School me on the Vehicle Speed Sensor (VSS)

[Marc311]

This has nothing to do with an engine conversion. I just need info about the VSS and the ECU to troubleshoot a problem. But maybe some of you folks that have converted to EFI engines can assist.

I believe my ECU is not getting the speed signal, or getting a poor signal, resulting in it not switching off the fuel cutoff solenoid. I had tested it per the FSM, but I think I need to retest it with an analog multileter to get a better idea of what is happening. I may also throw an oscilliscope on it, but I'm not exactly sure what type of signal I should see.

So my questions are:

If mine is bad, can I throw an aftermarket speed sensor on at the tranny and wire it up somehow? (rather than replacing my speedo)

What type of signal should I see with a scope (sine or square, frequency)?

My other thought is to just cut that wire to the ecu or short it to try to trick the ECU into thinking it is not moving. This would also mess up when air is blown into the exhaust pipe/air rail/bypass - but I could live with that. Any idea if shorting or opening the wire could "trick" the ecu?

 

[weak-link]

what engine,year,model

 

[RockDoc]

I'm guessing an 87 FJ 60 with a stock 2F.

 

[RockDoc]

This thread has some good info if you haven't come across it. It's discussing getting the proper signal for a 3FE in a 40..... but might help you out.

 

[Waggoner5]

Yes if your VSS is actually bad, you can use an aftermarket as long as it puts out the same pulses per mile as what the broken one did. I have not seen a bad VSS ever though except a physically broken one. Most GM stuff is looking for 4000 pulses per mile which is easy to find. There are VSS that bolt inline where your speedo screws in and the speedo screws to it. 2 wire hookup. easy.
G

 

[Marc311]

Thanks RocDoc! I had searched the 40 section and read dozens of threads - Don't know how I missed that one. I still need to verify mine is bad. But based upon those pics - it looks like I might be able to access it to clean it up if there is a problem or possibly swap it out without swapping the whole speedo.

And yes, stock 2F, but I figured the engine swap guys probably dealt more with VSS issues. in fact, i really didn't find much in the 60s forum about the VSS that wasn't engine swap or EFI related.

 

[RockDoc]

I remembered mmw68 having VSS issues with the 3FE 40 he got, so I went straight for that in a search.

 

[Marc311]

I was also reading your discussion in mmw's thread. In that you mention bypassing the VSV to give it vacuum all the time. Ironically, as I was writing notes to myself last night about possible solutions to my backfiring problem, I thought about bypassing the VSV. Still might have to try that one.

That was a great back and forth between you two.

 

[Marc311]

Well, bypassing the VSV(2) solved the backfiring problem. VSV(2) checks out fine, so that isn't the problem. So, like the decel fuel cutoff, it seems to be getting a bad signal from the ECU. However, I have tried 3 different ECUs. I believe both (VSV and FCS) are related in the schematic, but can't figure out what the issue is. So I'm pretty baffled, but happy I no longer have the "pop pop."

I still would like to get the whole thing solved, but I can put it lower on my priorities now.