SUBARU Automatic Transmission Solenoid A Dropping Resistor

July 2, 2011. Author:

The SUBARU automatic transmission dropping resistor is connected in parallel with duty solenoid A. Dropping resistor Duty solenoid A regulates the Subaru automatic transmission fluid line pressure. Like most of the transmission solenoids, duty solenoid A's duty cycle is varied by the transmission control unit (TCU) to control the line pressure.

The dropping resistor works with duty solenoid A in regulating the automatic transmission fluid line pressure. This resistor keeps a certain amount of current flow through duty solenoid A during the 'OFF' portion of its duty cycle. So in other words, duty solenoid A is never fully 'OFF'.

The dropping resistor is located on the right front shock tower, near the MPI (multi-port injection) fuel system dropping resistor. The photos at the right depict the Subaru automatic transmission dropping resistor.

If this resistor fails open, becomes disconnected, or has its wiring severed, one result might be increased shift shock. The reasoning behind this is that without the resistor in the circuit, the line pressure may be higher, since without the current passing through solenoid A during the 'OFF' portion of its duty cycle, it will tend to close further, and thus not bleed off as much pressure from the automatic transmission fluid pump.

If the resistor fails open or is disconnected, it should cause the ATF temperature lamp to flash 16 times on the next startup, since the TCU would be able to see electrically that the resistor is open.

This resistor was used on the SUBARU 4EAT phase I and 4EAT phase II automatic transmissions. It may also be used on the 5EAT though I have not confirmed it.

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Hello. I was having trouble with what felt like my transmission slipping. Someone suggested I disconnect this resistor. Is that a good idea?

Yes I have heard it suggested to disconnect the Subaru automatic transmission dropping resistor if the transmission is slipping. However, I do not think it is a good idea. First most automatic transmission control units (TCU), if they detect slippage, will automatically raise line pressure anyway.

Second, disconnecting the dropping resistor may cause small spikes in the line pressure. The spikes would be caused by duty solenoid A going either fully closed or nearly fully closed during off part of the duty cycle (whereas with the dropping resistor in place it would keep an amount of current flow through duty solenoid A). And when duty solenoid A is fully closed there is nothing to bleed off line pressure.

So the bottom line is, if the automatic transmission is slipping, its days are numbered. A simple thing to check, of course, is whether the Subaru automatic transmission fluid is at the correct level, since if there is not enough automatic transmission fluid in the transmission back can cause the transmission to slip. When an automatic transmission slips, it wears out the friction material on the clutches very quickly, so slipping must be addressed immediately.

When an automatic transmission's clutch packs slip, it is because the friction between the clutch plates cannot hold the applied torque. This situation can occur if the line pressure is not high enough, or if the wrong transmission fluid is used, or if the clutch packs are too worn out.

Every so often my 4eat transmission when shifting from 2nd to 3rd and 3rd to 4th revs up to approximately 4,000 rpm's before it shifts. This happens only under light acceleration.

Hello Ken and welcome to the site. What year and model is your Subaru? The 4EAT Phase II is known to have that sort of rpm jump sometimes. Subaru had issued a bulletin about it. But it was only for the first 3-4 shift I believe it was after sitting in a cold area afternight. It was something about a hydraulic circuit draining and then refilling on the first shift.

Otherwise, if you haven't already, you might want to double check your ATF level to make sure it is correct. Also, if the bands are far out of adjustment sometimes that can cause shift flares.

I have 2000 Outback, the trans all of a sudden stopped pulling like the bands started slipping and the at temp gauge started flashing. If you rev it real high it tries to engage, Is there any band adjustment on the 4eat trans ( the fluid is fine and clean )?.Is there a chance the dropping resistor caused this?, and finally, will the ECU cause a problem like this?, It shifted fine with no slipping up to the point it just went crazy. I would appreciate any input I can get before I go the "change the trans" route. Thanks, Scott

Hello Scott and welcome to the site. The AT temp light will flash 16 times at startup if the TCU - transmission control unit detected a problem on the last drive cycle. The first step is to read out the code(s) from the TCU. There is a harness under the dash where some certain pins have to be shorted together and some steps followed to do that. The codes that are flashed out will be helpful in troubleshooting. It could be a wiring harness problem, or quite possibly a faulty solenoid in the transmission. Most of the solenoids can be accessed on the valve body after removing the pan, provided you know which one is the target or is faulty. There is a band adjustment, but what you describe does not sound like a band problem. It is unlikely the dropping resistor is the cause of the problem. If the dropping resistor failed open, your line pressure should simply increase. If by chance the dropping resistor failed shorted that could be an issue but it is unlikely it would short. The next step is to get the codes flashed out of the TCU.

