Whats in the rev limiter? How does it limit revs?

[majcd213]

Hi All
Ever since I got my car running its had an intermittent problem...it would suddenly die and refuse to restart. This was a caused by a complete lack of spark. I would crank it embarrassingly at the roadside and after a wait of say 10 mins or so it would suddenly restart as if there had never been a problem. Probably the whole thing gets compounded by flooding caused by my attempts to restart. This problem usually strikes within the first 10 mins of running the car- when it goes away it may not recurr until I have left the car for a while and restarted say the following day.

During my investigations I have found several problems- running out of fuel (!), internally broken wire to the coil, detached coil spade connector, broken distributor wire (internal) and dodgy AB14 lead. I've fixed these and although its got better it is still happening.

I think I might now have found another problem in that during these periods of no spark, removing the lead to the rev limiter lets it start straight away. Reconnecting the lead doesn't however stall it. So my question really is what the hell is the rev limiter, how does it do what it does and could it account for this problem?
Well through my digging so far- I think its a Lucas ESL-2 (also used 6 cylinder motors) with a limit set around 6500 rpm. At this speed the limiter interrupts the spark for short periods so that the motor can't go faster.

But how does it do this? How does it detect engine speed and how does it interrupt the spark? Are there any moving points or contacts inside or is it solid state? Finally of course could it account for my problem if it fails? I would expect misfire at speed rather than total lack of spark but who knows? Finally is there a way to bench test it? New limiters are v expensive and a used one- although more reasonable- may simply have the same problem.

Cheers all
Mike

[amarshall]

Yes, it causes problems as it starts to fail.
Yes, it's sold(ish) state.
No, it's not easily repairable.
viewtopic.php?f=27&t=9151&hilit=autopsy

A limiter from an engine with more cylinders will set your limit too high. It's based on sparks per minute, not rpm. 6*6500 != 4*6500.

Get an Omex for under £100 and don't risk another failing 2nd hand unit.

[Lotus-e-Clan]

Limiters usually work by internally shorting the positive coil supply to earth within the rev-limiter circuitry. The pulses from the coil are counted by a chip (eg by using something like a 'decade counter' chip).

The target number of pulses would be set by a combination of capacitor and resistance values. If the resistor is variable (or other logic circuitry selects preset resistance values) then the target revs would be selectable - otherwise its value is fixed to a design target as is the case with the Lotus OE limiter.

When the target number of pulses is detected by the counter chip it will output a signal (or a burst of signals) to something like a miniature mechanical relay on the board which will close a connection (eg short to earth) when the signal is received from the chip. If the internal relay fails it is likely it will stick 'on' -shorting the coil to earth, or it will stick intermittently.

As Angus says -get an Omex Clubman rev limiter - I have one standing-by in case the OE Lotus rev limiter fails. I used the Omex on the Clan before upgrading to mapped ignition and everyone who has used one will agree that Omex has one of the nicest soft limiter on the planet. Although it is bad-practice, you want to hit the limit all the time - just to hear and feel the soft-limiter kick-in!

[majcd213]

Thanks guys- the autopsy was particularly informative... Not worth dismantling. It seems the only way to see if this is my problem is to replace it! Bite the bullet and reach for the credit card I guess...

[amarshall]

This is about the best price I can find at the moment: https://www.ebay.co.uk/itm/OMEX-Perform ... 0005.m1851

I think mine was about 15p cheaper when I got it two years ago.
It doesn't sit in the same place as the old one, but fits nicely in the driver's footwell - pefect size to go on the fuse box cover if you have the later large fuse box.

[Alan_M]

Just to throw another option in. If you are either in need of a new distributor or want an option for some mapping of advance curve. You could consider a 123 Tune distributor which has a built in adjustable rev limiter.

[CHRISYD]

This is about the best price I can find at the moment: https://www.ebay.co.uk/itm/OMEX-Perform ... 0005.m1851

I think mine was about 15p cheaper when I got it two years ago.
It doesn't sit in the same place as the old one, but fits nicely in the driver's footwell - pefect size to go on the fuse box cover if you have the later large fuse box.

im getting one of these. my limiter is disconnected currently. I'm going to limit mine to 5250rpm. (don't rev past that anyway, it sounds too stressed)

[majcd213]

Yep I'd seen that ad and have now got one. Hopefully this will solve my starting woes. Seems easy to fit but I was going to put it pretty much where the original was. Is that a bad idea? I was going to set it at 5.5k as I dont have any business revving that high anyway!

