Need Help from Electrical Gurus

[SmokingRocks]

OK So as many of you know I replaced my gauge cluster with a fully custom speedhut build. In doing so I ditched the old speedometer, which seems to be one of the only gauges in the combination meter that actually has an output signal needed by the cruise control computer, Trans control unit and ECU.

So I read the diagram and added a resistor between the speed sensor and the output of the combination meter (2 C in the photo below). Now my cruise control works great, and my transmission shifts fine but I still get a code 42 (vehicle speed sensor) from the ECU. The output of the combination meter (2 C) feeds all of the control modules so since the Cruise control is working I know there is a signal in the line.

I'm wondering if I'm getting a code because the output 13 B isn't hooked up to anything. But I can't tell if thats a diode or a transister.

Can anyone explain to me what the combination meter is doing here?

I'd really like to get rid of the code. (Also I'm not sure what the ECU does with the vehicle speed input, timing?)

 

[SmokingRocks]

Here is the combination meter

 

[Pin_Head]

It is a symbol for a magnetic sensor.

 

[SmokingRocks]

yes it is a transistor, I'm just trying to figure out what its output voltage needs to be. From what I'm seeing I need max 5 volts on the signal. I think I didn't need to add that other resistor after the speed sensor. I'll get my scope on the line and see what voltage I'm getting out of the VSS.

 

[SmokingRocks]

Just want to make sure that jives with the input voltage of the ECU

 

[SmokingRocks]

So this is the best I've found on a mini truck forum in regards to the 22RE 94 pick up. It sounds like the exact same setup as mine.

So how do I 'condition' the pulse wave? Getting it down to 5 volts isn't an issue

 

[SmokingRocks]

OK I think I've figured it out.
The 93-94 speedometer puts out a 4-6 volt signal to the ECU, Cruise CU & TCM. Where as the 95-97 puts out 9-14 volts. So I'm glad I found the Diagnostic EWD for the OBD1 ECU before I found the OBD2 version. I've attached both below.

So the VSS puts out 12 volt pulses to the speedometer, the speedometer 'conditions' the signal and puts it out as a 5 volt signal to the Control Units.

I will stop by radio shack on the way home tonight and pick up a prototype board, 2KOhm Resistor and a 5v Zener Diode, this should suffice for my signal conditioner.

This page helped me out

Specifically this section below is very helpful.

Also this was a great help
Zener Diode as Voltage Regulator Tutorial

 

[SmokingRocks]

So this works, no more code 42:

I haven't installed the Red leg yet but I know that works since it was working before.

Also note that the OBD2 ECU is looking for an input signal between 9-14 volts so this isn't nessecary.

 

[SmokingRocks]

OK so I need some more help.

The cruise control is not working, the light comes on but it does not set. I know its because its not seeing the VSS signal even though I've got it wired up.

Things I've tried:

1. I took the signal from the VSS to the resistor I pulled off the speedo circuit board (can't remember the band colors?!?) and the cruise worked. But the ECU TCM were getting a full 12 volt signal not 5v, so they didn't see it.
2. VSS --> Resistor --> Zener 5.1v Diode --> ECU, TCM, CCU (Cruise still inop)

In this picture you can see what I've tried, green highlights means that they are working Red means they are not.

Can anyone provide a photo of the big blue resistor on the back of the speedometer circuit board????

 

[SmokingRocks]

I made number 4 last night, I thought there might be feedback from the zener diode, so I added a diode upstream of the resistor in that line to block any feedback. So that means I've got a clean VSS signal (0-12v alternating sinewave) for the CCU... So I guess I just have my resistor wrong?

Pictures for documentation.
Version 3.5

 

[landcrshr]

I really haven't gone through all the information you provided, but it appears you still need to create a true signal conditioner, something that will "square-up" the pulses from the VSS. The circuit you have is merely a voltage clamper.

You should try the circuit with the transistor. This one:

 

[SmokingRocks]

I'll give it a shot thanks

 

[SmokingRocks]

@landcrshr So out of curiosity why would the cruise control work with the first circuit?

 

[landcrshr]

@landcrshr So out of curiosity why would the cruise control work with the first circuit?

The problem with the simple circuit you have is that there is nothing to drive the signal POSITIVE. Which is OK if the CC & ECU's inputs are equipped internally to "pull" the signal up, but perhaps they are not, and that's why they need the VSS signal to be conditioned. It may have been OK with just the CC connected, but when you added the ECU it quickly loaded down the signal. (My theory)

 

[landcrshr]

Cool project BTW

 

[SmokingRocks]

Thanks that make sense. The transistor takes a sine wave input and cuts (for lack of better words) the tails so that its a square wave? is that about right?

I've learned a lot about electronics over this last week! I feel like I could do anything!

 

[landcrshr]

The transistor takes a sine wave input and cuts (for lack of better words) the tails so that its a square wave? is that about right?

Pretty much. It's what this particular transistor CIRCUIT is doing. The connection to +12V is the crucial part.

 

[SmokingRocks]

Ok so if I break back into this I guess I should just do it exactly the way the FSM shows it right?

like this:

I can reuse the transistor that is on the board my only issue is that I don't have that resistor anymore. I took it off the board and used it in the first set up and then one of the leads broke off and it got canned. I'm kicking myself now since I didn't snap a photo of it.

But whats interesting about the FSM diagram here is that there is no zener or 12v on the back end.

 

[Pin_Head]

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.

 

[landcrshr]

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

OK I think I've figured it out.
The 93-94 speedometer puts out a 4-6 volt signal to the ECU, Cruise CU & TCM. Where as the 95-97 puts out 9-14 volts. So I'm glad I found the Diagnostic EWD for the OBD1 ECU before I found the OBD2 version. I've attached both below.

So the VSS puts out 12 volt pulses to the speedometer, the speedometer 'conditions' the signal and puts it out as a 5 volt signal to the Control Units.

I will stop by radio shack on the way home tonight and pick up a prototype board, 2KOhm Resistor and a 5v Zener Diode, this should suffice for my signal conditioner.

This page helped me out

Specifically this section below is very helpful.

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.