1 Vehicle Details:
• 2001 Honda Passport • 3.2 V6
2 The Problems
The customer reported that when driving the vehicle it felt like a ‘kangaroo’. The vehicle ran unstable and very erratic.
3 Diagnostic Investigation
There was no EML (Engine Management Light) on, however a scan tool was connected to the vehicle to see if there was any data or pending codes stored to give a quick idea what could be causing the problem. As we guessed by no EML there was nothing. We decided to road test the vehicle and get an idea of the customer complaint. The engine started fine, a quick blip of the throttle revealed that the engine would throttle without restriction.
We took the vehicle for a road test and we felt like we were sat in the pouch of a kangaroo, we saw exactly what the customer diagnosed, the vehicle became very erratic and unstable, we quickly slipped the vehicle into neutral whilst on road test and went WOT (Wide Open Throttle) and the engine again revved up without restriction. This made us think it had to be something with the Gear train i.e. Gearbox or even wheel speed sensors but possibly not the engine. We could be wrong but this was our hunch.
We opened the bonnet and plugged in our 4423 PicoScope, we thought as it was easy access we would quickly scope the Air Flow Meter; we figured this could give the symptom.
This looked perfectly normal to us, as we suspected, never hurts to be sure though especially when it’s such a quick test. While we were here, we quickly checked the throttle position switch, this as well as Air Flow could give this symptom. It was a 6 wire throttle motor so we read signals for the Potentiometer and the Motor at the same time.
Again this looked good to us, you can see the Motor is moving and the potentiometers are showing no drop outs or distortion indicating no interference along the resistive track.
At this point it was becoming clearer the problem was to do with the Gear Train. The vehicle was then put onto a ramp and raised in the air so we could get under it and take a look. The gearbox was showing signs of oil weeping around the gearbox case so it was fairly dirty. We quickly cleaned this up so we could look at what we had. The gearbox had what appeared to be the following sensors:-
• Input Shaft Speed
• Output Shaft Speed
• VSS – Vehicle Speed Sensor
We decided to scope the Shaft Speed sensors together; we raised the rear wheels off the ramp using the hydraulic lift and connected our scope into both shaft speed sensors. We then drove the vehicle on the spot and took a look at what both sensors were reporting. The vehicle was still kangarooing so we thought we would see this on our waveform.
The above is confusing, we were expecting to see some kind of irregularity, or problem. Yet the waveform looked good. We thought it was strange that the sensors had different outputs but we all know OEM’s like to confuse us sometimes. We then moved onto the vehicle speed sensor (VSS) whilst the vehicle was still raised off the ramp. We drove the vehicle on the spot and again the vehicle was erratic.
Now we could see from this, something didn’t look right. You can see by the detail at the beginning of the waveform, it looks all ‘blocky’. We then zoomed into the waveform to take a closer look.
This definitely didn’t look right, this output was definitely wrong. Earlier we mentioned the gearbox was covered in oil, we removed the multi-plug from the sensor and it was full of oil. We cleaned out the connector on the sensor, cleaned out the multi-plug on the wiring loom. We inspected the wiring as we thought if it had sat submerged in oil, it was best to inspect it for any damage etc. The wiring looked good; we reconnected the sensor and re-ran our static road test. The vehicle ran fine! No erratic behaviour. We scoped the sensor just to verify the output looked normal.
It certainly looked a lot better than before, and zooming in further proved the output was perfect.
We were more than happy with this. We disconnected the scope, got the vehicle off the ramp and took the vehicle out for a road test and confirmed that the vehicle was now driving normally.
We got the vehicle back on the ramp and topped up the gearbox oil.
We called the customer up and let them know the vehicle was ready and advised them to have the gearbox removed and resealed as this was only going to happen again. We are waiting to hear from the customer.
Case Study Information
Detail: Lee Hearnden & Richard Boyd
Author: Mike Valentine
Technical Author @ Pico Technology UK