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OP-amp help

HomerDodd

Sep 6, 2017
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I am new to this forum and really need some guidance. I am fairly good with electrical, but some electronics stump me.
My problem is that I need to install a small op-amp into my car, '95 Cadillac Seville. The transmission has an "Input speed sensor" that sends an AC voltage
signal to the PCM for monitorning, etc. The output voltage from sensor must never go below 5VAC, or the "Check Engine" light will come on.
This is my problem, since the car is due for a smog check in a few weeks and will automatically fail with the "Check Engine" light on. The car
otherwise runs and shifts properly. I just cannot take it to the smog station until the "Check Engine" light turns off. The major problem is that the sensor is a "$20 part
buried $2,000 deep in labor in the transmission", otherwise I would replace it. Some guys say the sensor itslef is fine, and the age of the signal wire
has caused high resistance, lowering the voltage. Maybe a bypass wire?
The guys on the Cadillac forum stated that a few of them were able to "fool" the computer into seeing 6,7,8 VAC signal by using the simple LM741 op amp cirucit.
I have asked a number of them for help, but I have no responses. I have researched the amp on Google, and it seems easy enough to build on my garage
workbench. I just do not know how to install it. It looks like it has a "voltage in" and "voltage out" with some grounds. Do I just insert in inline on the output
signal wire, so only 2 or 3 volts goes into the amp and much more than the minimum 5 goes out, and do I have to ground the circuit board, etc?
Please, and assistance will be a great help. I really like the car, runs and looks great, I just have to pass smog !! THANKS !
 

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CDRIVE

Hauling 10' pipe on a Trek Shift3
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It would seem unlikely that the sensor is buried within the transmission. It's more likely that it screws into the transmission outer casing. Can you give us a part number and or photo of it?

You should be able to test it with a DMM set to the 10VAC scale.

Chris
 

HomerDodd

Sep 6, 2017
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It would seem unlikely that the sensor is buried within the transmission. It's more likely that it screws into the transmission outer casing. Can you give us a part number and or photo of it?

You should be able to test it with a DMM set to the 10VAC scale.

Chris


The VSS (vehicle speed sensor) is on the outside and easily accessible. The ISS is most definitely on the inside of the trans case, which requires removal. I don't have a part number, only the GM trouble code, P056 (Input Speed Sensor) fault.
 

kellys_eye

Jun 25, 2010
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Have you used an ODB2 code reader to discover what the 'check engine' light is actually on for? The reader can be bought very cheaply these days and can also be used to reset the check engine light - this needs to be done after any fault has occurred anyway and the light may not be on for the reason you think it is.

Low sensor output could be caused by the pickup not being properly 'distanced' from the sprocket - volt drop in the cable is not really a consideration given the low current levels in use.
 

CDRIVE

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Buying from Horror Freight (ODB2)is a crap shoot. The up side is that their return/refund policy will be satisfied with a simple ... "It's a piece of crap!" at the register.

Chris
 

HomerDodd

Sep 6, 2017
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Gentlemen, I have done all of the troubleshooting. I have retreived the code (P056), cleared it multiple times and it has came back on. This is a very common Cadillac problem. Some guys tear into the trans and replace the sensor and harness, some ignore it since it cause no running problem, some bypass it with the op amp. I have to get the light off to clear smog. That is what I'm trying to do here. Please assist me with the op amp construction and installation.
 

CDRIVE

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The problem with this topic is it's treading on very thin ice. Don't be surprised if the Mods (officially) step in here. The odd thing about it is what the ISS has to do with smog? Doesn't your State use instruments to measure your emissions?

Chris
 

BobK

Jan 5, 2010
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The pollution control stuff is probably using the vehicle speed in its calculations. Or, maybe it is used to detect the smog test and change the operation of the engine, ala Volkswagen?

Bob
 

kellys_eye

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I don't think there's anything wrong with what the OP wants to do. A simple comparator circuit would probably doComparator.jpg

Vin would be the signal from your 'sensor' and the two 10k resistors could be replaced with a potentiometer to 'set' the trigger level.
 

CDRIVE

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Bob, thanks for the enlightenment.

Chris
 

mathphobe

Sep 3, 2017
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Check engine light or led display?? If it's a light can you not just pop out the bulb ready for the test and pop it back in once it's done?..
 

BobK

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I don't think there's anything wrong with what the OP wants to do. A simple comparator circuit would probably
The OP said it needed to send a 6 to 8V AC signal. (probably means square wave, which I am guessing the frequency indicates the vehicle speed). He just wants to amplify the <5V signal he is currently getting.