Thanks for the reply, I am very knowledgeable on most vehicles, subaru's are the exception, Is it possible to disconnect the negative post and reset the TCU as well as the ECU? What else would cause a pressure drop, as it seems to me to be the culprit, seals? or would you think the solenoids would be the thing to check?, Will the flashes on the at light be able to be deciphered by a book or who has a way of checking it, subaru , trans shop or a place like advance auto?.Thanks for your help!

Hello Scott. Leaving the battery unhooked for a while would reset both the TCU and ECU on that era Subaru. Of course, if there is a faulty solenoid or wiring problem, or the TCU is faulty, it won't make any difference. I have a list of the Subaru transmission codes buried in my files somewhere, I'm sure I could find it. Any transmission shop should be able to get it to flash out the codes and tell you what the code means. Of course as you probably already know, they will also want to rebuild your transmission, and a good transmission shop is hard to find. Advance Auto I doubt would know how to get it to flash the code.

Now, by chance, were you noticing a delayed engagement to drive before this problem started? If you started it up after a cool overnight sit, and put it into drive, did it engage within a second or two, or did it take much longer? I ask because that particular year 4EAT phase II, and also to some degree on the 2001's, have a known issue with a lathe cut ring on one of the drums that can 'prevent a rapid buildup of pressure', thus causing delayed engagement to drive. If that was the issue, many people have reported success using the Trans X additive.

The funny thing about it is it's acting just like my ranger did, it started slipping and finally wouldn't engage, I got underneath it and adjusted the bands and now it drives like new and haven't had a problem since, thus my reason for suspecting band adjustment.The outback will pull if you rev it, but I've noticed when I put it back in park, it has a sound like your moving while putting it in park, at a standstill. As I learned, never pull a trans prematurely, always diagnose everything first. Fluid looks good and I smell nothing strange, should I change the filter and fluid anyway?. I've heard not to change the filter, Is there a reason for that?.

The spin on ATF filter can be changed. Originally Subaru said it doesn't need to be changed, but many people change it anyway. It is an excellent design and keeps the torque converter and clutch material shreddings from plugging up the cooler in the radiator. The engine oil filter will fit in place of the trans filter, but the engine oil filter lacks a piece of metal screen that is over the bypass valve on the transmission oil filter.

The bands are adjustable on the transmission. It's hard to get at though, it's a double locknut type setup that is sort of on top of the transmission. A Subaru bulletin showed disconnecting the exhaust and using a piece of 2x3 to hold the exhaust out of the way to get to the bands to adjust them.

But anyway, if you're getting the flashing ATF light, it means the TCU has codes stored. Those codes could be very revealing. If you read the codes and then find out their meaning you might get something like Duty A low input or something that might implicate a certain solenoid or possible wiring issue, etc.

Thankyou, you're advice has been extremely helpful.

Pulled codes P0748 and Code 75 from the TCU.

Dropping resistor checks out good.

Trans drives for 1 wheel revolution in first gear and then slams into 3rd. It stays in third.

Torque converter locks up in 3rd just fine.

So the question is, wiring issue or solenoid issue?

Hello. I happen to have an subscription for that vehicle since I also have a 2000 Outback, so let's check what we have here..


And code 75 from the TCU would be DTC 75 DIAGNOSIS: Output signal circuit of line pressure duty solenoid or resistor is open or shorted

Looking at the procedure it appears it could be the dropping resistor is open or shorted, or the line pressure solenoid itself is open or shorted, or the wiring between them has an open or short. I'd probably start by checking the dropping resistor since that is more easily accessible, but I see you said you already checked that. Otherwise you have to start backprobing connectors and such to check the solenoid and wiring itself.

The slamming into 3rd you mention might be the TCU putting the transmission into a limp type mode so that the vehicle is still driveable in this condition. It slams because if the TCU can't control the line pressure, i.e. if the line pressure solenoid isn't opening, then the line pressure will be at the maximum and that is going to result in very hard shifts.

Yes the TCU is putting it into limp mode and the shift into third is very hard.

Where can I order a solenoid? Line Pressure Duty. 4EAT, 2000 Outback.

Hello.  You can get any transmission solenoid from a Subaru dealer.  They'll probably want to know your VIN to make sure you get the right part.  Local dealers can really mark up the prices on walk-in sales.  So look around, there's lots of dealers that sell parts online at reasonable prices.  I order pretty often from  It also helps if you have the Subaru Mastercard since you can earn $100 certificates for roughly every $3000 you charge on the card.

