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97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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groundzer0336
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Mar 18, 2013, 3:02 PM
Post #1 of 27
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97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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if you didnt know these cars have tons of issues with the combination meter/ instrument cluster and speedo not working. i first thought the vss was bad since it had a code but then learned about the common issues with the cluster. just tried to test the old vss and a new vss and its just getting a solid voltage and not on/off circuit like testing other hall effect sensors...so thats confusing me. also by looking at several wiring diagrams i see that the signal wire from the vss goes directly to the speedo and comes back out to the ecm. i figured i could splice the signal wire straight to the vss input wire at the ecm but nothing happened. basically i am trying to get enough ready codes to set so i can get this stupid car inspected and will deal with these issues after inspection when i actually have a plate on it and am driving it legally. any help will be greatly appreciated, im ready to roll this thing off a cliff.
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nickwarner
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Mar 18, 2013, 3:37 PM
Post #2 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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If you start splicing ECM reference wires on this car you are going to have a very very expensive issue on your hands, far worse than what you have. If you're using a multimeter to try to read the VSS signal it won't work. The pulses are too fast so the meter will just read an average all the time. You need an oscilloscope to be able to see the signal generated from this sensor and be able to determine if it is glitching.
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groundzer0336
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Mar 18, 2013, 3:43 PM
Post #3 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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i have the sensor out and am turning it really slowly so get the signal. i dont see any issue in splicing the wires since that is where it goes anyway, im just bypassing the speedo where the signal is possibly being lost, somewhere on the circuit board which goes bad on these. the sensor should go between a voltage and an open circuit and very slow revolutions.
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groundzer0336
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Mar 18, 2013, 3:56 PM
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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well im sure there is the possibility. hopefully i wont be one of those cases.
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Hammer Time
Ultimate Carjunky
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Mar 18, 2013, 5:03 PM
Post #6 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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I don't know where you're getting this nonsense from and your idea is ridiculous.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Mar 18, 2013, 5:08 PM)
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groundzer0336
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Mar 18, 2013, 5:16 PM
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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What nonsense and why is it rediculous?
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Hammer Time
Ultimate Carjunky
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Mar 18, 2013, 5:21 PM
Post #8 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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What's the code number?
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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groundzer0336
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Mar 18, 2013, 5:28 PM
Post #9 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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P0500
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Hammer Time
Ultimate Carjunky
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Mar 18, 2013, 5:29 PM
Post #10 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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Does the speedo work?
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
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Mar 18, 2013, 5:30 PM
Post #11 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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The VSS signal has to go through the combination meter because its signal from the VSS is conditioned inside the combination meter before going off to the ECM. Another words hooking the VSS directly to the ECM isn't going to work because the signal that is coming out of the VSS isn't what the ECM is looking for.
You really need a graphing multimeter or lab scope to see the signal at the combo meter. Don't know what it is supposed to look like but it is supposed have 4 pulse signals for every revolution of the speed sensor shaft. From the combo, the signal is going to change into a square wave.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Mar 18, 2013, 5:31 PM)
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groundzer0336
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Mar 18, 2013, 5:49 PM
Post #12 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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No the speedo doesnt work. Lke i said this is a very common prpblem in the sc. Thank you descretesignals i was thinking this but couldnt find any information but is the signal not already a square wave coming from a 3 wire vss? Im just trying to find a way to get my car to set fhe ready codes without replacing the cluster theyre really expensive and dont have that much money right now. Any suggestions?
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 5:53 PM
Post #13 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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I don't know what the signal looks like coming out of that VSS personally. What ever it looks like it isn't what the ECM is looking for, so the combo conditions the signal into some other type of digital signal. There could be differences in voltage amplitudes or maybe the signal format is different?
You could send the cluster out for repair. There are speedo shops that can repair those kind of problems.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Mar 18, 2013, 5:54 PM)
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groundzer0336
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Mar 18, 2013, 6:01 PM
Post #14 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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yeah ive seen a few places that will repair it for 2-300. theres one place that repairs 96 and below models for cheap but they dont do 97 and up at the moment but the owner is checking one of the clusters he has to see if he can repair. in theory though, if i had another cluster, not for this specific vehicle but maybe another lexus with the same type of speedo, i could possibly wire the vss through that speedo into the ecm. idk. this car is frustrating.
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 6:10 PM
Post #15 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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It's really not frustrating to fix something like that. Your making it frustrating because your trying to re-engineer it. 
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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groundzer0336
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Mar 18, 2013, 6:12 PM
Post #16 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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yeah i know, but im pretty broke at the moment and am just trying to make these ready codes complete without having the speedo working.
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 6:13 PM
Post #17 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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Ready codes? Are you wanting it to pass emissions?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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groundzer0336
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Mar 18, 2013, 6:15 PM
Post #18 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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i have to get a failed inspection paper so i can apply for a waiver. i have other issues im dealing with trying to get it to pass, but i can have 2 not ready codes present for the inspection. i got down to 3 before the speedo quit working completely.
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 6:28 PM
Post #19 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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Of course your monitors won't run if the speed signal isn't missing to the ECU. They won't run either if your setting misfire codes.
The EGR monitor is going to run, but soon as the test fails the light is going to come on and the catalyst monitor isn't going to run. It will probably halt the EVAP monitor too.
Your kind of up the creek unless you can get the P0500 to go away. Looks like you may need to save up some money. Swapping in another cluster from a different year and/or model might not work, so you willing to gamble with that?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Mar 18, 2013, 6:30 PM)
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groundzer0336
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Mar 18, 2013, 6:31 PM
Post #20 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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gotcha. guess ill start looking for a cluster or get mine repaired.
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Hammer Time
Ultimate Carjunky
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Mar 18, 2013, 6:33 PM
Post #21 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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If your speedometer isn't working either, there is a good chance that there is no VSS signal even reaching the cluster.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 6:38 PM
Post #22 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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That is a good possibility. That is why the signal to the combo has to be checked at the back of the combo using something like a lab scope or graphing multimeter.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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groundzer0336
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Mar 18, 2013, 6:42 PM
Post #23 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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it worked intermitantly when i got it and now has completely stopped. the odometer works sometimes now but will go off. apparently some of the parts on the circuit board of the speedo assembly fail and causes the speedo not to work. ill see if one of the guys at my work has a better multimeter i know they dont have a scope.
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Discretesignals
Ultimate Carjunky
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Mar 18, 2013, 6:50 PM
Post #24 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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You could use a volt meter but you have to back probe it into the VSS output circuit at the cluster.
There are three wires to the VSS.
Yellow should have 12 volts with key on
Red is ground back to the cluster, should be less than .03 volts.
Blue/Red is the signal out. Should be some type of digital signal when the vehicle is moving.
I looked at the diagram and the VSS is sending a voltage signal out to the combo. There is a transistor inside the combo that pulls a voltage low from the ECM. The ECM is watching the signal through a pull down circuit. Another words the ECM is sending some type of voltage to the combo and the combo pulls that voltage to ground to generate the special speed signal for the ECM. That explains why the VSS's signal can't go directly into the ECM.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Mar 18, 2013, 6:55 PM)
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groundzer0336
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Mar 18, 2013, 7:04 PM
Post #25 of 27
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Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm
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ahh i got it now. thanks. im trying to find a good place to get it repaired right now.
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