Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Rover V8 engine speed resistor

Hi all,

I've been trying to chase down a long term, extremely intermittent fault on my +8 - It occurs very infrequently and currently only after some harder (motorway type driving).
The issue is the engine will sometimes run really rough - possibly missing on one or more cylinders and then cut out when I stop. A reset of the ECU (remove/replace the red key) will usually get me going again. I have done all the usual checks and changes with no success. So I started to drive the car with a laptop connected and logging ECU data with Rover Gauge. Last weekend I think I finally caught the issue. Just before the engine cut out (as we approached a junction) the data shows the engine speed going to 5800rpm! It clearly didn't and the rev limiter would have cut in to prevent it going that high.

Following a bit of research I found there should be a 6K8 Resistor in line from the neg side of the coil to the ECU input pin for engine speed.
I took a look at the car and found two 6K8 resistors - both connected to the neg side of the coil.

The spade contacts all looked pretty manky and resistance measurements weren't as expected, so I cleaned them up and confirmed they then measured around 6K8 ohms. So this maybe the fix.
But I don't have any idea why there should be 2 resistors and the wiring doesn't seem to match anything I can find. I will continue to buzz out the wiring to see where it goes (White background blue stripe - one of the Resistors has a red sleeve over it as well.

Any advice or info would be most welcome.

Thanks

Kev

This may or may not be relevant but when I had problems with my CUX14 ecu the cause was traced to pin34 which tells the ecu what type of gearbox is fitted and what to do with the idle speed. Mine had gone resistive so was telling the ecu I had an auto box and it was doing all sorts of odd things. A rewire (brave) via. 510ohm resistor to watch changed the Rover Gauge to Manual and gave a nice smooth consistent idle at 850rpm.

Might be worth a check - I missed it for some time.

Thanks I'll take a look.

In the case of a Disco, with manual transmission, the second resistor provides an engine speed feed to a trip computer. I believe that it is used to allow the trip computer to determine if the engine is running

Again, in the case of a Disco, the pin 34 on the ecu is connected via a diode to the park/neutral switch on the auto box, which connects to earth when in park or neutral. Thus allowing the ECU to raise the idle when not in park or neutral. In the case of a manual box pin 34 (via the diode) is earthed using a 480 ohm resistor.

Thanks Mike - for both those snippets really useful.

I'm building up a number of thing to investigate when I next get a chance.