CaptainDean wrote:
layback40,
I don't have this from authority, but here is my rambling...
Case 1: The ECM used +5vDC as maximum in it's signals and measurements, so it would seem that +5v on the fuel pressure sensor would match up with ~25,000 PSI, and of course 0V with 0 PSI. That could correlate to a scale of 5,000 PSI per volt on the sensor. If that were the case, then a properly running engine would provide about 4.5v signal. (But I don't think that is the case. Also this scale does not allow much for sensing over-pressure situations.)
Case 2: However, if it uses 50,000 PSI as maximum measurement at +5v, then scale would be 10,000 PSI per volt. A measurement of +2v would correlate to ~20,000 PSI. (That matches up pretty closely to 1.8v signal indicating ~18,000 PSI which would probably get the engine running. And there would be plenty of measurement room to sense over pressure situations and shut down the engine for safety.)
Case 3: If the 1.8v were indicating 23,000 PSI, then full scale 5v would indicate 63,000 PSI. That is possible but I believe unlikely.
Dean.
Thanks Dean,
One of the many diagnostics I have read says that 1v during cranking is required as an output from the Rail Pressure sensor for the engine to start.
I get about .5v without cranking, that is no pressure. About the same with my failed Pressure Control Valve while cranking.
Most Bosch sensors don't use 0 to 5 v for output. More likely to use .5 to 4.5 v as it avoids any issues at the very ends of the scale. Not certain though.
Another PDF I have in the name of Tommy Atkinson & Clayton Renth (you probably have a copy), has a screen shot of a DBR 111 for Wide Open Throttle.
It shows fuel pressure set point of 23,196 PSI (nominal 23,000); I suspect that is programmed into the ECM.
Fuel pressure of 22,849 PSI ; probably the capacity of the CP3 at WOT flow of fuel at whatever speed the jeep is going at.
Fuel pressure volts 4.05v ; probably the rail pressure sensor output voltage.
Pressure control valve VLT (voltage?) 2.1v ; not sure about this as the Pressure Control Valve (Solenoid?) gets a pulse signal and at WOT the valve should be shut. Especially as the rail pressure is below the setpoint. Maybe it needs to get to say 4.5v before the signal is strong enough to open the valve?
Yet in another screen shot, for deceleration, shows the PCV voltage as .69v. I would have thought that with your foot off the throttle at speed, the PCV would be open to relieve fuel pressure as the injectors would not be using very much fuel. I guess the Fuel Quantity Solenoid on the CP3 must take care of that.
These values along with what I have seen on mine, suggest that .5v is 0PSI & 4.1v is 23,000PSI. This gives about 3,200PSI for a reading of 1v.
I don't know what the minimum pressure required for the injectors to pop is.
Maybe someone can comment (ww?).
I got scared when I saw the output of the rail pressure sensor get to 1.5v quickly, so I stopped cranking. Maybe when I get a chance I will try again & see how high it goes while cranking. It may start. I don't wish for anything to go bang.
The impression that I have had previously is that the Pressure Control Valve (Solenoid) on the end of the rail is basically a safety valve to prevent rail overpressure. The actual pressure is controlled by the Fuel Quantity solenoid.
Not sure if the CP3 has a mechanical over pressure valve as well.
Any comments are always appreciated.