Tapping into TCU wires with a digital multimeter?

I'm starting to get more serious (and hardcore) about some tricks that I'd like to perform on the TCU... The full thread can be found here .

To keep things simple (so that you don't have to read that huge thread), I want to modify the way that the TCU reads RPM information, thusly making it believe that the engine has a different RPM value than in reality. Since the TCU reads RPMs as a variable voltage amount, and then transforms it into an RPM value, I just need to change the voltage that the TCU "sees" at one particular wire pin. This is very simple electronics... The voltage just needs to be lifted or dropped 7% - 8%

The ultimate goal is for the engine to be at redline (7000 RPM), but for the TCU to think that the engine is at 6500. This will allow higher revving (given that the ECU's rev limiter has been modified, via something such as the ECUtune Stage 1 chip), and should also keep the transmission in each gear a little longer, whether you're redlining or not.

Anywho, I need to find a way to tap into the TCU wires with a digital multimeter, and drive the car around. I need to see if the voltage varies from 0 - 12, from 12 - ground, or if it has some other voltage quantity. This cannot be done with the leads that are provided with most multimeters, as I can't physically hold them to the TCU wires while driving the car. An ideal setup would be for the positive lead to hookup to TCU wire, and the negative to hookup to ground... I just need some neat idea of having the multimeter's leads stay there, allowing me to drive. I am open to purchasing a new set of multimeter leads.

Obviously, I am also attempting to leave the TCU wires unharmed.

I'm going to try reading some of the service manual stuff, to see if this voltage amount is documented anywhere.

I know there are some electronics wiz guys on this network (you know who you are), so help me out please!

[kuoh]

A small pin and an alligator clip would do the trick if you're looking for something temporary. However, this somewhat loose connection could cause spikes in the signal causing the TCU to falsely register error codes or operate erratically. A more robust method would be to use inline wire splicers but they would damage the insulation slightly. Honestly though, I wouldn't worry so much about damaging the wire, as they can usually be easily repaired, especially if they aren't normally exposed to the elements.

However, your method of extracting those few extra hundred RPMs sounds more risky and less reliable than just installing an existing chip upgrade.

KuoH

Originally posted by deruvian
Obviously, I am also attempting to leave the TCU wires unharmed.

Originally posted by kuoh
A small pin and an alligator clip would do the trick if you're looking for something temporary. However, this somewhat loose connection could cause spikes in the signal causing the TCU to falsely register error codes or operate erratically. A more robust method would be to use inline wire splicers but they would damage the insulation slightly. Honestly though, I wouldn't worry so much about damaging the wire, as they can usually be easily repaired, especially if they aren't normally exposed to the elements.

However, your method of extracting those few extra hundred RPMs sounds more risky and less reliable than just installing an existing chip upgrade.

KuoH

I thought about alligator clips, and they do seem a bit flimsy. This loose connection might be slightly insufficient, but no erractic codes will be thrown. The signal will still be going to the TCU, it will just be going to my multimeter as well.

And as for the chip upgrade that you mentioned... There is none. There is currently no TCU upgrade for the SVX. Perhaps my explanation earlier was a little funky, but I want to do this in conjunction with something like the ECUtune Stage 1 chip (which "modifies" the ECU). That chip changes the ECU rev limiter to 7400, but the 4EAT TCU still forces upshifting at 6500 RPM. I want to change it to 7000.

I'm currently not looking for a permanent way to attach the multimeter to the wires. I just need to do it for 10 - 15 minutes in order to see which direction the voltage signal for the TCU varies, and by how much.

[kuoh]

The problem is that the loose connection can cause noise spikes, even though it's still going to the TCU. Just tune to any AM radio station and tap on the antenna with a paper clip, the pops are like the spikes the TCU will likely see if the connection is too loose. You could solder the pin to the tip of the alligator clip for a more reliable connection.

Good luck, but you may find the electronic circuitry more complex than anticipated. You will need to isolate the signal from the driving source to prevent feedback from damaging components upstream from the TCU and the circuit will need to be very tolerant of noise on the supply line as well as not introduce any of its own.

KuoH

Originally posted by deruvian


I thought about alligator clips, and they do seem a bit flimsy. This loose connection might be slightly insufficient, but no erractic codes will be thrown. The signal will still be going to the TCU, it will just be going to my multimeter as well.

[b3lha]

What about making some sort of "extension lead" (for want of a better word). Buy a male and female connector and wire them together. Use it to extend the existing cables.

Then you can solder your multimeter wires onto the extension piece without disturbing the original wiring.

Phil.

[svxistentialist]

Best of luck with it, it sounds like a neat idea.

But, as the doctor says, both ECU and TCU are constantly evaluating a lot of signals other than just this particular voltage you want to modify.