[amarshall]

Seems easy to fit but I was going to put it pretty much where the original was. Is that a bad idea?

I didn't. The shell doesn't look quite as robust as the original. It's easy enough to run the 3 wires through the bulkhead grommet from the footwell into the engine bay.

It's also easier to set and check the limit with the limiter inside the cabin.

[Esprit2]

Mike,

If the budget card trumps all others, then consider two other options that have yet to be thrown on the table.

1) You have determined that disconnecting the rev limiter allows the engine to restart immediately. Okay, so simply install an on-off switch in that rev limiter wire you normally disconnect. Mount the switch in a discrete but conveniently accessible place in the cabin. Now disconnecting is as simple as flipping a switch. Heck, you could do that as soon as the engine dies, and re-start while coasting.

2) Go old-school. Delete the rev limiter and don't replace it. Generations of drivers got along just fine without rev limiters, now cars are considered un-drivable without them... I don't get that. You have a rev limiter, it's called your right foot... it's supposed to be connected to your brain. Re-connect that wire. Just a gentle jab

Regards,
Tim Engel

[Esprit2]

I think its a Lucas ESL-2 (also used 6 cylinder motors) with a limit set around 6500 rpm.

im getting one of these. my limiter is disconnected currently. I'm going to limit mine to 5250rpm. (don't rev past that anyway, it sounds too stressed)

The 907/ 912 is just getting rolling at 4000 rpm, and getting happy at 5250 rpm. It can sustain 7000 rpm, and it can touch 7500 briefly before shifting. A rev limiter set to 6500 is keeping you from the fun part of the rev range. At 5250 rpm, you don't need a rev limiter, you need cruise control.

Just my humble opinion, but you're being way too conservative with an engine that loves to rev.

Regards,
Tim Engel

[Redlotus]

[/quote]At 5250 rpm, you don't need a rev limiter, you need cruise control.[/quote]
or a diesel!

[majcd213]

Thanks Tim... I did wonder why it needed a rev limiter at all... Ive had plenty of cars without one, but i assumed its all too easy to rev to extreme with electronic ig rather than points? I think it came with a 6500 preset by Lucas and I doubt I ever rev close to that intentionally. I had assumed the limiter was more of a safeguard although maybe unnecessary and probably a hinderance on the track.

[Lotus-e-Clan]

Accellerator pedal/cable jams have caught out a few on this forum which unfortunately result in topend rebuilds for the unlucky. Loose footwell aftermarket mats have been cited. Incorrect acc cable return spring refits / damage is another source of woe.

On high rev distress ...my car wouldn't rev well past 5krpm when I drove it home for the first time nearly 20 years ago. After correcting multiple ignition faults mine revved smoothly to the limiter and did not sound/feel distressed. During 2004-ish after a PAS hose failure, I deleted the drag of PAS pump and water pump belts completely (manual steering /EWP fitted) which maximised torque and improved the free revving character of the 912.

I refitted the PAS last year, and although it now has a very light flywheel fitted and does rev better in the low and mid-range (now spins the wheels from a standing start if provoked!), the PAS drag is obvious at the top end. That said, my PAS system has obvous faults (need a new pump) and I've probably made the drag worse by adding very viscous PAS anti-leak fluid.

If your engine feels stressed at the top end and the ignition system etc is in good condition, then check out the PAS system as a possible source of distress. Are the belts running true? Is the PAS pump mounted firm and square (Holburn mounting bolts known to loosen / sheer) and is the pump running as free as possible?

[KevW]

I decided to fit an Omex Clubman limiter last week. https://www.ebay.co.uk/itm/OMEX-Perform ... 0005.m1851

I'd not noticed any problems with the OE limiter but considering it had been living low down in the engine bay for almost 30 years I thought it was probably a sensible bit of preventative maintenance. It was a doddle to fit. I ran the positive and negative wires through the speedo cable grommet and kept them both together with a length of heat shrink. The limiter's earth cable was fastened onto one of the fuse box mounting bolts and the limiter itself was stuck to the clear plastic fuse box lid as per Angus' idea. I've left the OE limiter in place but unplugged. My new rev limit is set to a sympathetic 6500 rpm.

Even though I thought the OE limiter seemed to be working fine I think the engine seems to pick up more smoothly from low revs after fitting the Omex. It could just be a placebo effect but bearing in mind I never expected the Omex to deliver any improvement in running I'm almost convinced. Maybe the OE limiter wasn't allowing a full spark?