Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Revs not going to idle

YK121 (yellow/pink) is one of the three wires from speed sensor and seems to be the shared signal. To this end it's joined to three other wires in a "bomb tail (as shown in Roger's pic) at a point 50 cm from the main chassis loom where stalk, ignition switch and immobiliser sub looms split off, so likely in the dash or fusebox area. This connection allows the speed signal on yellow/pink wire to be shared with ECU (yellow/white), TSM (yellow) and speedo (pink) wires.

The speed sensors other two wires, yellow/black and yellow/red, both go to ECU via chassis/engine connector as you have noted. Note there are NO chassis/ground connections directly related to speed sensor therefore.

Hope this helps Laurens.

Once more to all, thanks for the excellent advice.

As Chris advised, I swapped the Turn Signal module. No result.
Checked the "bomb tail", as Richard advised. No result.
Sprayed all connectors under the dash with contact spray. Reconnected and cleaned all grounds.
GLLOQ suggested spraying the intake manifold, but the idle goes back to normal as soon you are at a standstill.

During my test ride today, it happened that when driving downhill, I had a high idle and needed to brake all the time as the engine wouldn't brake as normal. Suddenly out of the blue, I felt the engine braking again, and the high idle was gone.

The other moment drove at 50 mph, put the engine in neutral, and there was high idling again. When in neutral and still rolling, I did rev up the engine to 5000 several times. High idle remained. Came to a halt. Idle dropped to normal. Start rolling again, and high idle started as well. Suddenly out of the blue, idling went back to normal.

During all my test rides, I switched the gear many times to neutral. 70% of that gives normal idling. 30% high idling. When at a standstill 100% normal idling.

I have the feeling it has something to do with the Idle Valve Control. But I swapped IVC already with the other car and swapped as well the complete manifold unit, including IVC, with the other car. Both had no results.
Perhaps the wiring from the IVC? But that goes directly to the ECU plug.
Secondly, how to check this wiring when it is intermitted a problem?
And what is the relation with the speed sensor, as all is normal at a standstill.

Time for a carburetor?

Forgot to mention that idling is all the time normal if I disconnect the speed sensor plug at the wheel. There is only no speed reading, no tacho reading and no automatic switch off from the turn signal. The rest is all OK.
This is to make the headaches complete.

During tracing the wiring I found some sensors shared connections (reference and/or return?) with the speed sensor. This probably explains why you loose tacho reading when speed sensor disconnected.

So by way of a bit of a summary and having re-read the Protune manual:

The IACV is driven by the ECU and if it's working normally without the speed sensor connected then that seems to rule out IACV itself being the problem.

The Speedo and turn signal cancelling works when the speed sensor is plugged in so the wiring between these would appear OK.

The ECU controls the IACV and at start-up it starts with a high idle and as the engine warms up it moves to a target idle speed set in the ECU. This idle is user set in the range of 900 - 1,100 rpm and as I recall is normally about 950 for our cars.

As you move off the idle speed is raised to improve drivability and it will return to the target value when you come to a halt. This is normal and your "good" car should do this if you coast to a halt in neutral.

There is an option for the ECU to use the engine temperature input to dynamically adjust the target idle speed, but I don't think this is used in our cars.

From the S&S manual - The vehicle speed sensor is routed first to the speedometer and then the speedometer creates a signal that
is sent to the ECU.

So if the speedo is working OK it could be the problem is in the wire(s) from the speedo to the ECU. I would suggest unplugging the speedo and ECU and then doing a continuity test on the connecting wires.

Fingers crossed and good luck

Tracing the wiring on chassis and engine looms I don't think the quoted routing from the S & S manual has been implemented by MMC Chris.

Just one of the three speed sensor wires, the yellow/pink is split in the chassis loom to feed TSM and instruments as well as being sent to the ECU. The other two yellow/black and yellow/red, route to engine loom and nowhere else in chassis loom.

As suggested I believe these latter two are common reference signal and maybe return to benefit just the ECU. This is born out by the fact that each split via bomb tails within the engine loom to provide common connections as follows:

YR14 ECU
YR15 speed sensor
YR16 MAP sensor
YR17 throttle position sensor

YB27 ECU
YB28 speed sensor
YB29 l/h lambda sensor
YB30 r/h lambda sensor
YB31 engine temp sensor
YB32 MAP sensor
YB33 throttle position sensor
YB34 air intake temp sensor

Given above I conclude that the speed sensor main signal is fed raw directly to TSM and speedo and probably referenced to ground since such is readily available. The speed signal does not appear to be routed through speedometer therefore since it's sent directly to ECU as well via the YK bomb tail.

The other two bomb tails (YR and YB) on engine loom may be worthy of investigation however. Also all sensor connections.

Failing that maybe another ECU could be tried. Indeed given the consistent elements of fault I'm beginning to doubt an intermittent connection.

Hopefully this explanation may help you in fault tracing of your own Laurens.

I checked the wiring to and from the speedometer, but all looked OK.
I searched for the two bomb tails; YB and YR finally found them as they were taped in the engine loom.
I took them out and checked them as well as possible, as they were in a very difficult-to-reach position under the battery.
But also that didn't bring any result.
During the test ride, I was coasting to a red traffic light. Idle was normal at 1000rpm. Waiting for the traffic light for 15 seconds with normal idle, suddenly engine light came on for a brief moment. Idle went up from 1000rpm to 1200rpm.
The traffic light went green, pulled up and as soon as rolling, switched to neutral, and idle was at 2500 again. One kilometre further idle went normal again and remained normal for the next 20 kilometres. And suddenly, high idle was back again.
So headaches are still there.

The engine light coming on should have triggered a fault code in the ECU. I really think you need to find a way to plug the ECU into a laptop and see what Protune says. It might point to the source of the problem.

I received a replacement sensor from China.