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

[buckallred]

Several years ago, installed the DIY4X competition dash with autometer gauges.

I didn't realize that in 1989 the vehicle speed sensor (VSS) was a small green plastic gizmo attached to the back of the OEM dash/speedometer unit. I tossed the VSS, and even if I had it, I don't think it's possible to make it work with an autometer speedometer.

Result: Everything looks great, vehicle runs fine, but the check engine light (CEL) pops on randomly. So far I have passed smog twice Just out of dumb luck. All that would take is for the CEL to pop on and that's an automatic fail as far as smog goes.

So I bought an aftermarket vehicle speed sensor (VSS) made for the 1989 Chevy 350 TBI system. It's a metal gizmo that attaches to the side of the transfer case and spins when the vehicle is moving, same as the speedo cable. Then the speedo cable attaches to it. The VSS has 2 wires that run up to behind the dash that essentially send a pulse to the engine control module/engine black box thing.

So I did all that. Problem is, it's not working. CEL still pops on.

The instruction page is attached. You'll note that it says to "connect the brown wire to one of the wires on your speed sensor. It does not matter which wire you connect to it (polarity is not important). Then it says to "connect the remaining wire on your speed sensor to an ECM ground wire."

See photos. I attached one of the VSS wires to a brown wire that I believe is correct. Although technically it is brown with a white stripe. So it may be totally wrong! It is one of 3 wires that had a plastic click type fitting on the end of them. I cut the fitting off. The other 2 wires in the plastic fitting are pink / black stripe and black / white stripe. As I'm sitting here writing this, it occurs to me that "brown with a white stripe" is totally different from "brown," and that's probably my error. I attached the second VSS wire to the black / white stripe wire that is located right next to the brown / white stripe wire, and was running to the same click type plastic fitting.

Ultimately, I need to find that brown wire. Then, I need to know what "an ECM ground wire" means. I can ground something to the vehicle's body, and a ground is a ground (No matter where located) as far as I know, but I don't know how to determine whether a given wire is "an ECM ground wire."

Any help is appreciated. Obviously I have no idea what I'm doing and trying to cram my hands around behind that dash is awful and I pretty much effing hate it. As much as I love the vehicle. And yes I hate California and I hate the smog system and I hate our governor and I hate everything about it all. Thanks!

 

[Wes Harden]

Are you sure that is the vss module wires you are working with? Look up the cruise control wire colors, and see if they match those, brown with white.

 

[dyeager535]

Don't ever go simply by wire color. I see two of the same color in just the pages you posted. You have the wiring diagram, it tells you which terminal on the ECM and opposite end connector that wire goes to, verify the LOCATION of the wire at the connector before cutting anything.

A10 is the wire that feeds the ECM the speed signal from the VSS. I don't know what they mean regarding ECM ground, the ground for VSS wasn't switched from the factory or anything, chassis ground should be fine, that's how the stock VSS was run IIRC.

 

[buckallred]

Thank you Wes & dyeager.

On further reflection, I'm sure that wire I connected to is incorrect.

Further dumb questions:

1) How do I identify "A10" at the ECM? Are the terminal things labeled in an obvious way? I'm going to have to remember where I stowed the ECM when I installed the aftermarket dash. It's stowed back there somewhere.

2) "Chassis ground." As far as I know, any wire connected to bare metal on the body is a ground. I really have only done this in the context of 12 volt stuff like switches. I think that should be fine for this VSS ground. Is that a good enough ground? Is there such thing as a "good" or "bad" ground? As I understand it, a wire that's connected to bare metal without any rust or paint is a good ground, and a bad ground would be something that's not securely connected or has rust or other material interfering with metal to metal contact. Do I have that right?

Thanks!

 

[dyeager535]

Thank you Wes & dyeager.

On further reflection, I'm sure that wire I connected to is incorrect.

Further dumb questions:

1) How do I identify "A10" at the ECM? Are the terminal things labeled in an obvious way? I'm going to have to remember where I stowed the ECM when I installed the aftermarket dash. It's stowed back there somewhere.

2) "Chassis ground." As far as I know, any wire connected to bare metal on the body is a ground. I really have only done this in the context of 12 volt stuff like switches. I think that should be fine for this VSS ground. Is that a good enough ground? Is there such thing as a "good" or "bad" ground? As I understand it, a wire that's connected to bare metal without any rust or paint is a good ground, and a bad ground would be something that's not securely connected or has rust or other material interfering with metal to metal contact. Do I have that right?

Thanks!

All connector bodies are labeled with a number or letter that corresponds to the cavity (hole) each wire is placed in. They are usually very small, but they are there. Been awhile since I messed with the ECM, but I believe in that case, A will be the first connector body (there are two) and 10 will be the terminal that wire is in. I believe the "A" will be cast into the connector body somewhere too.

