Solenoids - Replace or Reuse?


Solenoids - Replace or Reuse?

Solenoids can become a hot topic of conversation at transmission shops. Some say the best way is to treat them like a soft part and replace them on every rebuild, whereas other say that you can check them and only replace the ones that are faulty or worn out. The first group will say any reuse leaves you open to costly failure before the warranty period and can leave you with an unhappy customer. The second will argue replacing them all can be expensive, and on some models, you may end up with little or no profit left on a rebuild job.

Which is the stronger argument? Is there a right or a wrong way? Are both right?

Years ago, I was involved with the design of a high-end solenoid testing machine for an OEM Chrysler remanufacturer. They were specifically remanufacturing the rear wheel drive Chrysler units (42-48RE) and the machine we were commissioned to build would test the solenoids from all the cores that came in from the Chrysler dealerships across the country. Before we had even started the design of the machine, they had already completed a study to determined what percentage of the LU and OD solenoid assemblies were going to be recoverable and what percentage of the governor solenoids would be recoverable and could be reused once flushed and tested. The machine itself would perform a variety of OEM tests to the solenoids, and if they passed all of them, it would give green light that it was a good solenoid and could be reused. Since they were a large volume remanufacturer, the amount of money they would save by testing and reusing solenoids over the course of a year was in the hundreds of thousands and more than justified the cost of the machine.

If a scheme like this works for an OEM remanufacturer then it must be right for you? Well, there are a few things you need to consider.

First, you must properly test solenoids and verify their function. There are many inexpensive solenoid testers that may use air, are only an electrical check, or perform an incomplete hydraulic check. If they are not up to the task of properly testing solenoids, then it’s not a worthwhile investment. An incomplete test is almost as bad as no test at all. A high-quality hydraulic solenoid testing machine such as the Hydra-Test HT Sol are able to perform a consistent complete test on all modern solenoids. Having that capability does cost a set amount of money. Does your budget and volume allow for the purchase of a solenoid testing machine such as the HT Sol that can perform a complete, accurate, and repeatable tests?


Figure 1: Hydra-Test HT Sol Solenoid Test Machine

Second, do you have the technicians available to do solenoid testing and understand the process. Machines like the HT Sol can make the job easier since it uses a graphical overlay to compare the solenoid you are testing with a known good or known calibrated solenoid. There is still needs to be an understanding of the resulting graphs you get and what they mean when you compare them. Rushing through a test, not taking the time to understand how to test, or testing incorrectly is as bad as an incomplete test. Not only are you now paying for a testing machine, but you leave yourself open for comebacks because its not being used properly and eventually the test machine will migrate to the corner and collect dust.

Figure 2: HT Sol Comparative Graph

Third, and maybe most importantly, are the units you commonly see and work on going to give you payback that is worth the investment in both the technicians test time and the cost of a test machine? Not all parts of the country have the same mixture of vehicles and some shops specialize in certain units over others.

Let’s take a look at two different units. The first is a 4R70W which while it is an older unit, is still a relatively common in shops around the county. The second is the AS68RC/AS69RC, which is now the standard unit for Dodge trucks as well as used by other vehicle manufacturers.


Figure 3: 4R70W Solenoids | Figure 4: AS68RC Valve Body with eight solenoids

If we take my experience with the Chrysler remanufacturer, they had projections that upwards of 60% of the LU/OD (which are on/off) solenoid assemblies could be recovered and reused. On/off solenoids have fewer moving parts and realistically can last far beyond the life of the vehicle. The pressure regulating solenoid on the other had was closer to the 25% recovery rate. Since they typically are always moving they are more prone to wear and it was really the lower mileage solenoids that would still pass the tests and could be reused. These numbers are conservative so we will use them for the purpose of this example, assuming we can salvage 60% of on/off and 25% of pressure regulating solenoids.

If we look at a unit like the 4R70W, a complete set of solenoids off the shelf from a typical parts house runs about $50 for the on/off (shift) solenoids and $75 for the pressure regulating (TCC and EPC) solenoids. Averaging across all units at the percentages stated, you could potentially save $30 per unit for on/off solenoids and $19 per unit for the pressure regulating solenoids. At $49 savings per unit your payback time between a machine and a technician’s time could easily stretch into years. In this case it starts to make more sense to just replace the solenoids if the bulk of your work are 4R70W types of units.

If we look at the AS68RC, a complete set of solenoids off the shelf from a parts house runs about $360 for the on/off and $600 for the regulating. Again, averaging across all units at the percentages stated, you could potentially save $216 for the on/off (shift) solenoids and $150 for the regulating (linear) solenoids. This gives us $366 saving per unit. Suddenly the case to test and find and replace the faulty solenoids makes a lot more sense, especially if you see and service a lot of the later model units that have the higher number of these more expensive solenoids.

Another benefit for a solenoid test machine I have heard over the years from shops that test and reuse solenoids is that they also use the machine to test the new solenoids. Manufacturers, whether OEM or aftermarket, have extensive quality controls and test protocols in place to ensure that everything that goes out the door is a good quality product. However, there is always a chance that there could be a faulty part. There is also a chance that it was dropped in transport or somehow damaged before you received it. A quick test on a machine ensures that it does work and you can be confident that if you have problems once the transmission is back together, its not a solenoid issue as each one was tested and checked before you installed them.

At the end of the day it all comes down to dollars and sense. Is the investment in a machine and testing going to pay off and give you more profit down the road? You also need to have technicians that are comfortable using such a machine and are not afraid to scrutinize solenoids and err on the side of caution. Take a look at the units you commonly rebuild and run the numbers on the costs of solenoids. It should give a clear picture of what makes sense for you and your shop. At the end of the day the goals are to put out quality work, have happy customers, and be profitable.



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