (mass of air per unit vol-ume) are vastly different regardingpressure, temperature and volume. Oil,on the other hand, is a non-compressi-ble fluid, so its density remains prettymuch constant, and it isn’t affected bychanges in pressure, temperature andvolume.

Fluid Properties of Air The fluidic properties of air are

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interrelated, and may be predicted usinga mathematical equation known as theideal gas law. While this is beyond thescope of this topic, the basic idea is thata change in pressure, temperature orvolume will cause a change in the othertwo properties, due to the change in airdensity. This becomes even moreunpredictable when the air is moving,because then we have to consider airvelocity, volumetric flow rates, airexpansion through an orifice, heatingand cooling effects, and so on.

For example, if you heat a fixedvolume of air in a closed container, thepressure in the container will increase.This is because the air moleculesabsorb the heat and try to expand, push-ing against the walls of the container. Ifyou did the same with oil, you wouldend up with hot oil, but the pressurewould essentially remain unchanged.

Now take that same air in the con-tainer, and let it escape through a smallorifice. More air will escape under highpressure than under low pressure,because the density has changed, andmore molecules are compressed intothe same amount of space. Since oilisn’t compressible, the number of oilmolecules that can pass through an ori-

fice depends solely on the sizeof the orifice. The volumetricflow rate will be the same; whatwill change is the velocity atwhich the oil exits. So the sameamount of oil gets out, it’s justmoving much faster.

Air Testing Solenoids There are only certain

things we can learn from air test-ing a solenoid. Air testing canshow that a solenoid has the abil-ity to let air flow, or it can stopthe flow of air. But, since airmolecules are so much smallerthan those of oil, air testing mayindicate a leak that may notoccur with oil. The drop in airpressure and air flow rates willvary, based on air pressure values, airtemperature, air quality (moisture in thelines), test fixture volume, fixture tem-peratures, and any restrictions createdby the piping and fittings.

The more precisely you want to testa solenoid, the more these variationscan affect your tests. This becomes evenmore evident when checking PWM orproportional solenoids. Because theirdesigns are intended to balance andcontrol the flow and pressure of oil,their response to air is basically mean-ingless.

That doesn’t mean air testing iscompletely useless: It can be valuablewhen checking new solenoids thathaven’t seen any wear, contamination orabuse. But withused solenoids,air testing canonly showwhether thesolenoid canopen and close.There’s no wayto predict how aused solenoidwill stand upunder high tem-peratures, orhow long it willcontinue tooperate until theinternal compo-nent wear willcause a problem.

Even then,

air testing is only useful for testingon/off solenoids. PWM and proportion-al solenoids need a non-compressiblefluid to balance the armature positionand meter solenoid output properly.

Oil Testing Solenoids Oil testing can provide more reli-

able test results, if the test is conductedin the appropriate manner. The trans-mission solenoid is only a componentpart in a complete hydraulic circuit. Thereactions of other components withinthe circuit affect how the solenoid func-tions.

All of the hydraulic circuit compo-nents are related by pressure. Without adifference in pressure, there can be no

46 GEARS July 2003

Valve type EPC solenoid

Typical on/off solenoid. The wear pattern on thistype of solenoid differs

greatly from a PWM type of solenoid.

PWM type solenoids can cycle as many as 1,000 times or more for every eight hours of operation.

Solenoids: Test or Replace

solenoids.qxd 6/11/03 11:54 AM Page 46

Worn boost valve assemblyCAUSE:

®

SATURN TAAT(Type 2 Valve Body Only)

See your nearestTransmission Special-ties™ distributor & askfor Sonnax® qualityengineered products.

Also availablefrom Sonnax®:

Automatic Drive • P.O. Box 440

Bellows Falls, VT 05101-0440 • USA

802/463-9722 • 800/843-2600

fax: 802/463-4059 • info@sonnax.com and visit our website:

for Sonnax® reamers & other tools.

Visit our website dedicated to assist & guide transmission rebuilders

www.sonnax.com

Also:• Insufficient line rise• Soft shifts under load• Clutch failure

BOOST VALVE & SLEEVE

DELAYED OR HARSH REVERSE ENGAGEMENTCOMPLAINT:

This Sonnax valve can only be used with Type 2valve bodies which do not have a circuit here.

Part No.

95200-03K

CORRECTION:Replace boost valve assem-bly to prevent oil leakage.

