Okay guys what's the consensus on this as im sure many 300's have this problem.

 

Speedo works fine 70% of the time, then it drops to 0 and fires up to 180 then a couple minutes later the PAS goes heavy because the ECU thinks its on a go-slow. So two questions:

 

1) In my head it would make more sense that the signal from the VSS to the ECU is whats causing the problem (broken VSS, housing or wires), not the clocks themselves, as the ECU is throwing out the PAS based on the signal it's RECIEVING, not the signal it's SENDING out to the gauges. Does this sound correct?

 

2) Failing correcting this issue as I don't need a working speedo except for MOT time, is there a way of reprogramming the ECU, or wiring the pump sensor so it no longer has variable power steering and has a constant pump speed? (hicas is disabled on my car anyway so it doesn't matter so much)

Mine is knackered atm.

 

It might very well pass an MOT they may not find out it is US.

 

The signal path is not what you may expect, it is gearbox, speedo, PAS ecu, ecu.

 

It's probably the KPH to MPH converter if you have one, or something stupid gone wrong in the dash.

 

It'll be possible to fudge the PS to one speed but I'd try to fix it first.

 

It'll be easiest to have it stuck heavy which is fail safe becaude it assumes you're going fast not slow.

 

I'll report back when I fix mine

Okay guys what's the consensus on this as im sure many 300's have this problem.

 

Speedo works fine 70% of the time, then it drops to 0 and fires up to 180 then a couple minutes later the PAS goes heavy because the ECU thinks its on a go-slow. So two questions:

 

1) In my head it would make more sense that the signal from the VSS to the ECU is whats causing the problem (broken VSS, housing or wires), not the clocks themselves, as the ECU is throwing out the PAS based on the signal it's RECIEVING, not the signal it's SENDING out to the gauges. Does this sound correct?

 

 

 

 

 

2) Failing correcting this issue as I don't need a working speedo except for MOT time, is there a way of reprogramming the ECU, or wiring the pump sensor so it no longer has variable power steering and has a constant pump speed? (hicas is disabled on my car anyway so it doesn't matter so much)

 

 

So lets see what we have here, I am assuming VSS is the speed sensor fitted into the gearbox, to understand your issue you need to understand how the speed signal is distributed through the various modules. The speed sensor produces the speed signal and this goes directly to the speedo input circuitry, this same input signal is also taken to the Consult diagnosis plug.

 

Both the speedo needle and the odometer are run from the same signal, this is still the input signal.

 

The speedo circuitry then produces a clean output signal that then goes to the ecu for the engine, the ecu for the hicas / steering ecu, ABS, autobox and cruise control.

 

Given the signal goes directly from the speed sensor to the speedo cluster and that in general the speed sensors are robust and do not suffer common failure if the speedo stops working and the steering weights up after the problem is with the speedo not producing an output signal, this is very easy to check for, with the cluster removed but still plugged in, the input and output AC 1 to 5 volt signal can be checked, jack up both wheels just clear of the floor by an inch or so, make safe. Start the engine and in first gear or drive in auto`s allow the engine to tick over in gear so the wheels are turning slowly, now with a meter set to ac voltage check input and output, this of course will only work out of the problem is present.

 

 

Reprogramming the ecu is of little use when the speed signal is required by so many other units, the steering pump is already a constant speed one, there is a solenoid in the end of the rack that opens and closes according to road speed, this simply bleeds off some of the fluid pressure in the system to the front of the pump, this is how it reduces pump pressure and hence reduces assistance levels, ( heavier steering ) tracing and fixing the fault is the only workable option.

 

 

Jeff

 

Some more info here with pics

 

https://www.300zx.co.uk/forums/showthread.php?188358-Speedo-and-PAS

 

 

 

Edited November 19, 20178 yr by JeffTT

Very good post Jeff, thankyou! I didn't realise the VSS went straight to the speedo, then through to ECU. What's the approx VSS location on the gearbox, if it's easy enough to remove there's no harm checking for mechanical wear etc. It does indeed have a converter, the previous owner said he's been through 3 guage clusters and each one had the same issue, with and without the converter in toe. Are those rare-ish 300kph Jap speedo's likely reliable? Reading in kph doesn't put me up nor down.

 

Jeff didn't you used to do those generic ECU maps on a return basis or am I thinking of the wrong guy? :)

Very good post Jeff, thankyou! I didn't realise the VSS went straight to the speedo, then through to ECU. What's the approx VSS location on the gearbox, if it's easy enough to remove there's no harm checking for mechanical wear etc. It does indeed have a converter, the previous owner said he's been through 3 guage clusters and each one had the same issue, with and without the converter in toe. Are those rare-ish 300kph Jap speedo's likely reliable? Reading in kph doesn't put me up nor down.

 

Jeff didn't you used to do those generic ECU maps on a return basis or am I thinking of the wrong guy? :)

 

 

Here is a picture of the gearbox showing the speed sensor>

 

 

 

 

yes I did used to do generic chips but in the workshop only.

 

 

 

 

 

 

Jeff

Ah - much obliged! Haha, I used to have one in my old Z32 TT, my current one has M's FBC electronic boost controller which has a piggy-back into the stock ECU. I'm damn sure it doesn't run half as stout as my old one with 0.9 Mig Tips and your ECU.