Adding resistor to tranny temp sensor

[Trevor]

Hi Garret,

I see you missed my post by 2 mins. We must have been chasing each other for space! Only hope what I posted will assist. If it is a true thermocouple you will be provided with some interesting maths. and in any event you must take into account that bright old bugger Mr. Ohm and his law.

In my opinion this mod. is not on, even though I would dearly love to do it myself.

Cheers, Trevor.

[svx_commuter]

Originally posted by Beav


So does mine, I'd like to change that aspect and think of a way to enable TCC lock-up in third gear without resorting to a manual switch.

The service manual say lock-up occurs in 2nd and 3rd. I asked awhile ago and no one has ever seen lock-up in any gear but 4th.

[svx_commuter]

Originally posted by gcookaustin
AKA dropping resistor?

The tranny temperature sensor is attached to the bolts that hold the control valve assy to the bottom of the tranny. The tranny pan has to come off to get to it.

NOW get this...... I think it was in 95 that Subaru changed the location of the temperature sensor. I found out about this when I was asking about a replacement control valve assy.($500)

[Trevor]

I have checked this out further and as I suspected auto. electronics do not use thermocouples and use thermistors for temp. sensing.

The thermistor is connected in serious with a fixed resistor within the control unit so as to form a voltage divider. As the temp. rises the thermistors resistance in effect decreases and the output from the voltage divider drops providing the required proportional control signal. This provides a robust and not too critical arrangement. In order to make the desired adjustment the resistor within the control unit would have to be increased.

An external resistor in series with the sensor as has been suggested would have the opposite of the desired effect. A parallel resistor would give a false indication that there was constant temperature with no indication of relative zero.

[svx_commuter]

Originally posted by Trevor
The thermistor is connected in serious with a fixed resistor within the control unit so as to form a voltage divider.

Sometime a series connection can be very serious in deed But I really do think you meant a very strong electrical connection of the most enduring type

Anyway...... I measured 360ohms at 176F and 2,320 ohms at 68F. The SM states 280-360 ohms at 176F. This across pins b10 and a20 on the connector at the TCU. (SM pg 189 section 3-2)

The AFT temperature sensor is disussed on pg 75 of the SM section 3-2. It also shows the location of the ts inside the pan.....

Fig 35 shows the drop in resistance as the temperature goes up. It is not a linear curve and looks more like a power function. From the resistance I measured above....... Can you figure out what the resistance would be at 186F, 196, 206, 216, 226, 236F? For an example.

I am concerned about how this system gets calibrated. Down around 140F there would be a very big change in ohms as the temperature changes. This makes sense because everyone says the tranny TC locks up right around there. 145F

BUTwhat about up at the high end? It seems to me that with the expected range of 280-360 ohms at 176F would translate into quite a variation in temperature readings at say 230F because the curve has less slope.

What do you think?

Also, there is no harm in connecting a ohm meter accross these connectors? Right?

Also please tell me why hooking up a resistor in parallel across the leads would be bad. I am think I would like to drop the resistance from 360 to 280ohm at 176F. Then the TCU would
think the fluid was hotter.

Just trying to find out when that ATF light comes on and also to see if the tranny runs cooler.

Originally posted by Boone
Hmm.. Mebbe that's the same temp sensor that tells the TCU not to shift into 4th until it reaches a decent temp?

Hmm... Keeping this in mind?

[svx_commuter]

Originally posted by Boone

Hmm... Keeping this in mind?

It have a very small effect because the curve is steeper at that temperature.

I thought it would be the other way around, i.e. higher temperature would be higher resistance?

But anyway yeah (R1+R2)/(R1*R2) for a parallel setup is what we could use to determine what resistance to use. We need another data point somewhere, and I think that the curve would be a natural log.

[Trevor]

Hi SVX commuter,

Yes a very serious typo to be sure but lucky there was not a series of them ! Or is there refer your disussed? But who cares we are getting through I am sure.

We are starting to get into deep water here and typing for me is laborious and hard work. I do not have the data you have which I gather comes from a workshop manual describing trouble shooting.

As I see it a parallel resistor will upset the output curve which as you say will not be linear. If only one set point was involved this could be acceptable but it appears that the sensor is utilized for more than one measurement at different temps.

I agree that the non linear response does provide accuracy problems as the temp. increases. There is no way to calculate the temp. / resistance curve but this would be available from the manufacturer of the thermistor if one knew its origins and type.

The crux of the matter is that if the sensor provides signals for multi functions altering the calibration is simply not on as I see it.

There is no harm in connecting an ohm meter across the sensor. The only danger could be in respect of the meter not the sensor if the circuit is not isolated from the supply voltage.

If you wanted to play with a parallel resistor or even a preset potentiometer I canÕt see that you would damage the sensing components. However you would have to carefully monitor the effects on all else effected by the changed signal.

Hi Garrett,

A thermistor increases in conductivity i.e. has a lower resistance as the temp. rises but this is not linear. Furthermore resistors connected in parallel is not as simple as (R1- R2). However (R1+R2) does apply to resistors in series. When one is variable and the other fixed in a parallel circuit of the sort we are considering here the matter becomes rather involved.

The correct formula for two resistors in parallel is-- R = r1xr2 divided by r1+r2. For a number of resistors in parallel-- R = 1 divided by 1/r1+1/r2+ 1/r3+1/r4 etc. ( Rather hard to type correctly )

If you guys want to play go to it . I certainly do not wish to fumble with this key board any more.

Best of luck, Trevor.

[Landshark]

are all these equations just to get the tranny temp warning light to come on sooner? probably easier to just put a gauge in. i would be very paranoid messing with any transmission-related circuitry. BTW, i saw a cool gauge in the Summit catalog - had a programmable output. you could trigger a fan on your tranny cooler to turn on at whatever temp you want. pretty cool!

Alan

[svx_commuter]

Originally posted by Trevor
Hi SVX commuter,
and typing for me is laborious and hard work. .

As it is for myself also. Seems as though I type a short post in about an hour but it only appears to be 10 minutes