Boost Valve

CORRECTION:This retaining ring willnot expand, and will stayin place.

Part No.

36744-01

The retaining ring does nothave enough tension, causingit to come out of the groove

CAUSE:

OVERDRIVE PISTON RETURN SPRING RETAINING RING

E4OD4R100 ‘89 & UP

OVERDRIVE PISTON RETAINING RING POPS OUT OF GROOVECOMPLAINT:Also:

• E4OD, overdrive is lost• 4R100, catastrophic

failure

Boost Sleeve

Pressure RegulatorCushion SpringAlso Available 95200-01 4L30-E &

THM180P/N:

54701-03K3-TAB BI-METAL

THRUST WASHER &SELECTIVE SHIM KIT

MERCEDES722-3, -4, -5, -6

P/N:68942-01

SOLENOID PRES-SURE REGULATOR

VALVE SPRING

VW 01M, 01N,01P, 099

P/N:119940-01K

TCC BOOST VALVEKIT

Disc Spring

Housing

Overdrive PistonReturn SpringRetaining Ring

Type 1 valve bodies havean additional circuit here.

GRTSAD-07-03x.qxd 5/16/03 3:58 PM Page 1

oil flow. Spool valve areas, springloads, regulator valves, orifice restric-tions, filters, and so on, all affect thepressure in the circuit. Differences involume determine the rate pressurebuilds or drops. For example, it wouldtake much longer to pressurize a sealed55-gallon drum than it would to pres-surize the average accumulator.

Temperature also plays a majorrole, as the viscosity of oil decreaseswith temperature. So, as the oil heatsup, it becomes “thinner” and has lessresistance to flow. But at the same time,that “thinner” oil is more likely to leak.To test a solenoid properly with oil, allphysical conditions of the hydraulic cir-cuit must be present. And testing shouldbe conducted at the proper temperatureto ensure real life viscosity values.

PWM and Pressure Control Signals All PWM and proportional sole-

noids should be tested in oil. Reliability

of the results depends on the test fixtureand how it relates to the originalhydraulic circuits. Flow rates are not ascritical as pressure build and decayrates within the circuit.

Accurate electrical signals are crit-ical. With computer driver circuits pro-viding signals such as peak-and-holdsignals, two-stage variable PWM sig-nals, and current averaging, it’s impor-tant that the tester duplicate the controlsignal accurately to provide useable testresults. The difference of just a few mil-liseconds of control signal can make ahuge difference in how the solenoidregulates pressure.

Even if the tester replicates theelectrical signal and hydraulic circuitconfiguration accurately, no tester canpredict how much life is left in a sole-noid. Some PWM solenoid will see upto 600,000,000 cycles in 80,000 miles.So, even if the solenoid checks good,age should be a factor in determining

whether to reuse or replace a solenoid.

SummaryWhile there are many different

styles of solenoid tester on the market,of varying levels of complexity, noequipment exists that can guarantee theuseful life that remains in a used sole-noid. Some solenoid defects can beidentified with such equipment, butonly if those defects are evident duringthe test procedure.

Should you test or replace?Consider such issues such as the lengthof your warranty and how long the sole-noid’s been in service. And don’t forgetto factor in how much it actually coststo test a used solenoid, rather thaninstall a new one that comes with aguarantee.

48 GEARS July 2003

Which test equipment is right for you? Here’s a quick break-down of the Pros and Cons of each type of system:

Air Testing Pros✓ Less expensive

✓ Cleaner

✓ Will identify plugged or non-functional sole-noid

✓ Can identify a leaking solenoid

Cons✓ Doesn’t replicate vehicle conditions

✓ Won’t identify solenoid failure at high temper-atures

✓ Limited use on PWM and proportional sole-noids

✓ Can’t predict useful remaining solenoid life

Oil Testing Pros✓ Will identify plugged or non-functional sole-

noid

✓ Will identify a leaking solenoid

✓ Control signals are better approximations ofactual vehicle signals

Cons✓ More expensive

✓ Not as clean (although shops are used to oilanyway!)

✓ Hydraulic circuits aren’t adequately represent-ed in test fixtures

✓ Won’t identify solenoid failure at high temper-atures

✓ Data output may not have fine enough resolu-tion to predict response of fast acting sole-noids

✓ Can’t predict useful remaining solenoid life

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