Aftermarket veh. speed sensor misery (CA smog/check engine light). Help? 89 K5.

[buckallred]

 

[Wes Harden]

The ecm uses information from Coolant temp, park/neutral, TPS, and Distributor for EGR control so vss is not used.

Can we see the smog test results page ? I am interested in the rpm the 2 test are being run. I also assume this test is on the dyno and not the 2 speed stationary test.

 

[dyeager535]

You can also check any EGR pages,but from what I saw on posts on CK5 and the one below, originally out of the JTR book I believe, VSS does help with EGR control.

VSS Circut

VSS Circut

forum.73-87chevytrucks.com

 

[buckallred]

Here are the 3 smog test results.

 

[buckallred]

@dyeager535 That is very interesting reading, and my current problems make me agree with topp's argument. My truck has no VSS input, and therefore it's running lean / hot with high nox, AND the EGR is probably not being activated, so it's not functioning.

So back to that @#$ aftermarket VSS. I'm just about positive that I hooked it up correctly to the correct wire. As stated, I've grounded it three times, all tested with a test light. A bad ECU sounds so unlikely, but... I mean that could be it.

 

[dyeager535]

Thread has been going for awhile, so perhaps you already mentioned and I forget, did you contact the TPI Parts people and talk to them?

Those add-on type VSS boxes are pretty commonly used (well, or were, now that so few care about TBI/TPI), this SHOULD be pretty straightforward.

The '87-89 TBI truck ECM's (1227747 until 1989, not the same as cars) use the same VSS output/input for Cruise and ECM, which is 2000PPM from the VSS buffer, stock.

At its most basic, the ECM needs to see 2000PPM on pin A10. That will kill the code if the ECM is getting any compatible signal and the ECM is ok. Have you confirmed the VSS device is outputting a signal? Can you confirm the VSS "box" input is being turned by the speedometer gear?

 

[Wes Harden]

If you look at 4/29 test, truck passed at 25mph, test shows 2100+ rpm the other 2 failed at 25mph rpm was 1500ish. I think the gearing and tires is messing with egr function. See if tech will run the test in 2nd gear.

Does the speedo work ? I am not sold on vss effecting egr control.

 

[Wes Harden]

Quite the argument there. I do believe the egr should only open when traveling at a speed greater than 10mph. Does the computer use the vss for that strategy IDK, GM doesn't show that it does.

The 1 poster in that thread Topp did post some bogus info to back his argument. The throttle kicker plunger on carburetors was not implemented to prevent stalling when coming to a stop, it prevents high manifold vacuum when throttle plates close from drawing fuel out the carb circuits creating high HC emissions. HC emissions is catalytic converter killer.
Carburetors worked just fine for 70+ years w/o soft close the plunger. My truck had one, I kept it till last year, now I use a step motor that raises my idle rpm when ac is on. So now the throttle closes just like any 50's or 60's carb, no stalling. With out the ac in drive my truck idles at @580 rpm.

 

[buckallred]

Thread has been going for awhile, so perhaps you already mentioned and I forget, did you contact the TPI Parts people and talk to them?

Those add-on type VSS boxes are pretty commonly used (well, or were, now that so few care about TBI/TPI), this SHOULD be pretty straightforward.

The '87-89 TBI truck ECM's (1227747 until 1989, not the same as cars) use the same VSS output/input for Cruise and ECM, which is 2000PPM from the VSS buffer, stock.

At its most basic, the ECM needs to see 2000PPM on pin A10. That will kill the code if the ECM is getting any compatible signal and the ECM is ok. Have you confirmed the VSS device is outputting a signal? Can you confirm the VSS "box" input is being turned by the speedometer gear?

TPI parts: Yes, dude was helpful, but only so much he can do on the phone. At the transfer case, a "speedo gear reduction" box plugs in, THEN the "TBI box" plugs into that, then the speedo cable runs to the speedo. The speedo works, MPH is generally accurate.

