anesthes

iTrader: (12)

Ok.. I'm trying to understand something.

The "Magnetic" VSS on a firebird for example, I'm told is a "4K PPM AC generator".

So, let's look at some math.

edit: I chose 60MPH because the revolutions per MINUTE and per MILE will be the same. I.e, below 2,893 revolutions per minute will also travel 1 mile.

At 60MPH, with 3.73 gears, 26" tire, calls for a drive/driven ratio of 7 and 21.

In the 1:1 gear that happens to be 2893 RPM. We apply the drive/driven ratio and that puts us at 964 RPM on the actual speedo.

Now the speedo has 4 poles in it, so I'm assuming 4 pulses per REV, so 964 * 4 = 3856 pulses per mile.


-- Joe

Tuned Performance

iTrader: (92)

tbi and tpi maf use a 2k ppm vss to a vss buffer or cable drive to a optical sensor on the speedometer. On models without the cable drive tpi maf and tbi the 2k signal goes to the buffer out 4k optical to the speedometer and 2k opt to the ecm. The 90-92 tpi and 3.1 uses the 730 ecm as a buffer with a 4k ppm input. I just build a simulator for vss and rpm for my test bench using a Phase-Locked Loop and TTL 74LS04 inverter. you can get a dakota digital sgi-5 to change the ppm output.

anesthes

iTrader: (12)

Ahh. Ok. so my math is correct, the MAG VSS for the TBI/MAF cars are 2K, not 4K. I bet where people are confused is it's 4 pole.

I have a SGI-5 to change the PPM output for gear ratio changes.

What I'm trying to do is see if I can use an inverter to convert the AC voltage out of the VSS to 0-5v DC. Now I don't mean 0-5v square wave pulse, I mean straight voltage.

So for example, 5 volts might be 180mph, where 2.5 volts is 90mph. (.027 volts per mph).

If the VSS generates increasing voltage a simple inverter and a pot can be used to convert to where I need to be. Hence my question of what the VSS does when on a scope.

-- Joe

Tuned Performance

iTrader: (92)

I get 33.90 dc hz at 60 mph

anesthes

iTrader: (12)

I thought it was an AC generator??

-- Joe

Tuned Performance

iTrader: (92)

sorry, wish I had a scope im not sure what it does. I did use my fluke 78 and on dc hz it rises as speed in increased up to 255mph at 200.00 hz

Tuned Performance

iTrader: (92)

Your right ac sorry, I get the moron of the day award

anesthes

iTrader: (12)

Ok. You've helped greatly.

So it looks like,

15mph 8 hz
30 mph 16 hz
60 mph 32 hz
120 mph 64 hz
240 mph 128 hz
255 mph 136 hz (I think you went faster than 255mph, but exceeded an 8 bit integer)

So I'll have to use a frequency to voltage converter.

I'm glad the math works out.

Thanks again!

-- Joe

Tuned Performance

iTrader: (92)

I think your correct, I was going faster than 255mph my laptop literally flew off the table.

anesthes

iTrader: (12)

I'll probably end up making a small conversion circuit, like guys do that want to run an LSx (frequency based) MAF on a 0-5v AD (i.e, maf pin).

-- Joe

EagleMark

1.The two-pulse (2000 pulses per mile) square wave (D.C. current or direct current) used on all TBI engines through 1992, all computer-controlled-carbureted engines, and on 1985–1989 TPI engines.

2.A four-pulse (4000 pulses per mile) sine-wave (A.C. current or alternating current) signal is required by the 1990–1993 TPI, 1992–1993 LT1 engines, and 1990–1993 Camaro 3.1/3.4 V6 engines.

Then there are differant buffers, VSSB/DRAC.

http://www.jagsthatrun.com/V8-chapte...ed-Sensors.pdf

anesthes

iTrader: (12)

Are you sure the VSS itself on a pre '90 TPI is DC and not AC sign wave?

I know the output of the VSSB/DRAC is DC square. It looks like the VSSB/DRAC converts the ac frequency into a DC pulse with a pulsed transistor.

I don't know that I trust that link. It says the early TPI and TBI cars are 2 pulse units, then shows a 4out buffer that it says is for 4 pulse. All the early TPI and TBI cars used that buffer.

The math seems to shoe that a 'four pulse' sensor is in fact 2000 pulses per mile. If you calculate the RPM of the speedo and multiply it by 4 poles you arrive at 2,000 pulses per mile.

-- Joe

EagleMark

Sure? No! That is all cut and paste info from Jaqs that run.

The key is the buffer/VSSB/DRAC converts VSS whatever in to signal needed out. Buffers must be in newer PCM like LT1 and OBDII stuff, no VSSB/DRAC and paremeters for pulse change. 95 LT1 Manual is 2, well 1.91 in bin. Auto is always 40.

