SD TPI Swap yeilds no speedometer! :-(
06-06-2003, 11:43 AM
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SD TPI Swap yeilds no speedometer! :-(
Sorry for the book, but I wanted to get it all in here...
Ok, I had a carbureted LG4 engine previously and upgraded to an L98 with a SD TPI Setup. Despite other posts I've seen saying that pre-89 didn't use electronic VSS, mine does. No speedo cable, just the two VSS wires running to the buffer. Ever since I did the swap, the speedometer stopped working. After looking through articles, I found out I had pulled my VSS buffer. My setup used a yellow external buffer, which I hooked back up. Still no speedometer.
I think I have it figured out, though...the 730 ecm uses an internal buffer, so the yellow box is not needed. The output pin B11 on the ECM should be wired into the speedometer and pin C1 to cruise control. If anyone who has done this before can verify this, please do. I am going to try anyway, but wanted to post the info here in case someone else has similar problems.
Thanks in advance! (The information below I do not personally know the validity of, just pulled it from searches on the subject.
--------------------Info below found in various other posts----------------------------
VSS Connector - A Yellow / B Purple (Although this polarity doesn't matter to the ECM as it is an A/C signal. The A/C is a changing voltage or current that is referenced to one side or the other.)
90-92 ECM
VSS 4000 PPM - Yellow Wire ECM Pin B10
VSS 2000 PPM - Purple Wire ECM Pin B9
The purple wire goes to ground inside the computer, while the yellow wire splits inside the ECM. One leg goes to VSS input and the other leg goes to 12 volts through a pull-up resistor.
The current flows from B9 (ground) through the VSS to B10 (positive). Once inside B10, the resistor shunts the current to keep from overloading the VSS input circuit. The VSS input is taped in front of the resistor between the VSS sensor and the resistor. The VSS sensor adds its modulated current to the current flowing from B9 and the ECM reads the pulses as the current flows into B10.
The ECM then sends signal to the speedometer (4000 PPM) and the cruise (2000 PPM)
Ok, I had a carbureted LG4 engine previously and upgraded to an L98 with a SD TPI Setup. Despite other posts I've seen saying that pre-89 didn't use electronic VSS, mine does. No speedo cable, just the two VSS wires running to the buffer. Ever since I did the swap, the speedometer stopped working. After looking through articles, I found out I had pulled my VSS buffer. My setup used a yellow external buffer, which I hooked back up. Still no speedometer.
I think I have it figured out, though...the 730 ecm uses an internal buffer, so the yellow box is not needed. The output pin B11 on the ECM should be wired into the speedometer and pin C1 to cruise control. If anyone who has done this before can verify this, please do. I am going to try anyway, but wanted to post the info here in case someone else has similar problems.
Thanks in advance! (The information below I do not personally know the validity of, just pulled it from searches on the subject.
--------------------Info below found in various other posts----------------------------
VSS Connector - A Yellow / B Purple (Although this polarity doesn't matter to the ECM as it is an A/C signal. The A/C is a changing voltage or current that is referenced to one side or the other.)
90-92 ECM
VSS 4000 PPM - Yellow Wire ECM Pin B10
VSS 2000 PPM - Purple Wire ECM Pin B9
The purple wire goes to ground inside the computer, while the yellow wire splits inside the ECM. One leg goes to VSS input and the other leg goes to 12 volts through a pull-up resistor.
The current flows from B9 (ground) through the VSS to B10 (positive). Once inside B10, the resistor shunts the current to keep from overloading the VSS input circuit. The VSS input is taped in front of the resistor between the VSS sensor and the resistor. The VSS sensor adds its modulated current to the current flowing from B9 and the ECM reads the pulses as the current flows into B10.
The ECM then sends signal to the speedometer (4000 PPM) and the cruise (2000 PPM)
06-06-2003, 10:22 PM
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For anyone's future reference that has this issue, my original post did fix my problem. I had a painless harness with no wire coming from where the speedo (B11) and cruise (C9) wires should be. Fortunately, I also had a VATS wire I didn't need. I pulled the pin (first pull the plastic clips/combs off of the connector, and then use a paperclip or thumb tack from the end of the connector that the computer's pins go into. I saw the thumb tack idea on this board, but didn't have one, so a small paperclip did the trick!)
I just popped the VATS (well, previously VATS...now speedo) wire into the B11 pin, and spliced the other end into the green/black wire from the old buffer connector, which feeds the speedometer. I can now see how fast I am going! (More importantly avoid tickets!
) I'm going to try a tech article on this with some pictures, as I had some issues such as figuring out exactly how to get the ECM harness pins pulled without breaking anything.
I just popped the VATS (well, previously VATS...now speedo) wire into the B11 pin, and spliced the other end into the green/black wire from the old buffer connector, which feeds the speedometer. I can now see how fast I am going! (More importantly avoid tickets!
) I'm going to try a tech article on this with some pictures, as I had some issues such as figuring out exactly how to get the ECM harness pins pulled without breaking anything. 
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