I have not confirmed the VSS device is outputting a signal. I am an idiot, first of all, but I understand that would require a test meter of some sort hooked up to the output wire from the VSS device. . . and while the wheels are turning, the test meter should show an alternating (on/off) pulse-type signal. Do I have that right? And do you know what type of (hopefully cheap) test meter I would need?

Stupidly, I have not unhooked the battery (to clear out the ECM/error codes). Doubt that will change anything, but I will try.

Also: if the thing would pass, I would gladly pay the $300 for a new cat converter. Any idea if that would help? THANKS for all of the help (you too @Wes Harden ).

 

[Wes Harden]

For some reason I had in my head this was an electronic speedometer.

 

[dyeager535]

So did I. But it's an aftermarket gauge, so that's why no stock VSS buffer.

I would expect you'd need an oscilloscope to see what the output of that sensor is. I have no idea what else may exist that would let you "see" it's output.

If you had winALDL or Tunerpro and a cable you'd be able to see that the ECM is getting vehicle speed or not. But that would just confirm what the CEL is already telling you, so minimal help IMO.

At this point I'm fairly confident you have it hooked up properly (two wires after all, and if the vendor verifies your thoughts on wiring...), I doubt it's a wiring issue, it seems to me that for whatever reason the signal isn't getting to the ECM. Or the ECM isn't processing the signal.

 

[Blue85]

Let me explain to you why this doesn't work. We have a couple of threads on the topic, but I don't see them breaking down the root of your problem.

https://ck5.com/forums/threads/generating-a-vss-signal.329462/
https://ck5.com/forums/threads/i-ha...-come-with-both-did-they.346338/#post-4260592

Your new 2-wire sensor is very likely a VR (Variable Reluctance) sensor, which is more or less just an electromagnet. As it passes teeth on a reluctor wheel somewhere (in this case the little adapter box), the magnetic field rises and falls, creating a voltage in the sensor. If you get the polarity backwards it may still work, but they are designed to give a sharp rising or falling edge in one direction vs the other. If you wanted to test it, you could set your DMM to AC Volts. Then turn the driveshaft by hand. If your meter has frequency mode and you can have it hooked up while you drive, that can let you verify the PPM.

However, the ECM is not designed to read the zero crossings of this AC signal. It's looking for a square wave transitioning between fixed "high" and "low" levels. The VR voltage varies a lot with speed, so it's possible that occasionally you get a speed reading, but it's never going to be right all or even most of the time.

NOTE: I am not 100% on this, but: the OEM VSS had a third wire for power. The aftermarket has NO POWER SOURCE, just 2 wires (hot and ground).

The 3-wire plug in the wiring diagram doesn't go to the transfer case, it plugs into the little box on the back of the speedometer.



https://www.ebay.com/itm/185867429865?chn=ps&_trkparms=ispr=1&amdata=enc:1vrcQUKACQJW4lIPjc1qZ3A34&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=185867429865&targetid=4580153136941814&device=c&mktype=&googleloc=&poi=&campaignid=437225722&mkgroupid=1224856224320848&rlsatarget=pla-4580153136941814&abcId=9300906&merchantid=51291&msclkid=22b92b12669a18fcb88f4f499c5d4b33

That uses the 12V power because it is an active light sensor, creating pulses as the magnet in the speedometer spins (at the speed of the cable). If you still had a stock speedo, you could just plug this in and BAM! VSS.

So instead, you'll need an adapter to process your VR into a hall-effect or open-collector type of signal. I'm not sure if this is the right unit for a TBI PCM, but it's the general idea: https://sirhclabs.com/product/cortex-ebc-speed-sensor-adapter/ Hopefully somebody has experience measuring this signal and can help.

1. Aftermarket speed sensor is attached per the above. I grounded it (all confirmed w/test light) to the body, to the battery, and now to the engine block. The check engine list still pops on and OBD1 system gives "no signal from speed sensor" issue. I tested it with the paperclip trick.

