88 Toyota Transmission - Modulator?

toyota
camry
transmissions

#1

I have a 1988 Toyota Camry S/W, 2.0, 250k miles, automatic with electronics (electronic controls for overdrive and for power vs. economy settings). When accelerating the car will go right from 1st to 3rd - skipping 2nd gear. To allow for proper driving, I drive it as a “manual” auto - shifting from 2nd into Drive once I get up to the proper speed. Years ago I heard a Cartalk caller with the same problem and the answer was the transmission “Modulator”. Well, I can’t find any information about this part - is it external to the tranny or do I have to take everything apart to get to it? Thanks


#2

I am pretty sure you don’t have a vacuum modulator. I think on this year you only had the Throttle Valve (TV) cable. I think the TV cable just sets the line pressure in relation to the throttle position. On the electronic transmissions, the computer uses the engine sensors, like throttle position sensor, MAP sensor, ignition sensors, etc. to set the shift points. It is possible that the second clutch pack or its related one way roller clutch is worn out because manual second uses the back up clutch pack. You don’t indicate if the engine speed goes really high before the shift to third which would be indicative of a commanded second shift but mechanical in ability. Have a transmission technician scan the computer for codes and check the line pressure standing still and while the 1-2 transition is occuring. He might see something. Also he should check the 1-2 solenoid and valve to see if there is a problem there.

Let us know what is found.


#3

Look for a vacuum hose going from the engine to the trans. The connection on the trans will be the modulator. As researcher stated, you may not have one.
If you have a TV cable instead, make sure it moves freely.


#4

I have not had a chance to scan for codes, but as for the shifting, at the normal spot it would shift into 2nd, it goes right into third - the engine does NOT rev high or anything, it just losses power and acceleration once it shifts out of 1st and slowly gains it back once it gets into the normal range for third - just like in a manual if you would shift too early. And as for codes, as this is pre-OBD2 do I read the transmission codes the same way I read the engine codes or is there a seperate procedure for tranny codes? Finally, is the 1-2 soleniod external to the tranny or internal? Thanks


#5

I have not had experience with getting the codes from an OBD1 computer but they will most likely be stored in the engine computer since it uses the same sensors for the transmission. That early shift to third is puzzling it could be a stuck 2-3 spool or leaking solenoid valve i.e when the computer commands the 1-2 shift the 2-3 spool already or quickly blocks the oil flow to second clutch and feeds the high clutch. I don’t have access to the oil passage circuitry diagram so I am only guessing at the failure mode. This is where you might have to enlist the more knowledgeable help of the dealer or AT technician. The solenoids are mounted to the valve body inside the transmission pan. If you go in there you might be adventurous and remove the valve body and check the status to the 1-2 and 2-3 valve spools, try sliding them against their spring and see if they snap back correctly. At the same time you can do an air pressure check on the second clutch pack (the one that locks the second roller clutch) not the coast second lock clutch.

Again, when you learn the cause of this failure, share the information.

Curious