Bob
 

kellys_eye

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Yes Bob, and the comparator would work like a buffer with a variable sensitivity input depending on the comparison voltage level set. The output would be close to the supply - as good as the comparator can manage wich will be no worse than a volt or so under the supply voltage - but I suspect it wouldn't need to be much higher to get it to work.

I'm not sure if the original sensor output is differential though - if referenced to chassis (ground) it won't be a problem using the comparator circuit shown - otherwise some slight modification may be necessary.

If, as I suspect, the signal is a squarewave as derived from a hall effect pickup on a toothed shaft then the grounding of one side of the signal has to be determined to prevent short circuits.
 

HomerDodd

Sep 6, 2017
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I don't think there's anything wrong with what the OP wants to do. A simple comparator circuit would probably doView attachment 36051

Vin would be the signal from your 'sensor' and the two 10k resistors could be replaced with a potentiometer to 'set' the trigger level.


Thanks Kellys eye. Now I am getting some idea's.........Hey how about this one below? I have cut output signal wire and inserted the very basic Op amp inline. Thinks this might do the job? Remember, I only need to boost the output signal from 1 or 2 VAC to over 5 VAC, which would then meet the parameter. Just wondering also about the op amps ground circuit, and if it is OK to ground it to the vehicles ground? Thanks !
 

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HomerDodd

Sep 6, 2017
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Check engine light or led display?? If it's a light can you not just pop out the bulb ready for the test and pop it back in once it's done?..


No, I wish it was this simple. The "Check Engine" light has to pass a functional test during the smog test. The smog tech turns on the ignition, verifies it works, then checks for any codes. That is what would fail me. If I could simply boost the output voltage from the the Input Speed Sensor to 5, 6, 7 or more VAC, it will serve the purpose and turn the light off, clear the code and allow me to pass smog. THANKS !!!
 

HomerDodd

Sep 6, 2017
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The OP said it needed to send a 6 to 8V AC signal. (probably means square wave, which I am guessing the frequency indicates the vehicle speed). He just wants to amplify the <5V signal he is currently getting.

Bob

Exactly right. The Cadillac guys actually recommended it in an earlier post, and I have been unable to reach them for the help. One guy said he built the op amp from all Radio Shack materials, less than $30 and it got him passing smog and no "Check Engine" light. I am simply not certain how to rig it into my signal output line; I have an idea, but not positive. Thanks.
 

kellys_eye

Jun 25, 2010
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Some 'problems' with your schematic - first the 'input signal' to the shaft speed sensor. You have to determine what this is and I suspect it will be a simple DC supply voltage (either 12V or 5V).

The signal is not 'AC' - it is a simple squarewave with a reference voltage of zero (ground) - but that's just semantics....:p

Your circuit doesn't show the supply rails to the op-amp and, on the basis of the design as-is would require a 'split rail' power source i.e. +12V - 0V and -12V. The usual method of powering an op-amp for this purpose is a single rail supply (i.e. +12V and 0V) would requires you to bias the input to half the supply rail.

The link (scroll down a bit) shows how to develop the circuit and bias the op-amp accordingly:

http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

The op-amp method, whilst workable, introduces some (minor) complications that are unnecessary which is why I suggested using a comparator. The following link explains the purpose of a comparator and shows the implementation.

http://www.radio-electronics.com/info/circuits/opamp_comparator/op_amp_comparator.php

I realise you want a solution that is just a 'follow the build instructions' but those articles are easy to read and understand and having that little extra knowledge makes the chances of your building such a circuit that much more successful - after all, KNOWING what you're doing is the key to understanding, especially if/when things go wrong.
 

BobK

Jan 5, 2010
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Hmm. The diagram posted by the the thread starter indicates a coil and magnet are used for the pickup, which means the signal might really be AC. It also explains why the signal might degrade. A Hall effect signal would not likely degrade to a lower output voltage, but if clearances changed, the coil and magnet would.


Bob
 

kellys_eye

Jun 25, 2010
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The diagram posted by the the thread starter indicates a coil and magnet are used for the pickup, which means the signal might really be AC.
That diagram also shows the terminals labelled as 'S' and 'V' which I took to be 'signal' and 'volts' which corresponds to how a hall effect device may be utilised.

If the OP measures 'V' to be a fixed DC voltage and 'S' to vary in frequency (many multimeters these days will read frequency directly) then we could clarify the matter.
 

BobK

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Agree. Not enough info yet.

Your comparator idea might work in any case if it is basically the frequency that matters, which seems likely.

Bob
 
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