1985 Limited PB Sensor Replacement

[Rednaxs60]

Have found a replacement for the PB sensors (qty 2) on my '85 LTD - from the CX500-CX650 forum. The fellows over on that site have done a lot of research/testing and have found that the Suzuki sensor - Brand: Denso. Type number: 100798-5630. Suzuki ordering number: 15620-35F00. Suzuki description: Sensor Boost, IAP (Inlet Air Pressure) sensor, is an almost direct replacement for the Honda OEM PB sensors on their 500/650 Turbos.

One of the issues that was found is that the internal construction/connections was less than stellar (they took one apart) and since the connections were not soldered, the internal connections would come apart such that vibration would cause the PB sensor to fail. This is a good hit/miss scenario when troubleshooting.

Did some investigation into the difference between the Suzuki sensor and the OEM PB sensor(s) on my '85 LTD. I took some measurements today. It was mentioned that static testing may not be indicative of dynamic testing; however, it is a starting point.

The power source for this was 3 - AA batteries in series giving me 4.8 VDC. I hooked up a test circuit with multimeter. Here are my readings, read in 3 columns - inches of Hg/Suzuki sensor - VDC/Honda OEM PB sensor - VDC:

0 inch of Hg/3.45/3.63
5 inch of Hg/3.1/3.14
10 inch of Hg/2.65/2.59
15 inch of Hg/2.18/2.0
20 inch of Hg/1.73/1.43
25 inch of Hg/1.26/0.88

I have checked the vacuum at the PB sensors and at approximately 3000 RPM the vacuum to the sensors is 10 to 12 inches of HG and remains steady at these vacuum values as RPM increases past 3000 RPM. At 12" of Hg - Suzuki sensor - 2.46 VDC.

I have installed the Suzuki sensor(s) and did a road test. Seem to work well, bike idle is steady and it pulls nicely up through the RPM. Will keep installed for a few weeks and check fuel economy with these installed. Also cooling down coming into fall, so will be interested how the bike starts and idles in the cooler weather.

Here are a few pics of the testing and Suzuki sensors ready for install.

Suzuki Sensors

PB sensor test rig

Used the vacuum pump from my brake bleed kit

Power source for test - 3 AA batteries giving 4.8 VDC

Suzuki sensors mounted for install - wire connectors came from a Subaru at the local auto wreckers - $2.00 for two

Schematic for wire install

Still have some testing to do. Would like to test the sensor(s) when engine is started. Would only have to add one wire to the harness.

More to follow.

Cheers

[Rednaxs60]

Started the bike this morning and it is running a bit rich, but this was also reported on the CX forum by a few members. Once the bike got up to operating temp it seems to go away. It ran a bit rich even with the original OEM PB sensors installed so there is something else just not quite right, but on a 34 year old bike it probably can be expected.

Did some more research into what the PB sensors actually do. These monitor the intake pressure, at idle and steady throttle - greatest vacuum, vacuum drops as throttle is applied. Makes sense. Here is a site that is more detailed than I am being: http://www.aa1car.com/library/map_sensors.htm

When throttle is applied more fuel is injected into the cylinders and this is caused by the ECU taking the signal from the PB sensors and increasing the duration of the fuel injector. It also retards the timing so there is no pre-ignition. When the bike gets up to speed and throttle is reduced the PB signal to the ECU is such that fuel delivery is reduced, timing is advanced and maximum fuel economy is achieved.

I have noticed this with the travel computer readings. The fuel usage readings fluctuate with throttle application, uphill/downhill/flat running, and at idle. At idle there is very little fuel flow such that it does not register nor is there a read out.

Virgilmobile in an old post detailed: "The sensor connection fine tunes fuel delivery.there are 2 of them,and it will overload the engine when unhooked. The sensors expect about 9" of vacuum at a idle and report a corresponding voltage to the ECM. It uses that voltage along with the throttle position sensor voltage, ambient air temp and engine temp sensor to determine the pulse duration of the injectors."

Interesting to learn how the various sensors interact. The PB sensors affect fuel delivery and timing. The Ns (crankshaft) sensor is used for timing. The Gr/Gl (camshaft) sensors for fuel injection. Put these three signals into the ECU and let the black box work its magic. Mustn't forget the throttle position sensor (TPS) that also influences the fuel delivery and vacuum.

Cheers

[Rednaxs60]

I have changed out the OEM PB sensors and put the Suzuki boost sensors, IAP (Inlet Air Pressure) in place of. Bike has been running well and have just did the first fill up since these were installed. City/highway driving and the bike is averaging 16.28 Km per litre (38.29 USMPG or 45.98 IMPG). Pretty much in line with what I was getting before with the OEM PB sensors. Going to leave these in for the time being. A road trip would really decide the efficiency and benefit.

Cheers