You don't ground a VR sensor because it's an AC signal. You'll want twisted pair wire going straight to the adapter box.

 

[Blue85]

What's confusing is it says the adapter is for 85-89 TPI or 88-92TBI and specifically says "square wave". However, it's designed for cars (it says "transmission" and TPI didn't come in trucks), which had already transitioned to the 2-wire sensors. The trucks didn't make that transition until 90 or 91. https://www.rockauto.com/en/moreinfo.php?pk=10319652&cc=1035672&pt=10634&jsn=1727

It looks like TPI parts has stopped selling the SEN-820, but if you find a cached version of the web page, it includes:

Note - this sensor will not work on applications that use a VSS buffer box. If you will be using a VSS buffer box, contact us so we can assist you in identifying what VSS you need.

It's possible using the wayback machine or Google you could find what part number could/should work and swap it out.

 

[buckallred]

What's confusing is it says the adapter is for 85-89 TPI or 88-92TBI and specifically says "square wave". However, it's designed for cars (it says "transmission" and TPI didn't come in trucks), which had already transitioned to the 2-wire sensors. The trucks didn't make that transition until 90 or 91. https://www.rockauto.com/en/moreinfo.php?pk=10319652&cc=1035672&pt=10634&jsn=1727

It looks like TPI parts has stopped selling the SEN-820, but if you find a cached version of the web page, it includes:

It's possible using the wayback machine or Google you could find what part number could/should work and swap it out.

Jesus . . . at this point, I would be thrilled to toss the TPI parts thing in the trash and (gladly) buy a different plug-in unit that would actually work. . . does such a thing exist?

 

[buckallred]

Really hoping one of these is the correct solution. The one I have (or identical to it) is listed:

Speed Sensors

Why You Might Want One. Without a Vehicle Speed Sensor (VSS) and the reluctor ring that feeds it signals, many engines including the Chevrolet TPI fuel-injected engines we specialize in will not run correctly and will not pass a smog test.Background on Our VSS Products. JTR Stealth Conversions...

jagsthatrun.com

 

[buckallred]

**** me, I think this might be it. . . .

OEM GM VSS (Vehicle Speed Sensor) for 1989 4x4 trucks

Stock GM Speed Sensor, GM # 15547452, New OEM. Used with Pigtail, aftermarket replacement for GM part 12085498, sold separately here. Part Number: GM-Sensor-Old-StylePrice: $44.81 (Each)Material: MetallicWeight: 3ozMade in USA

jagsthatrun.com

 

[Blue85]

This adapter would probably work, but it's out of stock. https://sirhclabs.com/resources/cor...peed-sensor-adapter-v2-instructions-2-0-0.pdf.

Maybe there are others. I don't see how another 2-wire sensor is going to help.

I wonder if you could swap to the ECM from 1991. I don't know if there are other differences, but they should be set up to read the 2-wire sensor. I suppose with a Dakota dash going back to stock speedo isn't an option. Why not call Dakota? It looks like they used to have some related products. https://www.dakotadigital.com/pdf/sgi-2.pdf

 

[buckallred]

dude (I called him) said this will work. It looks similar to what I have. . .

Speed Sensor 2PRS

Two-pulse square wave speed sensor. The two-pulse square wave unit for all passenger car TBI installations, all computer controlled carbureted engines, and 1985–1989 TPI engines. 7/8–18 thread, .104" square drive. Part Number: 2PRSPrice: 120$(Each)Material: Aluminum & BrassWeight: 9ozMade in USA

jagsthatrun.com

 

[Wes Harden]

It may work for the speedo, but Dakota isn't really concerned your ecm is happy.

 

[buckallred]

It may work for the speedo, but Dakota isn't really concerned your ecm is happy.

The dude (from that website, not Dakota) definitely said it would keep the ECM happy . . . that said, I have no idea how, if at all, it differs from the VSS I already have . . .