| Author | Message | ||
     Lars Linders New User Username: olazz Post Number: 5 Registered: 03-2026 |
Hi all, I have a 1981 RR shadow that has had an SU conversion from Solex, done by the previous owner. The car runs beautifully smoothly when cold, but after prolonged driving at constant speeds, and when very hot, If I try to accelerate past 60mph, or put more load when approaching an incline, the car becomes very lumpy and the speed starts to fall away, almost as if there is pronounced engine breaking. No amount of throttle helps and the car just continues to slow. If I lift off throttle and the speed drops below 50mph, or I dap the brakes, it will pick up again and run smoothly, provided I don’t exceed 65mph. (quite scary when overtaking a large truck and the car just slows to under 50mph with the truck barreling up behind us. ) My first thought was fuel starvation…. but it doesn’t do this when it’s cold… So next thought was the coil could be breaking down as it gets’ hot. It is running a replacement Opus ignition ( 2 years old) I have no idea of the age of the coil. Just returned from a 900 mile trip, and no other issues with it. It runs smoothly below 60mph. Probably the best way to describe the effect is if you have cruise control on, and the speed exceeds the set limit... the car auto slows with some gusto... if that make any sense. My first thought was fuel starvation... but it doesn't do this when cold... so next is possibly the coil breaking down... Or could the temperamental cruise control's electronic box, be interfering with the engine and braking it? I dont use the the CC as it doesn't always work and is usually in the off position on the stalk. Would anyone have any other ideas for me to look at? PS. the previous owner also installed a modern electric fuel pump. | ||
| Lars Linders New User Username: olazz Post Number: 6 Registered: 03-2026 |
Just to clarify, this is a RR Corniche 1981 not my Shadow, apologies for the error | ||
| David Hughes Experienced User Username: wedcar Post Number: 190 Registered: 07-2004 |
Hi Lars Others will know better than I, however, first suggests a faulty coil. Regards David | ||
| Jeff Martin Frequent User Username: jeff_r_1 Post Number: 763 Registered: 07-2018 |
Also suspect the Opus module... I'm going to assume that the replacement was used and not NOS. Installing a used Opus module is defeating the purpose. Start with the coil first, and only the coil. | ||
| Lars Linders New User Username: olazz Post Number: 7 Registered: 03-2026 |
JM, I've recently acquired the corniche and the previous owner said it was replaced with an upgraded Opus with modern components 18 months ago. I will be replacing it with a Pertronix igniter. I fitted one to my Shadow when it FTP earlier this year. Do the ballasts ever break down like a coil? I understand how that ballast works but could that also be a culprit? I will also change the condenser, once I've done the coil first. Any other advice would be greatly appreciated | ||
| Jeff Martin Frequent User Username: jeff_r_1 Post Number: 764 Registered: 07-2018 |
Ballasts are just a simple resistor, if they fail, the car won't run at all. They're going to be either an open or a closed circuit. Old coils fail because the varnish on the thousands of windings on the secondary windings (the primary windings too) breaks down and causes internal shorting. This can get worse when the coil gets hot because so much internal resistance build up, it won't won't produce "spark" anymore. | ||
| Jeff Martin Frequent User Username: jeff_r_1 Post Number: 765 Registered: 07-2018 |
You may find this useful: https://www.rollsroyceforums.com/threads/how-to-bypass-the-ballast-resistor-ue37053.36692/?post_id=240915#post-240915 | ||
| Lars Linders New User Username: olazz Post Number: 8 Registered: 03-2026 |
That's extremely useful Jeff, many thanks for your input. | ||
| Lars Linders New User Username: olazz Post Number: 9 Registered: 03-2026 |
UPDATE ! I have only recently acquired this Corniche and going through the raft of paperwork last night, found a number of things. Coil, HT leads, Opus (exchange), rotor, fuel pump, had all been replaced 16 months ago. My concern about a failing coil looked unlikely then. On further reading, I came across a 4-page report about the speed control. It had stopped working and according to the “specialist” who carried out the repair, they found 6 severed wires by the gearbox and similar in the loom. So they apparently repaired the wiring and concluded that the speed control worked. Well it does to an extent…. It disconnects when you use the stalk or touch the brake, but won’t resume speed at the stalk. I did have a nagging feeling that the car’s faltering was very similar to a cruise control speed interruption for overspeed, but didn’t think the car had an electronic speed module, that cut ignition and as it only happened when engine was hot, I naturally dismissed that idea. The cruise control was switched off anyway when this happened. At 0300 this morning I woke with a thought, what if the problem is indeed speed control linked. I found the car does indeed have a Speed control "Module" With nothing to lose, I decided to remove the CC fuse and take the car for a drive and Bingo! Pulls like a train with zero problems or issues. I drove 10 miles and tried it a dozen times from different speeds. Clearly the UD25279-A Speed Control "Module" is interfering with electronics somehow, or was stuck at set speed. I have replaced the fuse and the problem has gone (assume SCM has reset) and the car repeatedly pulls without hesitation. So I think I have found some, if not the entire problem. I have not tried the cruise again and will leave that to the weekend when I have more time to investigate it. Now to learn about how this module functions and what it does, or do I have different electrical problem from the previous repair. Thank you all for your help and ideas on this, thus far. Lars | ||
| Jeff Martin Frequent User Username: jeff_r_1 Post Number: 766 Registered: 07-2018 |
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