What is this?

[tucsonjwt]

I am in the process of reattaching everything under the dash and came across this. A yellow plastic box marked AC and a three wire harness coming out of it going to what appears to be a small light (?). Does anyone know what it is and where it attaches? I have an 83 C20. Thanks

[sweetk30]

look like the optical sensor for the back of the speedo for VSS input for cruise / or other functions .

if it has three wires ( red / ? / green ) on it i see in the back ground thats what i guess it is .

and it looks a little screwed up also like its been ground up . . . or could be the angle of the pic .

[SunSoaked]

Red/black/green wires. As k30 said, it's the optical sensor that attaches to the back of the speedo in guage clusters. Mine drove the cruise control, which I recently removed when I put the Vintage Air system in.

[tucsonjwt]

Thanks for the information. I see that the part number on the yellow module is AC 25007227 and may still be available as NOS, and it is called "sensor buffer." I do have cruise control and it works only intermittently, but I don't know if this module is ok or not. My sensor has no apparent physical damage, unless there is supposed to be something attached to the end of the black part, which looks like a small light or two?

Where does this actually connect to the back of the speedometer in the gauge cluster? Is the little light (?) in the black part supposed to attach to the speedometer at the instrument cluster somewhere? If so, where does it attach? The yellow module is just supported with a zip tie, and the black part is hanging loose under the dash. I did not notice this sensor when I removed the instrument cluster, and I did not notice anything on the speedometer gauge or speedometer cable that looked like it was missing anything.

[tucsonjwt]

The wires going to the yellow module appear to be black, pink, and yellow. The wires going to the black piece appear to be green, red and blue (or black?) The black piece has a different part number (25007166 2402 - made in Mexico.)

[SunSoaked]

If I recall, all you will see is a hole (may not be round) that those lights see thru. That plug will be held in place with a small screw. It's like a reluctor ring, it's just measuring the rotational speed of a spinning disc (or whatever is in there?).

[SunSoaked]

The wire colors may vary slightly. Mines an 85. All my red wires look pink, black look brown, etc. Lol. That sensor uses photodiodes, just like a CD player.

[tucsonjwt]

I found a listing for this part number online which states:

"Choose from Digital or Analog. Color is light green for analog Yellow for digital. Original GM AC Delco. 25007227 Digital dash unit. ( yellow ) 25007463 is for analog gauges ( green ) Part number 25007463 is for analog gauges. Part number 25007227 is for digital dashes and fits just about everything with digital dash."

I have analog gauges, so is this the wrong one? Did GM trucks ever have digital gauges?

[SunSoaked]

Hmmmm, good question. Mine is lite green. I know it works. If you need it, let me know.

[tucsonjwt]

So, the black sensor just attaches to the speedometer cable itself, not the instrument cluster? How would I go about attaching this sensor and where would it attach? Is that countersunk hole for the attachment screw? Can I just use a small radiator clamp to secure it to the cable, or is it possible to get a factory style attachment ring? Thanks.

[sweetk30]

1 tiny screw in the back of the speedo head just next or above the cable click on point .

it views the fly weight in the speedo head with the optic sensor to determine the speed for its computer use to hold the servo for the cruise control .

parts speedo i have . shows window hole with 3 little holes and the top center hole holds it to the body . you can see the flay weight in the square hole that rotated slightly in my pic . this is what its watching .

now if someone changed the speedo unit out they might not have hooked it back up or even used a speedo with no opening for it .

[tucsonjwt]

I don't think my speedo head has anything like that - the cable just sticks into the back of the speedo head. Maybe the previous owner(s) changed the speedo head and just left the sensor dangling. It looks like I have the wrong sensor anyway, and maybe the wrong speedo head. One more strike and I am out.

[tucsonjwt]

From an ebay listing for the green part:

"The speed sensor senses how fact the speedometer magnet shaft is turning, with relation to possible pollution control. This was a failed attempt for improved emissions that was eventually discontinued. The sensor appears as a connector behind the dash and plugs into an opening in the back of the speedometer."

