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Pin_Head said:
You said that the red circuit with the 68 ohm resistor used to drive the CC alone, so I suspect that the circuit impedance is too low when you add the second branch with another 68 ohm resistor. You could try upping the resistance values. 68 ohms seems like it would sink a lot of current for TTL level signals.
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I think it was a 68 ohm resistor can't remember for sure. I thought the same thing last night so thats why I reduced the resistance in my final attempt.

landcrshr said:
Ok, so I read up on the info provided here:


The simple Zener circuit is the solution for a switch-type speed sensor, but the Land Cruiser uses a Hall-effect sensor instead, needing the transistor circuit to condition the signal.

Your diagram of the combination meter does not show any pull-up resistor (to +12V), so I assume that function is done in the ECU's. The transistor's function is to act as the switch-to-ground like in the NON Hall-effect example.
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Very helpful thanks. I'm still trying to find out what the resistor value needs to be....
 
I don't know what type of sensor the Cruiser VSS uses, but the circuit needs to match its impedance requirements. In one of the circuits above, it uses a 100K resistor for a Hall effect sensor. Hall effect sensors need some kind of amplification to drive TTL level circuits.
 
Right. The transistor circuit IS the amplifier (which is part of the "conditioning").

And matching is critical, and that 100K base resistor seems way too big to fully turn on the transistor. Depending on the transistor used, that resistor should more likely be in the 1K to 5K range.
 
OK I built and bench tested the circuit.
IMG_0657_zpsssx7xthn.jpg


I'll have to test it on the rig tomorrow.
 
Can you trace out the circuitry related to the speedometer that's in the combination meter and create a diagram? Otherwise it seems like you're trying to work with incomplete information given in the electrical wiring diagram. Nothing in any of the circuits proposed above really does any waveform conditioning, which according to the 95-97 speedo diagnostics you included in post #7 converts the VSS waveform "into a more precise rectangular waveform by the waveform shaping circuit inside the combination meter." According to the waveform diagram on page 2 of the same PDF it's a pulsed waveform, but not a square wave. If true and you want to recreate the same shape waveform you're going to need a slightly more complicated circuit.

The D2033A transistor isn't anything special, although it's hard to believe it's driving anything close to 1.5A with just connections to the ECU, CC, and F/R diff lock ecu. But it's the output transistor for additional circuitry that needs to be diagrammed. Once you've determined that recreating it on your perf board should be straightforward.
 
I've tried to map the circuit out but cant.

There are two chips on the board that I can't find any pinout for. TB9213p 16 pin (8 per row) & TD6905s 9 pin single row. Both toshibas.

Also there are plenty of other components on the board too. I think some capacitors among others. I'll send a photo when I'm back home
 
The other thing to consider is that since I installed circuit #4 with the zener as a voltage regulator I haven't had the CEL 24 (speed sensor fault) code come up again.

Since the ecu compares the speed sensed from the vss with the secondary speed sensor and only alerts code 24 if there is a variance between the two I believe the the ecu is receiving a signal it can work with.

It's just not one that the CCU can use
 
Right, @TrickyT makes a good point, but you may still be able to create a signal that works, even if it's not perfect.

But your new circuit worries me since there is no resistor for the transistor's base. How did you bench test it?
 
landcrshr said:
...But your new circuit worries me since there is no resistor for the transistor's base. How did you bench test it?
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As @landcrshr points out, if you don't limit the base input current your transistor will be toast.


The other way to design the circuitry is to measure the waveforms on a functioning combination meter. The output waveform is easy to get at by probing the main ECU connector and the signal from the VSS goes through enough connectors that there should be a convenient point to tap in to. Then you can just design a circuit that converts inputwave waveform at some voltage level to output waveform at (maybe a different) voltage level.
 
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I used a fused 8v power supply and tapped the B input of the resistor with the E grounded, applied 5 volts to C.

Measured voltage across C and E while tapping 8 volts to E.

From what I can tell the vss goes through its own reduction via resistor so I figured 8 volts was safe.

When 8 volts was applied to E voltage at C dropped to 0, when voltage was removed from E C returned to 5 v
 
SmokingRocks said:
I used a fused 8v power supply and tapped the B input of the resistor with the E grounded
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Uh-oh

Without the resistor, you could damage the transistor with less than 1 volt.
 
Are we all on the same page here? I thought the D2033A transistor @SmokingRocks referenced is from his old combination meter. I.E. this one:
speedoTransistor.webp


Although this diagram may give the appearance that the VSS is directly connected to the base of this transistor, I'm sure that's not the case. There's additional circuitry beyond what is indicated in this simplified diagram. It's this circuitry that does the waveform conditioning and that limits the base current in this transistor.
 
TrickyT said:
Are we all on the same page here? I thought the D2033A transistor @SmokingRocks referenced is from his old combination meter. I.E. this one:
View attachment 1404009

Although this diagram may give the appearance that the VSS is directly connected to the base of this transistor, I'm sure that's not the case. There's additional circuitry beyond what is indicated in this simplified diagram. It's this circuitry that does the waveform conditioning and that limits the base current in this transistor.
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Sure, we're on the same page, and I agree on all points, but I am curious if the VSS (with its internal resistor) coupled to the transistor's base will yield a usable signal, even if not perfectly conditioned.
 
Of course it is a long-shot because that resistor would have to be just the right value.
 
I think your right @landcrshr I need the exact resistor.

Ok here is the back of the board.

Its hard to see but I'm pointing to the location that i took the transistor out or. It at the top center of the board. Black is E, Green is B and Violet is C.

Also I marked an arrow on the tach / odo driver so that you can reference the board when its flipped over.
IMG_0659_zpswbojamsm.jpg

IMG_0660%201_zpsmaki3tkm.jpg
 
Thanks for the pics. I think I see what's going, mostly, but it's kinda vague.

Can you identify the four screw-head electrical contacts on the board?
They should be: plus, minus, speed, & tach.
 
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