That should be the phase II 4EAT.  The Subaru part# for the line pressure solenoid is 31939AA191.  They go for a bit over $100 as a good dealer price.  Also, don't neglect salvage yards.  Since some number of this transmission developed the slow to engage into drive issue, the solenoid is likely available in a number of salvage yards.  That part # is also the same part # as for the 2-4 brake solenoid for the Outback,  1999-2004.

i am having trouble finding the dropping resistor on my 2001 legacy B4 sedan.. any help with pics would be appreiciated..

Subaru line pressure dropping resistor locationHello Charitha and welcome to the site.  The normal location for the Subaru automatic transmission dropping resistor (to help with line pressure control) is near the right hand strut tower. There are a couple other items on a metal bracket at that location such as the PSS - pressure sources switching solenoid. The dropping resistor itself is behind this bracket, sometimes it is covered with a metal plate (probably to act as a heat sink) which can make it hard to find. Here's a photo of the typical location from my 1996 Legacy:

Please let me know if you have any other questions!

I have a Subaru B4 V reg with a trip tronic gearbox. When I brought it took a while to engage drive so i replaced the gearbox & remove engine. I've put it all back and it now pulls away fine, but jumps into 3rd gear. I've had it looked at and was told code 75 line pressure duty solenoid circuit, so i replaced all solenoids from old box, but there was no change. Can anyone help? Which gear do I put it in to check the oil level?

Hi Steven, welcome to the site. That sounds like a nice Subaru. All automatic trans Subaru's I've seen always have the fluid level checked on a level surface, in park, after shifting through all the gears.

It sounds like you might have a wiring or connector problem on the gearbox. If it is indicating pressure duty solenoid circuit, and you already replaced the solenoid with a known good unit, there could be a bent pin in the wiring harness or a pin that is not making good contact.  First and third gears are the popular 'limp mode' gears.

Hello, since mine is a phase II 4EAT I think it doesn't have a dropping resistor. Thanks for the reply.

Hello Claritha.  I believe the 4EAT Phase II does have a dropping resistor in the same location as the phase I.  I could double check on my 2000 Outback.  I'm pretty sure I've seen it in a metal shroud near the RH strut tower.

Hi, i have a 92 Subaru Legacy AWD. For a while the transmission would only shift from 1st to 3rd. Even if you put it in 2 it would stay in 1st. Also it would never engage the final gear when in D. Two days ago it started to jump in rpm's out of nowhere. Just trying to do 35mph was a struggle. It was like I was driving a manual transmission and put the clutch half way in while driving. The RPM's shoot up but you aren't going any faster. Any ideas what could cause this all of the sudden? I have checked my fluids already as well.

Hello Jason. The strange shifting you describe sounds like a possible transmission solenoid problem, possibly the 2-4 duty solenoid. The RPM's jumping is not good. It is possible the line pressure is not sufficient to hold the clutches from slipping. If you already checked the transmission oil level and it was good, then the slipping is a bad sign. A slipping automatic transmission will shred the clutch materials very quickly. Once that starts happening, the problem will only get worse, until eventually it will not even be able to move. How many miles are on the vehicle? The automatic transmission friction materials do degrade over time. Every time it shifts it loses a little bit of material in the clutches. Normally there is plenty of clutch material to last the lifetime of the vehicle, but once it wears out, the transmission must be replaced or rebuilt.

My power lite flashes 16 times at start. In a previous comment the drop out resistor was mentioned causing this to happen. Does a 94 Legacy have a dropping resistor and if so where is it located and what would it look like. Would this problem cause stalling at idle and a shuttering when turning?

Hello Bill. The dropping resistor is just one of many reasons that could cause your power light to flash 16 times at startup. That flashing indicates the TCU detected a problem on the last drive cycle. The '94 Legacy does have a dropping resistor. It should be in the same location as the pictures I posted above for the 1996 Legacy (right hand / passenger strut tower area). It may be enclosed in metal as a heat sink.

It is unlikely the dropping resistor would cause stalling or shuttering when turning. Stalling, if it is transmission related, could be the torque converter lockup duty solenoid. The stuttering when turning is most likely torque bind. The transfer clutch in the tail housing of the transmission can get sticky and get grooves worn into the drum so that it doesn't completely release, resulting in binding while turning. Or the 16 flashes could be indicating a faulty duty c solenoid, which would also result in binding when making turns.

There is a sequence to go through to get the light to flash out the actual codes to help troubleshoot the problem. The 16 flashes simply indicates it has encountered a problem.

My 94 Subaru Legacy had a problem with shuttering while turning that cleared up a few years ago by itself and now it's started to do it again. Assuming the 16 flashes might indicate a duty C solenoid issue. Would this likely be the duty C solenoid and if so where would it be located and what would it look like? Is there a way to test this?