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Old 10-11-2011, 03:28 PM
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LT1 VSS Sensor -- non working speedo, odd voltage at PCM

Hi guys,

My speedometer isn't working and I'm wondering if there are any additional checks I can run. Right now, I have a 1994 Camaro tune for M6 -- I'm trying to see why I'm not getting any signal at the speedometer.

When i scanned the PCM i pulled code 97, which is VSS output circuit not giving me 4000ppm. I was reading at 0mph with the key on, and the engine off. I checked the voltage at the Green/White wire (output wire) in the cab and it read .75v -- I had read on a previous forum entry that it is supposed to read between 6.5-7.5v. I checked the Green/white wire at B8 on the PCM and got a voltage reading of 11.8v -- weird, huh?

Does that mean the VSS is outputting a faulty signal (indicating faulty VSS or connection at the VSS)? Why else would my output wire be so high?

What else can I check? What are the voltages supposed to be at the VSS itself? perhaps I can track the problem down from there.

Any help would be very appreciated!

Tyler
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Old 10-11-2011, 04:24 PM
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Quote:
Originally Posted by tylerwerrin
Hi guys,

My speedometer isn't working and I'm wondering if there are any additional checks I can run. Right now, I have a 1994 Camaro tune for M6 -- I'm trying to see why I'm not getting any signal at the speedometer.

When i scanned the PCM i pulled code 97, which is VSS output circuit not giving me 4000ppm. I was reading at 0mph with the key on, and the engine off. I checked the voltage at the Green/White wire (output wire) in the cab and it read .75v -- I had read on a previous forum entry that it is supposed to read between 6.5-7.5v. I checked the Green/white wire at B8 on the PCM and got a voltage reading of 11.8v -- weird, huh?

Does that mean the VSS is outputting a faulty signal (indicating faulty VSS or connection at the VSS)? Why else would my output wire be so high?

What else can I check? What are the voltages supposed to be at the VSS itself? perhaps I can track the problem down from there.

Any help would be very appreciated!

Tyler
if you have a different voltage at 2 ends of the same wire the wire is damage or broken.
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Old 10-11-2011, 04:28 PM
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Vss

my 88 silverado has a speedo problem that it works in cold mornings but not as the day warms up. i put in a new VSS but it did not help. any idea ????
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Old 10-11-2011, 04:30 PM
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That's what I thought -- the voltage is coming up as wrong from the PCM though -- it should be 6.5v with key on engine off but instead it's coming up at 11.8 (battery's voltage).

Would this be indicitive of a faulty VSS outputting an incorrect signal? And if so, why aren't I getting a code 24?
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Old 10-11-2011, 04:38 PM
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Quote:
Originally Posted by tylerwerrin
That's what I thought -- the voltage is coming up as wrong from the PCM though -- it should be 6.5v with key on engine off but instead it's coming up at 11.8 (battery's voltage).

Would this be indicitive of a faulty VSS outputting an incorrect signal? And if so, why aren't I getting a code 24?
If a wire is broken the voltage can float. In order to check the voltage at the PCM you would need to have the same situation as the one you referenced. If they took that reading on a car with the sensor in the circuit it can and will make a difference. If you are sure it is the same wire, to test just patch in a new piece from the sensor to the wire at the ecm and see if it works. If it does you can do a permenant repair by running a new wire or finding the break in the old one.
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Old 10-11-2011, 05:20 PM
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I'm going to try running a new wire from the PCM to the speedo and see if that does the trick -- the voltages are indeed different, and different from what the shop manual says they should be at. Running a new wire shouldn't be too difficult.

Thanks for the input.
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Old 10-12-2011, 11:03 AM
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Very strange -- i ran a new wire from the B8 terminal on the PCM straight to the Speedometer and got very low voltage readings -- 75mV. I should be getting 6.5 key on, 7.5 engine running.

Anyone have any experience with the DTC 97?
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Old 10-12-2011, 12:04 PM
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Sensor is probably shorted
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Old 10-12-2011, 03:21 PM
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UPDATE:

So, here's what I've done so far:

Checked voltage at the PCM (B8) -- had odd floating readings, but when I found a good ground it stayed at .26v.

Ran a new wire directly from B8 directly to my speedometer and checked the voltage -- still at .26.

Checked the voltage at the VSS harness connector with Key On -- came back with 4.9v -- which is what I should come back with. OK.

Since i had another VSS, i figured I would just swap it out and see what happens. I swapped in the new VSS, voltage readings at the PCM still remained the same (.26v)

According to shbox's pinouts, I should be getting 6.5v with key off at B8 and 7.5v with the engine running. I'm getting neither of these. The speedometer still is displaying nothing.

I can try swapping in a different PCM but I don't think it will really make a difference. Other than doing that, is there anything else I can do? I am at a bit of a loss here.
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Old 10-12-2011, 04:34 PM
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Vss

Tyler, I don't have the Helms Manual with the trouble shooting procedure and all the wiring diagrams, but you need to check the VSS output from the transmission to assure there is a signal. It should be an AC sine wave which can be read with an oscilliscope or with a Fluke or other digital multimeter as frequency in hertz. The voltage level and the frequency will vary with speed. Rear end on jack stands with front wheels blocked, check for output with the driveshaft spinning. I THINK there is a GM signal buffer between the VSS and the PCM input. The output goes to the speedometer, cruise control module and the radio for SCV. If the circuit wire that was supposed to be connected to these other items is stuffed under the dash it could be shorted to ground somewhere. Don't know how much of the GM harness you used in your conversion. I seem to remember that you had a '93 engine and t56 now, the '93 t56 with the real low first gear used a different pulse than the later models. Read this for pointers, and a good breakdown on all the different VSS;
http://www.jagsthatrun.com/V8-chapte...ed-Sensors.pdf

Also, I have read some of Joe G a member here and he seems to be well versed in figuring out VSS problems. Hell, you got it running, who needs a speedo, use the tach! Ha ha
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Old 10-12-2011, 04:41 PM
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Ha, normally I'd be all for driving around without a speedo -- but it's like a white whale -- you just want all the little gauges working! Plus, I'll need a working speedo for the odometer and my nifty little 1980s trip computer/gas mileage calculator :-)
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Old 10-12-2011, 06:54 PM
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Actually the T56 I have is the "good" 1993 model (M29) with the 17 tooth reluctor gear -- so the pulses that I'm getting should be the same as one of the later T56's.

The engine/transmission + harness are all chevy. I had to add a Speedometer converter that dampens the chevy signal going to the Jag tach -- however, this shouldn't be what's causing the problem as I tested the voltage before the converter box (just tested the wire) as well as after the box and still got weird voltages.
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