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SN95 Cobras
Oil Pressure gauge reads BACKWARDS???
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<blockquote data-quote="mwolson" data-source="post: 10106824" data-attributes="member: 16006"><p>My new 3MHz function generator arrived today, so I was able to do some more testing of both the speedometer and tachometer.</p><p></p><p>The speedometer requires a sine wave of at least 7.5V peak-to-peak in order to drive it over the entire range of the speedometer. It can detect the VSS signal down to less than 0.5V p-p under 50 MPH, but the voltage must go up at higher speeds or the speedometer will malfunction. The speedometer expects the AC signal to start slowly and have continuously increasing, decreasing or steady frequencies. If you hit it suddenly with a high speed signal, it will not work. If you stop the signal suddenly, the needle will drop back to the proper side of the pin if it is in the first 180 half of the needle rotation, or it will go all the way to the back of the pin if you remove the signal with the needle in the second 180 degrees of needle travel. To get the needle back under control, you need to start out with a slow speed signal, then increase it to where the needle will be more than 180 degrees from the pin. At that point, the needle will go to the correct speed, and, if you sweep the signal back down, the needle will go back to the proper side of the gauge.</p><p></p><p>The bottom line, the speedometer expects a gradual increase and decrease of the VSS signal, with higher voltage at higher speeds. If it does not see such a smoothly changing signal, the speedometer may behave strangely.</p><p></p><p>The tachometer signal is a pure digital signal. I was able to drive the tach to any RPM with no problem. If I took the signal away at any point, it would drop to zero as it should. I was unable to duplicate your problem on my bench.</p><p></p><p>But at least I now have the ability to drive both the tachometer and speedometer with complete control. It should make debugging a lot easier.</p></blockquote><p></p>
[QUOTE="mwolson, post: 10106824, member: 16006"] My new 3MHz function generator arrived today, so I was able to do some more testing of both the speedometer and tachometer. The speedometer requires a sine wave of at least 7.5V peak-to-peak in order to drive it over the entire range of the speedometer. It can detect the VSS signal down to less than 0.5V p-p under 50 MPH, but the voltage must go up at higher speeds or the speedometer will malfunction. The speedometer expects the AC signal to start slowly and have continuously increasing, decreasing or steady frequencies. If you hit it suddenly with a high speed signal, it will not work. If you stop the signal suddenly, the needle will drop back to the proper side of the pin if it is in the first 180 half of the needle rotation, or it will go all the way to the back of the pin if you remove the signal with the needle in the second 180 degrees of needle travel. To get the needle back under control, you need to start out with a slow speed signal, then increase it to where the needle will be more than 180 degrees from the pin. At that point, the needle will go to the correct speed, and, if you sweep the signal back down, the needle will go back to the proper side of the gauge. The bottom line, the speedometer expects a gradual increase and decrease of the VSS signal, with higher voltage at higher speeds. If it does not see such a smoothly changing signal, the speedometer may behave strangely. The tachometer signal is a pure digital signal. I was able to drive the tach to any RPM with no problem. If I took the signal away at any point, it would drop to zero as it should. I was unable to duplicate your problem on my bench. But at least I now have the ability to drive both the tachometer and speedometer with complete control. It should make debugging a lot easier. [/QUOTE]
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Oil Pressure gauge reads BACKWARDS???
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