What will happen to the transmission maps and the fuel maps when the ECU sees discrepancies between the signal you are generating and say information from the injector timing map?

What will it default to? Throw a wobbly?



Joe

[Bwana]

Cant you just have a friend hold the leads while you drive? (would it work in neutral?)

p.s. I have no idea where the TCU is...

Originally posted by svxistentialist
Best of luck with it, it sounds like a neat idea.

But, as the doctor says, both ECU and TCU are constantly evaluating a lot of signals other than just this particular voltage you want to modify.

What will happen to the transmission maps and the fuel maps when the ECU sees discrepancies between the signal you are generating and say information from the injector timing map?

What will it default to? Throw a wobbly?



Joe

I realize that the TCU takes other factors into consideration, but I am assuming that the RPM value plays a large role in shift points.

The ECU, however, will not see any changes in the RPM value. The ECU feeds RPM value to the TCU. Modifying this connection between the ECU and TCU will not change anything that the ECU sees, regarding RPMs.

Originally posted by Bwana
Cant you just have a friend hold the leads while you drive? (would it work in neutral?)

p.s. I have no idea where the TCU is...

It might work in neutral. In that case, I could hold the leads to the TCU whilst pressing the accelerator with my hand.

The TCU is under the driver's side dash, right next to the ECU.

[oab_au]

I use the pin in the wire, to check signals. Sometimes you can poke the probe in the back of the plug. But if you are going to use it on the road, I would poke a pin through the wire,and use an alligator clip around the wire and pin, to hold it solid.

When you go to measure the signal, use a DC setting first, if it just stays the same, it is a pulsed signal. To read this signal, use a frequence setting to see the frequence change. If you don't have frequence setting, use AC.

As this signal is from the crank sensor, through the ECU, it is most likely a pulse that the TCU counts, to limit the maximun gear change. The box does not use this signal for normal changes, it uses road speed and throttle opening to change. As far as I know it is only used for the change when the gear lever is in any other posiion other than D.

If it is a pulsed signal, you may be able to turn it off, when you are changing gear with the lever.

Harvey.

[CigarJohnny]

Why not use those plastic wire-tap connectors they use to splice into an existing circuit? They make a small hole in the insulation of the TCU wires bit can easily be covered up with electrical tape when done. You can make a solid connection wo the leads that you splice into the lines with the taps. Hope I worded it all so it makes sense. Here is a picture to illustrate the tap...



Johnny

[Earthworm]

Only problem with those connectors is that they usually cut part of the wire as well.

Well, from Harvey's explanation, it sounds as though it may be a different signal than varying voltage. Bummer. That puts my idea into the can.

[rmjjensen]

If you want to simply change the tach rate that the TCU sees you can use a voltage divider circuit for the TCU's tach signal.

The voltage divider circuit consists of two resistors and will work on both AC and DC signals. You just need to use the divider circuit and use that as the reference tach signal into the TCU.

So - currently, the voltage @ 6500RPM will be at the maximum, but you'd like to change it so the maximum is actually 7000RPM.

6500/7000 = 0.92857 <- Ratio

So, our voltage divider circuit needs to have this ratio.

...Vin(R2/R1+R2) is a basic voltage divider.

so, R2/(R1+R2) = 0.92857

..........we want a high impedance to draw low current, so let's just make R2 = 10k ohms

Simplifying the equations yields R1 = 769.23

Bang .......get a 10K ohm resistor and a 770 Ohm and you have a perfect voltage divider circuit that WILL trick the rev limiter.

But - before you get into this you have to determine if the RPMS vs Voltage is a linear plot. If it is not, then this circuit is useless

Originally posted by rmjjensen
If you want to simply change the tach rate that the TCU sees you can use a voltage divider circuit for the TCU's tach signal.

The voltage divider circuit consists of two resistors and will work on both AC and DC signals. You just need to use the divider circuit and use that as the reference tach signal into the TCU.

So - currently, the voltage @ 6500RPM will be at the maximum, but you'd like to change it so the maximum is actually 7000RPM.

6500/7000 = 0.92857 <- Ratio

So, our voltage divider circuit needs to have this ratio.

...Vin(R2/R1+R2) is a basic voltage divider.

so, R2/(R1+R2) = 0.92857

..........we want a high impedance to draw low current, so let's just make R2 = 10k ohms

Simplifying the equations yields R1 = 769.23

Bang .......get a 10K ohm resistor and a 770 Ohm and you have a perfect voltage divider circuit that WILL trick the rev limiter.

But - before you get into this you have to determine if the RPMS vs Voltage is a linear plot. If it is not, then this circuit is useless

Thanks for the thorough explanation... That's my current plan.

Determining what the RPMs are vs. the voltage is the next step, and which is why I need to find a way to hookup my multimeter to the ECU>TCU RPM feed.