AdsoYo

After a cold start, the check engine light will turn on when I first start moving. The scanmaster shows it as the vehicle speed sensor. After driving for a few seconds, the light turns off and the speedometer accurately works with or without the light on. This happens 95% of the time. If I start it after the car's been running, it might do this 50% of the time.

I replaced the sensor and that did not fix the problem. I checked the wires for continuity and there was continuity. I'm not sure what's left in the equation here.

A little history: after my engine swap, there were a handful of times where I was accelerating and the speedo needle would freeze for a second or two, then snap to where it should be. That was during the first month or two after the swap, June and July. It's been throwing the check engine light since August or September.

Injuneer

Harness connector pins making good contact? Any chance you crushed the wire when you installed the engine, causing an intermittent circuit?

AdsoYo

If it were either of those possibilities, would the check engine light still go away so predictably once I start moving? I figure the sensor itself can be ruled out, I forgot to mention it did this with both my PCM's, sorry about that. That must leave the wires as the culprit but it seems like the light should stay on if it were the wires.

meissen

My VSS wire went bad entirely - over time the wire just corroded away and i lost continuity. If you can get your car in the air, it's fairly easy to run a dedicated wire from the PCM plug down to the VSS and drive around for a few days to see if that fixes your issue. If it does, then you know it's the wire for the VSS that's bad and you can do a real fix to it.

94Z28LS1toLT1

after i did my swap.. my speedometer would move then it would freeze, causing my SES to pop up then come off etc, but i checked the wire that led to VSS and the wire was losing connection.i just bought a terminal kit and reconnected it and now my speedometer is great

Injuneer

When the circuit is broken, the code sets and the light comes on. When contact is re-established, the SES light goes off. When the circuit is broken, the speedo will stop moving. When contact is re-established, the speedo will resume operation.

To run the diagnostic, the following conditions must be met:
-no codes for MAP, TPS or ECT
-No codes for idle system or IAC (OBD-II)
-coolant greater than 0*C (=32*F)
-engine speed greater then 1,000rpm
-throttle between 5%-100%
-MAP between 40-100kPa
-above conditions met for more than 2 seconds

AdsoYo

Good info guys, thanks!

AdsoYo

Well, I disconnected the harness at the transmission and pulled the wires up above the engine so I could inspect the whole thing and there was no damage that I could see. I also tested for continuity and both the ground and signal wire were good although the signal wire didn't light up my tester as bright as the ground did. The plastic on the connector that goes into the sensor was kinda loose, but I zip-tied it down and thought it was on there nice and tight. Despite all this, I still have the dtc 24 popping up.

blown94

go to the junkyard and get a used connector and splice it on with solder and heatshrink. if this doesnt work you likely crushed a wire when doing the swap and although it has continuity, ot will cause a voltage drop in the wire and it might only be hanging on by a few strands. If this is the case you will need to run both wires from pcm to sensor, thats the easiest way to go about it. If I had to guess, you've opened up one of the wires somewhere and it is intermittently grounding out somewhere. In this case, since you cant find it, I would just run 2 new wires and be done with it. This is by far the fastest way to fix these intermittent type of problems.

Injuneer

Shoebox has the part # for the VSS pigtail:

http://shbox.com/1/Parts.htm

AdsoYo

cool, thanks guys

AdsoYo

I replaced the pigtail and ran two new wires from the PCM to the VSS with no luck. Just got a dtc 24 again yesterday.

Hopefully this is helpful: it will throw a code pretty consistently if I'm idling and rev the engine. It also does it pretty consistently if I make small movements with the car like adjusting in a parking space or back into my driveway after driving home.

The PCM connector pins weren't bent when I looked at them, and the connector in the PCM itself didn't look dirty or anything. I've got it plugged in nice and tight. Could there be another wire or connection affecting my VSS?

bobdec

Do you have any scanner or logging capability. Here is the DTC text for an A4 right out of the '94 F-Body service manual. " Transmission output speed less than 250 RPM and engine speed greater than 3000 RPM and not in park or neutral and not DTC's 21,22,28,33,34 and MAP above 20 and below 99 and for 3 seconds" If it sets when you are idling and revving the engine then the VSS should be inactive, and engine may be over 3K not sure if you held it at 3K for 3 seconds, but the park/neutral signal to the PCM should block the DTC in neutral. TTS DataMaster has a P/N status bit displayed, other scanner s/w may have it also. With a Datamaster trace you could you see if VSS, RPM, MAP and P/N are all where they should be when the DTC is set ?

AdsoYo

I managed to catch the dtc 24 on a datalog. It happened after I revved the engine for a second or two. The rpm was at ~1500, map was at ~10, the p/n switch had a green "0" the whole time and there were no other dtc's. Does that help at all?

bobdec

AdsoYo, I apologize, I just re-read your sig and noticed you have a 4l60E to T56 conversion. I gave you the DTC 24 causes for an A4 in post # 13. I assume you reloaded the PCM with a M6 load after the conversion. In an M6 the DTC 24 sets when " Transmission speed less than 3 RPM and engine speed greater than 1250 RPM and TPS less than 1% and MAP between 11 kpa & 21 kpa and DTC's 21,22,33,34 not set all for a period of 6 seconds " The P/N status switch does not come into place in an M6. What seems strange is your low MAP with engine over 1200 and speed near,or at, 0 and TPS just about closed your low MAP/very high vacuum (10 kpa = 3 lbs/Hg , 2 lbs/atmosphere) looks like the culprit. Are you sure you MAP is reading correctly. Not sure I ever saw a MAP as low a 10 kpa, but you do have a heavily modified setup.