Electronics can apparently get a bit weird depending on how the ground is arranged, so I don't think it's 100% accurate to say a ground is a ground. Not super good with electronics, but in the case of VSS, take a look at the factory diagram...where does the black wire go? It probably goes directly to the chassis for ground, in which case nothing to worry about, just ground that wire and go.

 

[buckallred]

OK, I took the dash apart, well sort of, trying to figure out what's what.

I believe I located the ECM, which is attached to the underside of the dash on the passenger side. I definitely don't want to try to get it loose and I don't know if that's even possible without fully disassembling the dash. I was able to identify the outside (facing toward the passenger side) of the ECM) based on the color/sequence of the wires plugged into it (based on looking at the diagrams I have). Per the diagram, those (facing the passenger side) are the OUTPUT of the ECM. I need the INPUT. Is the INPUT on the opposite end of the ECM unit/box?

There is only 1 ECM, correct?

Trying to find/see/locate anything behind that dash is an effing nightmare . .. sorry for the continued idiotic questions.

 

[Wes Harden]

iirc there are 2 connectors both are he same side of the ecm.

 

[buckallred]

The schematic appears to show two different ECMs. Or is it two ends of the same thing? I do not understand it. The vehicle speed sensor is controlled by circuit number 437, brown wire.

The top diagram (ECU thing, identified as item number 12047946) has only 12 rows. You can see where the brown wire (437, speed sensor) runs into circuit number 437 at row 10.

The bottom diagram, identified as item number 12045575, looks very different because it has 16 rows in total. The ECM that I'm barely able to see under the passenger side of my dash appears to be the bottom diagram, because it has orange and blue wires entering it at row 16.

Does my vehicle have two separate ECM objects? Or one ECM and then a separate box that controls certain functions? I note that the top box (12047946) controls the vehicle speed sensor as well as the spark retard control/485, Park neutral switch/534, serial data/461, assembly line dag./451.

Thanks. I'm trying to avoid removing the dash because messing with the air conditioning plumbing behind it is awful. Really all I need to do is figure out which wire was running to the vehicle speed sensor which was attached to the speedometer. Seems to me I should be able to figure that out and hook it up instead of disassembling the dash and trying to locate or fiddle with the ECM box or boxes.

 

[K85 Octane]

Where is your check engine light with the new dash/gauges set up? Seems like there’s a much easier solution to this CEL coming on.

 

[dyeager535]

Just one ECM, two connectors, side by side. There is one 10mm screw that holds the ECM in it's holder via a metal strap over it. I don't have AC, so mine is easy to see with the glovebox out of the way.

Does A10 go through another connector before it gets to the VSS? On my setup it does but mine is a retrofit from a car, so could be different. That connector is easier to access.

I don't disagree, if your wiring is stock and in the stock locations, it should be pretty easy to find the unconnected wire that was used to feed VSS to the ECM.

And no, removing the CEL doesn't fix the potential stalling on deceleration you can have from a non-working VSS, nor will it pass a visual of they check that the CEL works properly initially.

 

[buckallred]

@K85 Octane CEL is located in center of new dash. It is required to exist & function correctly in order to pass smog.

@dyeager535 the vehicle runs & idles fine, but the CEL pops on randomly from having no signal from the VSS. My choice is to look again to see if I can locate/identify another connector that A1 passes thru before connecting to the ECU . . . Or to start over at the driver's side to find that A10 wire. Ideally would be best to find it at the ECU, but it is extremely hard to find/see much of anything without fully removing the dash . .. thanks!

 

[dyeager535]

They generally run and drive ok without VSS. The only condition Ive ever heard anyone having a VSS problem is decelerating without throttle.

I've never seen any data showing there is any other downside to it not working. Of course, the random CEL lol.

The wiring diagram should show a break in A10 is there is another connector.

 

[Wes Harden]

In your picture the top connector right row of terminals is the "A" row, left is "B".
The low connector is "C"and"D".
Number 1 cavity is at the top and 12 at bottom of the upper connection.
That picture is the connection unplugged and you looking at the pin side.
Highly recommend removing the ecm from the mounting location, so you can manipulate the connectors.
Use a good quality digital volt ohm meter.
There is possibly a body control module (BCM) in your truck, but has nothing to do with vss.

 

[buckallred]

Jesus God in heaven, don't let me jinx this, but I might have gotten it. TL/DR, I connected the aftermarket VSS to the ECM by locating and cutting wires very close to the ECM in the passenger footwell / dash area

As mentioned above, the 2 diagrams of the ECM (above) indicate 2 sections of cavities. One large plug of wires goes into each cavity/receptacle. The one cavity I needed, A10 for circuit 437 (vehicle speed sensor), is located in the cavity section / plug receptacle located toward the forward of the vehicle (and the plug plugs into the outside of the ECM, meaning the passenger side of the vehicle).