VSSB/DRAC has to match VSS, if they don't they let the magic smoke out and never work again... but if they match then they can be adjusted by jumpers for output.

Tuned Performance

iTrader: (92)

hey mark, lol love your description

anesthes

iTrader: (12)

As far as I can tell though (unless you know otherwise), the buffer/vssb/drac all output a square wave. None output variable voltage (like a MAP/TPS sensor).

-- Joe

EagleMark

They put that magic smoke everywhere! If you let it out and don't notice it's a bitch to diagnose...

But seriously this is when I found the VSS output is differant, not just a 2k, 4k, ? But AC to DC... then realized the buffer/VSSB/DRAC are differant as well.

? I've gotten way better at matching parts from the start and making them work then understanding the electronic characteristics. Just thought the info would help smarter guys like you find what they are looking for. Or at least like I did and realize all VSS are not the same and it's not just the pulse count.

RBob

iTrader: (1)

All of the 3rd Gen f-body magnetic VSS's are 4-pole. They provide 4K PPM, A/C sine wave. The actual voltage varies from as a low a 6 volts to as high as 100+ volts.

The TPI Firebirds went to magnetic VSS's before the Camaros did, sometime in '86 or '87.

The 4-out buffer used in the early TPI and all of the TBI cars provided digital pulse streams of 2K PPM and 4K PPM. Note that you will need a pull up resistor to get an actual pulse stream from them. They interfaced to a 4K PPM magnetic VSS.

The '90-'92 TPI & MPFI f-bodys have the ECM doing the VSS buffering. Providing that same functionality.

The 3rd gen f-bodys with cable driven speedometers had a 2K PPM optical pickup on the back of the speedometer head. It provided two 2K PPM streams. One for the ECM and the other for the cruise control (when required).

Can easily use a LM2917 freq-to-voltage convertor to get a linear voltage output. Did that on a '78 El Camino I used to have. Drove a small panel mount LCD voltmeter to get a correct MPH reading. The stock speedometer needle shook like a tree in the wind and was wayyyy off.

RBob.

RBob

iTrader: (1)

Engine RPM will be 2893. Which will be the same at the output shaft with a locked TCC. Had a few cars long ago with rear ratios and stick 4-speeds such as that. No fun on the highway.

RBob.

anesthes

iTrader: (12)

Yeah, that seems to be the general idea. I've got a few different designs I downloaded. Most guys omit the 4out buffer and pick the signal up directly off the VSS, and others are utilizing wheel sensors for traction-control purposes.

-- Joe

anesthes

iTrader: (12)

Ugg.. I miscalculated my tire size. doh!

So why the heck did tunesperformanc get 33hz at 60mph?

60 mph = 3856 pulses per minute, so 3856 / 60 = 64.4 pulses per second (64hz).

(unless he was looking at the OUTPUT of the 4out buffer?

-- Joe

RBob

iTrader: (1)

Engine RPM will be 2893, divide by 60 = 48.2166 RPS

48.2166 RPS divided by 3 (21 / 7) = 16.072 rev-per-second of the VSS

16.072 RPS * 4 (poles) = 64.288 Hz @ 60 MPH from the VSS.

The 33 Hz is for a 2K PPM input. The actual PPM is 2002 and 4004, which is why some calculations don't always line up exactly.

RBob.

Tuned Performance

iTrader: (92)

A old thread , But I had someone inquire on the vss rpm generator schematics.
Although not labeled the test bench lights indicate ECM power, injector banks, mil, iac a & b , fuel pump power.

tedpants

This is great info. I've replaced my 87 L98 ecm with EBL P4 Flash ecm. Does the optical VSS pickup in the instrument cluster output the correct signal for the new ecm? I'm not getting a speedometer reading in the WUD so my tune is missing torque converter lockup, egr, and this is the only Service Engine code(24 VSS).

Tuned Performance

iTrader: (92)

The brown wire that went to your A10 gets repined to the optical input pin C6 of the 730. Optical flag needs to be set. I’m sure if different from $8d hopefully rbob will let you know.

RBob

iTrader: (1)

The C6 input is the correct one. Then clear the MagVs option flag:

Option Word 3 - Bit 4 - MagVs

And set the PPM to 2K:

VSS - Pulse Per Mile

RBob.

tedpants

Perfect. My optical pickup at the speedometer cable in the gauge cluster appears to be working. I'll do some more VE Learn sessions but it hasn't hiccupped yet and the idle has already smoothed some. I'll be back to paint prep in no time! Many kudos and thanks!