AC voltage to speedo

[moesterman]

I am still sorting through my new BD PDM and what is causing the MPH to rise 3-5 mph with increased RPM's (Bike sitting stationary). I just tested the circuit at the speedo bucket pin connector. The circuit has three leads; red green and black with the green being the one sending the signal to the speedo. I had a reading of almost 7.0 volts AC and thinking I have a bleed somewhere in the harness which causes the mph increas with increased rpm's. It is zero at an idle. Anyone know what the AC reading should be, I bleieve it should be no more than one-volt AC, but not sure. Any ideas where it coiuld me bleeding from? This harness thing has been a real PITA!!!!

 

[JimNubs]

Shouldn't the reading be in DC volts?

 

[ChopperJeff]

No, it's not a DC input. It's more like AC, and more accurately a square wave, with the frequency varying with the speed. The amplitude of the signal is in the area of 7 volts.

Basically the faster you go, the more pulses are sent to the speedo. I know this as I once built a circuit that simulated the output of the speed sensor. If you build a square wave generator with about a 10 kHz signal, then the resulting speed is somewhere around 70 MPH. I did this in order to "spin up" my new speedo that was shy a few tens of thousands of miles.

Granted, this is what applied to my '05 Chopper, but I would imagine the other models and years work the same... though I could be wrong.

 

[BigDogBro1]

:up:

Connect an oscilloscope to the signal and you'll see the waveform. The magnetic sensor in the trans is sending pulses from the gear teeth passing by it.

A DVM meter will read any frequency waveform signal better when set to AC.

What type speedo are you running? Any needle type speedo either driven by stepper motor like BDM's, or the old style magnetic speed cups are somewhat susceptible to vibration. It could be just vibration causing the minor fluxuation of 3-5 MPH.

 

[nobbyjohn]

That kind of knowledge is impressive!

 

[Moespeeds]

How about just cutting the green wire up at the speedo, then running a new one directly down to the speed sensor. Obviously cut the existing signal wire off the speed sensor. If you know you have a ground or bleed between point A and B, the simplest fix is to just run a new one. What's one more wire when you already got 5000 in there.

 

[moesterman]

No, it's not a DC input. It's more like AC, and more accurately a square wave, with the frequency varying with the speed. The amplitude of the signal is in the area of 7 volts.

Basically the faster you go, the more pulses are sent to the speedo. I know this as I once built a circuit that simulated the output of the speed sensor. If you build a square wave generator with about a 10 kHz signal, then the resulting speed is somewhere around 70 MPH. I did this in order to "spin up" my new speedo that was shy a few tens of thousands of miles.

Granted, this is what applied to my '05 Chopper, but I would imagine the other models and years work the same... though I could be wrong.

With that said, should I be receiving a 7volt AC signal at the speedo signal circuit at 2500 rpm's while in neutral? I tested it at the connector in the speedo bucket.

 

[moesterman]

:up:

Connect an oscilloscope to the signal and you'll see the waveform. The magnetic sensor in the trans is sending pulses from the gear teeth passing by it.

A DVM meter will read any frequency waveform signal better when set to AC.

What type speedo are you running? Any needle type speedo either driven by stepper motor like BDM's, or the old style magnetic speed cups are somewhat susceptible to vibration. It could be just vibration causing the minor fluxuation of 3-5 MPH.

I just installed a new Dakota Digital BD-140 when I installed the BD PDM.

 

[moesterman]

How about just cutting the green wire up at the speedo, then running a new one directly down to the speed sensor. Obviously cut the existing signal wire off the speed sensor. If you know you have a ground or bleed between point A and B, the simplest fix is to just run a new one. What's one more wire when you already got 5000 in there.

I thought of that already but for obvious reason I want to be sure it is actually a problem before I start cutting and running wire to find out it will do the same. Has anyone with a BD PDM retro kit and a DD Speedo watched their speed indicator while in neutral and increased rpm's to 3000rpm's I found it after the install and was checking the tach function. Al

 

[Moespeeds]

The speedo is a very simple system, the speed sensor sends a signal and the speedo interprets it. If your speedo is increasing with RPMs, something is bleeding somewhere, possibly in the PDM, or a crossed wire which is very possible since new wires were pulled through the backbone and you had all that rooting around to sort out the other problem. I'd also look inside the speedo housing. Trace the RPM signal wire back to the IGN and snip it, and see if the problem goes away. If it doesn't, then your issue is somehow in the PDM or just a bad speedo. In the 2 PDMs I've done, the owners did not report this problem, but I'll have them check.