So, does this mean that this sensor is related to emissions control only, and does not affect the actual operation of the cruise control? My CC works some of the time, but if I go up or down a hill or turn on the a/c then it does not work. So, apparently, the CC can function without it being connected to the speedo. My 83 C20 BB 4 speed granny has some pollution controls and not others. It does not have a vacuum canister or catalytic converters, but does have a smog pump and all of the vacuum hard lines in the exhaust manifold area.

[USSkoval]

No, it's not for the emissions. If it actually gets hooked up to the speedo where it belongs, your cruise control should work properly.

[Wgesnerjr]

I think digital vs. analog refers to the signal output, not the gauges.

This is just guess on my part. I have no real experience with this item.

[tucsonjwt]

I can't find the part that attaches the speedometer cable to the speedo head. Does anyone know where I can get one?

The green (analog?) sensor is apparently available NOS, so should I replace my yellow (digital) sensor with the green one?

[tucsonjwt]

If anyone has the part number for that connector that would be helpful. Thanks.

[hatzie]

As has been said that's the Optical pickup for a VSS signal to the Cruise III Cruise Control. There should be a hole next to the speedo cable to insert that sensor on most 82 and later clusters.

The Yellow or Green box is a VSS buffer/amplifier. Not sure where Digital vs analog comes from. Yellow and Green do the same thing. The output signal is a square wave with a frequency of 0.556 Hertz/MPH.

Some of the cars used the signal for the early electronic fuel injection so the boards have two output connections. I believe the 87 and later trucks had a pass-through gear drive VSS in the transmission but don't quote me on that.

Download the Cruise III textbook from my manuals post. There's a bunch of info there. If your Cruise III is otherwise knackered good luck finding parts.

[tucsonjwt]

Thanks for the info. I notice that the cruise control manuals are through 1982, then start with another manual at 1984. The cruise control III does not show my particular cruise control stalk control (mine does not have the intermittent wiper control on the stalk.) Is there a separate manual for the 1983 cruise control only?

[tucsonjwt]

If the VSS must be connected to the speedo in order for the cruise control to work, then why does my cruise control sometimes work without being connected?

[tucsonjwt]

I am posting pics of the back of my speedo head. Can someone indicate where the VSS "bracket" to connect the VSS would attach? Should there be a bracket held down by the silver colored screw to the right of the rectangular window? Thanks.

[USSkoval]

That screw is not the original type, but that's what will mount your optical sensor to your speedo.

You said when your cruise works, it doesn't maintain speed if something like a hill causes the truck to lose speed. That's because the sensor hasn't been mounted up, so the truck doesn't know it's slowing down.

[hatzie]

Originally Posted by tucsonjwt

Thanks for the info. I notice that the cruise control manuals are through 1982, then start with another manual at 1984. The cruise control III does not show my particular cruise control stalk control (mine does not have the intermittent wiper control on the stalk.) Is there a separate manual for the 1983 cruise control only?

That textbook is what's available. They might've published one for 83 but I do not have a copy.
The wiper control has no bearing on the operation of the cruise control. IIRC the 1983 wiper switch is on the Dash. The set resume etcetera on the cruise control switch should operate the same as the later switches in the cruise III textbook.
The 1983 service manual probably has a section on cruise control. If the 83 system is very different from the 84 and later it will be extremely difficult to get parts for it.

[USSkoval]

I'm very curious now, what does the '83 cruise control stalk look like? Is it just a single SET button on the end like the older models, or was there also RESUME/ACCEL like the '84 and later multi-function switch?

[hatzie]

Originally Posted by USSkoval

I'm very curious now, what does the '83 cruise control stalk look like? Is it just a single SET button on the end like the older models, or was there also RESUME/ACCEL like the '84 and later multi-function switch?

83 is a transitional year for the interior controls.
If it's Cruise III with a column mounted dimmer the stalk should look just like the later ones without the wiper controls.
I don't have the wiring diagrams in front of me and I don't remember if the 83 dimmer is on the column or on the floor.
If the dimmer is not on the column the switch stalk probably looks like the late 70s very very early 80s Berlinetta Camaro with the last of the cruisemaster systems that had resume.
A picture would be darned intresting.