So while I'm laying down in the passenger side footwell looking up at it, I could see the larger cavity section / plug receptacle but could not see the smaller cavity section
/ plug receptacle which is located toward the forward of the vehicle and therefore is blocked from vision by the air conditioner fan box etc.

I was able to locate (by feel and sticking my cell phone camera up in the area) the bank of wires that plug into the smaller cavity section / plug receptacle. Based on the diagram, I was pretty sure I had found the brown wire corresponding to the vehicle speed sensor (cavity A10/circuit 437). I therefore cut that wire and spliced in the signal from the aftermarket VSS. The other wire from the aftermarket VSS I spliced into wire 450, identified as "system return." I thought that sounded like a ground.

After that, the vehicle ran fine but the Check Engine Light (CEL) was totally dead and did not even illuminate when the vehicle was started. So that was bad.

I then re-connected wire 450 ("system return") wire and connected the other wire from the aftermarket VSS to what I believe is a ground. It's a bolt that goes through the body of the vehicle and holds the speedometer cable on the underside. I know that this was an awesome time to learn that my automotive test light was non-functional... **** me!!

At any rate, after reconnecting wire 450 and connecting the other aftermarket VSS wire to a ground, the CEL illuminated at startup as it supposed to, and then during a quick test drive the CEL did not pop on. I did not have time to do an extensive test drive, but I'm cautiously, cautiously, very ****ing cautiously optimistic that leaving wire 450 reconnected and simply connecting one wire from the aftermarket VSS to the brown wire/cavity A10/circuit 437 and the other wire from the aftermarket VSS to any ground on the vehicle is the correct solution.

Thank you all for your help and your patience with my idiotic questions. Surprised to note that no matter how hard I looked at the wires behind the speedometer area, I could not find that brown wire for cavity A10/circuit 437. I don't remember what in the world my buddy & I did with it when I installed that dash and autometer gauges. It should be sitting back there, capped off or just unused but I could not find it despite looking for a very, very long time.

So Californians, don't mess with the dash. Don't mess with the speedometer. Whatever you do, don't mess with the vehicle speed sensor!

 

[Wes Harden]

I should have remembered earlier, if you remove the dash pad( not Hard ) and the glove box you get a great view of the ecm and wiring.

Anyway finger crossed you got it licked

 

[buckallred]

I should have remembered earlier, if you remove the dash pad( not Hard ) and the glove box you get a great view of the ecm and wiring.

Anyway finger crossed you got it licked

The DIY4X aftermarket dash changes all of that. Another thing I wish I hadn't messed with...

 

[dyeager535]

There is a factory ground location on the driver's side, behind the dash, probably right about where the AC vent is. It's below the A-pillar, but mine had a bunch of empty spots to lock a connector on to, so that was handy.

You got it though, congratulations. I know AC is really nice at times (even necessary depending where you live), but man does it make getting at stuff under dash difficult.

 

[onetonbb74]

I just left the stock 89 speedometer guage gutted under the dash, face plate removed and bezel gone. The speedometer cable is screwed in with the stock wiring hooked up. I have no idea what my speed is, but the ecm shows a speed reading (tech 2) and I can pass smog if I needed one. If i need to see the vehicles mileage, I have to look under the dash, which i've never needed to do. My autometer speedo guage isn't hooked up. I use a GPS speed app now.

I am thinking of trying this so my autometer Guage would finally work in the dash, granted I would need a new cable as the 1989 speedometer ends are not the same as autometer.

 

[buckallred]

I just left the stock 89 speedometer guage gutted under the dash, face plate removed and bezel gone. The speedometer cable is screwed in with the stock wiring hooked up. I have no idea what my speed is, but the ecm shows a speed reading (tech 2) and I can pass smog if I needed one. If i need to see the vehicles mileage, I have to look under the dash, which i've never needed to do. My autometer speedo guage isn't hooked up. I use a GPS speed app now.

I am thinking of trying this so my autometer Guage would finally work in the dash, granted I would need a new cable as the 1989 speedometer ends are not the same as autometer.

here's the gizmo to adapt a normal (click-type) GM speedo cable to the Autometer (screw-in) speedo fitting:

https://www.amazon.com/dp/B00BE5U1KE?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[Blue85]

Result: Everything looks great, vehicle runs fine, but the check engine light (CEL) pops on randomly.

So how does torque converter lockup work without a VSS? You can overheat a 4L60 running in 4th gear without lockup.

my automotive test light was non-functional... **** me!!

You shouldn't use a test light one ECM/electronic signals. Use a voltmeter/DMM - they have high impedance.