 

[moesterman]

The speedo is a very simple system, the speed sensor sends a signal and the speedo interprets it. If your speedo is increasing with RPMs, something is bleeding somewhere, possibly in the PDM, or a crossed wire which is very possible since new wires were pulled through the backbone and you had all that rooting around to sort out the other problem. I'd also look inside the speedo housing. Trace the RPM signal wire back to the IGN and snip it, and see if the problem goes away. If it doesn't, then your issue is somehow in the PDM or just a bad speedo. In the 2 PDMs I've done, the owners did not report this problem, but I'll have them check.

Thanks, I'll wait to hear from you. My next step is to check the voltage at the sensor connection under the battery. I really hate to snip and solder any more wires. AL

 

[moesterman]

Further info on the ghost reading with the Dakota Digital speedo. All I did tonight was plug the stock BD speedo back in at the bucket and determined it performes the same as the DD. When I increased the rpm's the needle would bump up to 5-7 mph. Yet another harness/PDM issue! I am receiving a bleed somewhere and will have to tear into it again. I willl start at the sensor tomorrow to eliminate the problem in the backbone. Any thoughts would be appreciated. Al

 

[ChopperJeff]

With that said, should I be receiving a 7volt AC signal at the speedo signal circuit at 2500 rpm's while in neutral? I tested it at the connector in the speedo bucket.

Good question!

It would seem that in neutral your shoudn't be getting any signal to the speedometer regardless of the RPMs. At least for the '05 Chopper, the signal from the speed sensor that comes from the tranny goes electrically straight from the sensor to the speedo. I say electrically as it does go into the EHC, but it just goes in one connector and out another without anything in the way of electronics occurring. I guess if you suspect some sort of bleeding or crosstalk is happening along the backbone, then you could run a wire straight from the sensor into the speedo and see what happens. I may even have some spare connectors laying around if you need some. Using connectors would mean no soldering and/or splicing needed.

 

[Moespeeds]

BD runs them through the computer so they can track rev limiter hits. Get out the BD wiring diagram, take off the tank, and trace that wire from the speedo back to the PDM, then from the PDM to the IGN. If you have no breaks then it's most likely a problem inside the PDM. You could also pull the plugs at both ends of the harness, put a continuity tester on the RPM wire, then test every other wire in the harness to see if they are crossing. Seriously man, just bypass it. I'm not busting your balls, I'm talking from direct experience. Once you've put a PDM or WP in the bike, it's a bandaid on top of an already seriously flawed electrical system, and troubleshooting is a nightmare. There is absolutely no downside to just running the wire straight from the pickup to the speedo. It's how any basic wiring harness is run, and it's one less thing running through the PDM that can break later on. There is no harm in cutting wires and soldering in a new one, as long as it's properly shrink wrapped.

 

[ChopperJeff]

Perhaps I'm a bit out of the loop with this PDM stuff. In fact, what is the PDM?

That aside, I understand that the computer in the BD bikes tracks the RPMs for rev limit hits, but isn't this totally different than the speed sensor? My understaning is that a signal is sent from the speed sensor to the speedo so it can display the speed, and then there's the seperate signal from the sensor that is tracking RPMs that is sent to the "computer" for tracking purposes.

Please excuse me for somewhat hijacking this thread a bit for what may be my ignorance in this matter. Just trying to understand what's being said here.

All that aside though, I fully agree that running a seperate wire from the speed sensor to the speedo could yield some good info.

 

[Moespeeds]

Sorry I should have clarified. Since the speed is rising with the RPMs, you're probably crossing the RPM feed somewhere.

 

[BigDogBro1]

You could be getting coil/ignition induction noise into the speedo signal wire. Are you running resistor type spark plugs? The resistor type plug will decrease the EMF energy around the coil, wires,and plug area. You also need to run GOOD condition plug wires to reduce interference into the on-board control electronics.

 

[Moespeeds]

BD Bro if he was getting noise from the coils, wouldn't he have other issues in the speedo as well, like flashing idiot lights? Those wires all run together right? Changing plug wires is easy and worth a try...

 

[ChopperJeff]

If you suspect that there's a crossing of the RPM feed somewhere, how 'bout simply disconnecting the RPM feed sonewhere down the line, preferably as close to the source of the RPM feed, and see if that affects the MPH reading.

 

[moesterman]

If you suspect that there's a crossing of the RPM feed somewhere, how 'bout simply disconnecting the RPM feed somewhere down the line, preferably as close to the source of the RPM feed, and see if that affects the MPH reading.

Correct me if I am mistaken, but isn't the rpm feed signal DC? The only AC source signal I am experiencing would be from the ignition I am assuming. I did change plugs and considered new wires but I find it hard to believe that the interference from the plug wires could be causing the 7volt AC reading on the sensor wire I am experiencing. I plan on checking the resistance on the sensor wire tonight and go from there.
As always, any ideas/thoughts are appreciated. Al