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GM E38 Engine ECU speedo output signal. Any ideas?

EFI Wiring Fundamentals

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Hi all, First a little bit of back story, so please bear with me! I have a 2005 Holden Rodeo dual cab ute with a GM LS2 6.0lt engine from a 2006 Holden SS Thunder ute installed. I have had it running and driving for about 2 years and am finally getting to sorting out speedo and few other "gunna do" details.

I have speed signal to the engine ECU from transmission OK and I can see this working in the HP tuners scanner. The issue I am having is sourcing a usable signal from the engine ECU to power the speedometer in the stock Rodeo dash. I have found some info through HP Tuners forum suggesting pin 57 on X1/J1 connector on ECU; this pin also pops up in several other info sources on several other different GM vehicles. I have also changed to speedo output to "vehserial" in the speedo settings as suggested by HP Tuners. With engine running and wheels raised running at 20km/h I just get battery voltage (13.8volts) at this pin 57 this does not change with variations in wheel speed, which would suggest to me it is not the speed output. It sounds hard to believe with the many LS conversions done around the world, but I have struggled to get clear info on engine control wiring for this model ute as the VZ SS Thunder was a cross between the VZ and VE model utes, hence I have found that they use bits and pieces from both vehicles. If you have read this far - Thankyou - and hopefully someone can steer me in the right direction.



Cant help with the ECU output. But perhaps you could look at a non ECU derived speed source. You can buy VSS converters that take an ABS wheel speed signal and turn it into a VSS output. And im pretty sure you can GPS VSS aswell.

Yes, that is a possibility that I have considered, and it is looking like it may be simpler to maybe pick up the VSS from the transmission sensor and use a converter box to adjust the signal for an accurate input at my speedometer. My thought was that if the VSS is already at the ECU, and I can find a usable output from the ECU, that I may as well use that. I have done a lot of research online and have not found anything definite with regards to the ECU VSS output, so I figure there must be either a lot of LS conversions running around with no operational speedo or they have used a VSS source such as the ones you have suggested. It is also on my to-do list to investigate getting the cruise control operational too, but I believe that may be a little tricky with the DBW throttle as the BCM in a Commodore manages cruise and the Rodeo cruise is a standalone system.



Have you tried paralel wiring the instrument to the VSS.

On Subarus (pre08), the VSS signal goes to both the ECU and the instrument cluster. Not from VSS>ECU>Cluster.

I Haven't tried that yet, but will try that and see what sort of signal I get at the speedo and, if necessary, tune it with an adapter box to get it accurate.

Tried connecting speedo directly to VSS output without success. Both the VSS wires have approx. 2.5 volts when checked with a multimeter with engine running and wheel raised and rotating. This value does not change when wheel speed is varied which does not make sense to me as I can see the speed vary in the HP Tuners scanner. I would have expected one of these wires to have 5 (or 12) volts and the other to have a varying signal as wheel speed changes.

Have you got access oscilloscope David? You're not going to be able to get much useful information out of a digital multimeter in this situation unfortunately :-/.

Thanks Zac, this is something that has entered my mind as far as getting a better understanding of the signal I am trying to use. I don't have access to an oscilloscope, but it would be a useful addition to my arsenal and am happy to purchase one. Do you have any suggestions on what features I would need to be useful in instances like this and/or other automotive electrical diagnosis and possibly a brand which you might recommend. Bearing in mind that it is not something I will probably use a lot, so a cost effective option would be great. As a long shot, I do have HP Tuners; is there some way I can utilize the HP tuners scanner to visualize/measure the speed signal from transmission or the ECU speedo output signal (I suspect its not that simple). Thanks in advance for your advice.

Sorted out the speedo issue!! Used the speed signal output from the ECU, pin 57, which is great as it means I can adjust the speedo accuracy if needed. Connected directly to the stock Rodeo instrument cluster speed input.

As it turns out, with the current settings, it reads about 3 to 4 km/h too fast (compared to GPS) which is fine.

Hope this info is useful to others.

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