Code 24 and speedo reads 0
 

I'm troubleshooting a speedometer reading 0 and also I get a service engine soon light about 1/2 mile down the road. Code is 24, so I first checked ckt 437 at the VSS connector for 12V to gnd. All I got was .422V. Do I have a bad ECM??? I have a 1986 4+3coupe.

RE: Code 24 and speedo reads 0
 

Code 24 - Only valid if vehicle moving. Check connections at ECM Check TPS setting. Possible ECM failure.

RE: Code 24 and speedo reads 0
 

Service engine soon light comes on only when driving (I get about 1/2 mile). Question is: If I disconnect the VSS connector (ckt 437)at the tranny (4+3)and measure the voltage fromthe connector, brown/white wire(looking back to the ECM) and Ground, it appears I have .422 VDC, but from what I see in the service manual, I should have between 10-12 VDC.

I've ordered an ECM From Eckler (A/C Delco remanufacture). I was woundering if anyone else had similar trouble, and did the ECM Fix it?

RE: Code 24 and speedo reads 0
 

Voltage should be created at the vehicle speed sensor then be sent to the ECM. It will only generate voltage when the drivshaft is rotating.
I do not remember which type of vss this car uses, it may be a permanent magnet generator, or ig may be an induction coil type. The PM generator is larger and usually has a gear on the end that is driven by the transmission, like an old speedometer gear. If you have the other type, it is just a magnet with wire wrapped around it that passes by a toothed wheel on the transmission shaft. The fitrst tpe would generate DC volts,(as welll as I remember) but the second type should generat AC volts. It would be unlikely for this problem to originate inthe ECM, it usually is because there is a broken of shorted wire between the ECM & the vss, or a fault in the vss.

RE: Code 24 and speedo reads 0
 

I'm having a similar problem. My speedo goes to zero after driving for a while and then the check engine light comes on also. It will pop back on now and then. This also messes with my 4+3 overdrive, I believe the OD thinks the car is at zero so doesn't shift into OD. I plan on replacing the speed sensor this weekend ($70), that's my best guess but believe me I'm not expert. Also, note that my speedo stays lit but reads zero. Let me know what you find. Good luck

RE: Code 24 and speedo reads 0
 

Every GM product I've worked on since the 80's has a magnetic pickup speed sensor that generate a/c voltage. I agree with Tacky, you either have a bad speed sensor or an open circuit to the speedo head. 1 way to check is to unplug sensor and with back wheels off ground, put your voltmeter on a/c voltage, touch the 2 terminals and have someone rotate your tires to see if you get a/c voltage that increases as you spin tires. If so, then you have an open circuit somewhere. Good Luck.

RE: Code 24 and speedo reads 0
 

The speed sensor on my 4+3 looks like and encoder, with a green tooth gear. The service manual indicates the 12 VDC Signal is sent to the VSS encoder. when the tires turn, it opens and closes a switch, sending a 12 v pulse to the speedometer. I don't see a magnet. Maybe a Doug Nash 4+3is different than the700r4.

The ECM will not be here for a day or two, But I'm having the same trouble as pointed out bydans87vette.ie; no OD, speedometer reads zero,...intermittantly works...etc. So you think its the VSS? From the looks of the diagram, the VSS looks like a switch that opens and closes...sending pulse's to the speedometer.

I just tried to ohm it out, and I see what you mean. I have the VSS sitting on the bench with the ohm (DMV)meter hooked up to the two pin connector. I turned the green gear, expecting to see an open/close pattern, but did not see a thing. I switched the meter to VAC, and turned the meter about 1 turn a second (3 turns), and see about a 1.8-2.1VAC signal. Now I'm confused. What is the VSS Signal on the ECM? is says 12VDC in the manual.

I need a VSS...think one of the dealers will have one???



dans87vette
"I'm having a similar problem. My speedo goes to zero after driving for a while and then the check engine light comes on also. It will pop back on now and then. This also messes with my 4+3 overdrive, I believe the OD thinks te car is at zero so doesn't shift into OD. I plan on replacing the speed sensor this weekend ($70), that's my best guess but believe me I'm not expert. Also, note that my speedo stays lit but reads zero. Let me know what you find"

RE: Code 24 and speedo reads 0
 

The a/c signal feeds directly to instrument cluster. I am posting a schematic. If you notice, there is a rotating mass that a magnet picks up on. The VSS picks up on the interutption of the magnetic field the mass creates which generates a small a/c voltage that the cluster interperates for speed. The one you have on the bench came out of your car? If so, then it is good and you have an open circuit to the cluster. I think probably the ECM samples the signal at one point probably through the cluster and it sees no signal, hence the code. It is also quite possible your cluster is failing too and will cause the same code because, I think, the VSS signal is not being sampled through it. Best to pull cluster and verify a/c signal input to it also. [:o]

[IMG]local://upfiles/1427/FCB42C51D8864E76861A6D20F5C45F7A.gif[/IMG]

RE: Code 24 and speedo reads 0
 

Thanks for the schematic post, very helpful........much better than the one I have for my 87. Does anyone know the sequence of operations for the 4+3 overdrive signal. Does the speed sensor send a signal to the speedo and then from their to the 4+3 O.D. If that were the case, then the speedo cluster could be the problem.......or does itgoe to the ECMfirst and then distributesthe signal??? When my speedo goes out and shows "0" mph, it make my O.D. system stop working. I believe this is the clue to finding this problem if anyone knows that sequence.

RE: Code 24 and speedo reads 0
 

Any luck yet "Ironhead 86". I discovered what looks to be bad news, when I crawled under the car for a look at the VSS it looks like pretty new as if the previous owner had alread tried to fix this problem. Let me know if the ECM replacement works. Other options after is the speedo cluster itself or as Mech29 suggested an open circuit in the wiring of the VSS. Keep me updated.