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

Hi Laurens

Yes it sorted itself out, totally baffled.

Hi Laurens, an explanation based on further investigation and info from yourself.

Fuses 7, 8, 9, 10 and 11 are all fed directly and exclusively from ignition switch, all using green/yellow wires.

With ignition off you tell me operating hazard switch fed by power from fuse 2 (purple/black wires that also feed horn) allowing all indicators to flash as normal. Unfortunately this also causes fuel pump to run and oil pressure warning to light. You also tell me that removing fuse 9 stops this unusual behaviour.

My first thoughts are that somehow power from the permanent hazard switch feed is looping through to the circuit(s) fed by fuse 9 (green wires, stalk switch and instruments) thus allowing a back feed through this fuse to all the other ignition controlled fuses mentioned above, making them all live! Fuse 10 then passes power to the engine relay coil (as it would with ignition on) and the ECU completes the relay coil circuit with a ground, energising it. This puts the permanent power feed into relay on to the fuel pump. With the engine relay energised the oil light comes on as well since engine not running. When headlamps are switched on fuses blow as all power with ignition off is passing through the 7.5 amp fuse 9.

I know this doesn't offer an answer to your issues but (assuming above scenario correct or nearly so) gives you an area of the wiring to concentrate search on. I would be looking for a failed component or wiring causing a short of permanent feed to another circuit.

ETA: fuse 10 is another possibility for the back feeding and that normally feeds the infamous TSM and flasher relay. The results would be the same.

Hi Richard and others,
Again thanks for the reply and explanation of this difficult matter.

I did today a few test rides without fuse 9 installed. All worked normally, regarding lights and switches. So, for now, I will leave fuse 9 out as long as everything works well.

Yesterday I thoroughly inspected again the cable from the wheel speed sensor to the main chassis loom. Didn't see any damage or chaffing on it.
During the first test ride today, the high idling was much more present than before (but still intermittent).
So I gave that sensor cable an even more thorough third inspection by bending and repositioning the cable.
Went for a ride again. Now the high idling did get worse. Still intermittent as before, but the idling is now around 3200 rpm!

This makes driving downhill almost impossible and dangerous, as you can't control your speed anymore.
When going downhill, you feel directly when the high idling is present because it comes and goes out of the blue without any signs that could make it start or stop.

When I take the connector from the wheel speed sensor apart and go driving, idling works normally again! But by doing so, there is no speed indication, and the turn signals remain on after doing the turn.

Is it possible for you to explain to me how the wheel speed sensor works?

There is a WB 120 and a YR 119 wire which goes both to the engine loom connector. The YK 121 wire goes to the SP 5 wire bomb with Y100-K101-YW102.
I searched for that SP5 bomb as it is taped in the big loom under the battery. Opened the taping and inspected the bomb, but all looked solid pressed to each other.

I already swapped the wheel speed sensor with the other car, but that didn't solve anything.

Laurens, have a look here at my post several months ago concerning speed sensor. Note that only one of the three wires from this sensor, yellow/pink, is used by the TSM and speedometer itself via the chassis loom distribution bombtail you refer to with different colour wires. From your explanation both these devices appear to be working correctly though. The other two are connected exclusively to the engine loom via its connector and are shared with other sensors so assume they form part of a network bus benefitting the ECU.

Some monitoring of the signals may be appropriate if it's possible whilst driving using Protune. Failing that just measuring signal voltages when idling normally and not and comparing with same on your other car.

Finally and a long shot, has the adaptive map got messed up and the ECU gone into limp mode, if there is such although would then expect MIL display. I remember some very similar symptoms with high and intermittent idle when the throttle stepper motor on a Mitsubishi partially seized.

ETA: as a temporary workaround and if car runs ok without speed sensor, you could disconnect just the yellow/white wire from the chassis distribution bombtail. This should leave the ECU without speed reference but TSM and speedo should still benefit from sensor.

Laurens, just a thought have you inspected the pins inside the wheel speed sensor multi plug attached to the suspension. Mine was showing signs of surface corrosion so I cleaned them up with a small screwdriver and switch gear cleaner and then WD40 on re-assembly as my speedo had stopped working which had affected the cooling fan and indicators.

On reading all of this it seems to me that the issue is in the wiring from the speed sensor to the TSM and ECU, either a break in a wire somewhere or a loose/dirty connection. I suppose the best solution would be to clean the connectors then if no improvement obtain a new wiring loom, not sure how easy to obtain or expensive that is though.

According to Laurens last post above he has thoroughly checked out speed sensor and associated wiring and connectors. Note he reports the speedo works correctly and TSM cancels the indicators when moving. The ECU is obviously receiving something as well since the idle problem dissapears when he disconnects the speed sensor. This of course interrupts the speedo reading, hence my suggestion of just disconnecting the ECU speed feed as a temporary measure where there is a three way split at a bombtail.

BTW please note this present discussion relates to pre-2014 M3W's. Among others there are many differences to the TSM on the later models most notable being the integral indicator relay and a different connector with more pinouts. Main loom is obviously different to accommodate, plus of course cooling fan, heated seats and likely many more minor changes.

Would it be useful to get hold of a complete wiring loom, dissect it and get a correct wiring diagram layout with colour and cable routes etc. compiled? As I assume the only available wiring diagrams are the ones shown in the parts book?

The wiring schematics included in the parts books introduced across the Classic and M3W range from around 2006 onwards are difficult to read initially, having a degree of OCD helps though . Given the huge increase in electrical and electronic complexity they are probably one of the few practical solutions by effectively compacting the detail into the display of the physical looms that carry all the cables and their connectors, whilst adding loom dimensions not available in the old format. Each cable run is uniquely identified at each end with colour code and number. Multiple connection/distribution points (bombtails) with cable identifiers are also shown with location on loom.

The introduction for the 3.7 Roadster of 25 or so additional old style wiring diagrams covering in isolation engine and body aspects from battery to devices, was a big step forward and greatly eased cable tracing and specific fault finding. Sadly it seems never to be repeated for other models.

I would add I've found relatively few errors in the schematics which also include fuse box layout and wire codes. I admit to finding cable ends that appear to vanish but without exception find the other coded end eventually . I simply don't see the need therefore to reverse engineer a full wiring loom when it's effectively been done and for each variant.

I agree, from what Laurens says the speed sensor, TSM and ECU appear to be working although not as they should which possibly indicates an erratic signal input throwing off the components. Having experienced over 44 years of Morgan erratics especially electrical I can't remember how many times it came down to poor connectors, bad earths, shorts or an invisible break in a cable that produced intermittent connectivity. Most recently my temperature gauge on the +8 started reading hotter than normal then back to normal temps and was always reading about 10 degrees above actual temps, the thermostat connector seemed a bit loose so cleaned and tightened it up now consistently reading 5 degrees cooler; but of course I bow to better judgement.