Lucas AMM adjustment

[philb]

Hi all, after searching the forums, I decided to go and adjust the amm to the factory 336 ohms. It was set at 502 ohms. I've searched for topics about lucas amms, and I can't see a consensus on whether reducing the resistance leans or richens the mixture. I started the car and it seems to idle fine, haven't got a chance to take it for a run yet. I'm hoping for more fuel economy, but I may have just richened the mixture. Can anyone shed more light on this?

[Fred]

Hi,

I'm afraid the factory manual doesn't say which way is which on Lucas AMM setting. Very brief reference at bottom of one page

But what it does say is that this setting is ignored when you have Lamda sensor and later ECU. So it shouldn't make any difference. All the LPT's (including mine) have Lucas and Lamba. There's a wire missing from the harness on these later cars. It the wire to pin 6 on the AMM. See snippet of wiring diagram attached.

I've never seen a non lamda Lucas system though they do exist.... But if I do I'll try tweeking mixture.

Fred

[ShadowWorks]

I have the Lucas AMM and i know i have a lamda sensor somewhere as there is a fuse for it and when i pull it out my T16 runs like a dogs dinner but i cant find the Lamda anywhere and Alan also had a look?

The lucas has a wide range setting i think from 390 to 500 ohms, but i have never messed with mines so i dont know how it affects the fueling?

[Kaas]

Less ohm = more lean
More ohm = more rich

Sha: Your lamda sensor is placed in front of your turbo exhaust bend, you need to get your head down to the batteri, follow the small wires from the top of inder wing and under the batteri!

[philb]

Thanks, since mine is a 91 lucas lpt, it would be the same as yours Fred, so it makes no difference

[nbell]

I have a 93 car, so not sure if any adjustment would help mine. I think its running rich so wanted to try the adjustment anyway. Checked and got 476 between the 3 and 6 terminals, if I am using my meter right.

I tried adjusting the screw, but it took 12 anti-clockwise turns of the black plastic screw to get it to 426, and then further turns did not help. In addition the screw seems kinda loose.

Anyone can comment on this? Did your Lucas AMM need that many screw turns to adjust? I expected it to need much less turning. Car seems to run ok, will check plugs and mpg after some miles.

[Kaas]

You need to messure pin 6 and 1 mate! And you will see diffence right away you starts to turn the screw!

[philb]

Just as a followup, I'm pretty sure that it has helped economy, now getting 30mpg to a tank, mixed city and long runs, fair bit of welly.

[Matthew]

A common fault with AMMs is that they don't respond to adjustment. Despite that being classed as terminal AMM failure, Boxman told me that the fault is repairable.

Dunno if he posted the repair though.

12 turns for 50 ohms reduction in resistance is wrong.

[nbell]

Originally Posted by Kaas

You need to messure pin 6 and 1 mate! And you will see diffence right away you starts to turn the screw!

Whoops! The info I read said pins 6 and 3 I will re-adjust tomorrow. Fortunately it looks like I can lay my hands on a replacement AMM too, will be nice to see if I can improve the mpg (21-22ish, thats 320 miles to a tank).

As always, cheers for the help guys.

[nbell]

Adjusted impedance between pins 1 and 6 on the AMM to .38 on my meter, and changed the plugs, miles went up from 320/tank to 380 Recalculating the mpg more accurately I got 380 miles on 52.69 litres. I make that 32-33 mpg (imperial gallons).

Hopefully have another '93 AMM coming this week, so will be able to test and compare and try and improve this figure.

[Matthew]

33MPG is decent mileage from a C900...

[nbell]

Yes, I am going to get the turbo leak resolved tomorrow (exchange turbo) so will be interesting to see how better it runs with nice clean plugs. Going to use that spark plug guide to try and get them right.

Also going to stick some redex injector cleaner through the car, see if that helps. But going from 320 to 380 on a tank (with a lot of fast motorway miles in that) I was really pleased with.

I also read with interest that some posters prefer the Sainsburys or Tescos high grade petrol. Thats on the list for testing too.

[Matthew]

Recommend against using injector cleaner. I have read too many "engine problem/breakage after using injector cleaner" stories.

In one case, the cleaner damaged an injector which leaked fuel into the cylinder overnight and hydrolocked the engine

If the injectors need cleaning, then there's really only one way to do it properly. They should be removed and taken to a specialist, where they'll be ultrasonically cleaned and flow tested. Costs around £15/injector.

You could always snag a spare set of injectors from a scrapcar and get those cleaned, so you can swap them straight over with no time off the road.

[nbell]

Originally Posted by Matthew

Recommend against using injector cleaner. I have read too many "engine problem/breakage after using injector cleaner" stories.

Ooooo, OK, thanks Matthew.

[ShadowWorks]

Originally Posted by nbell

Whoops! I can improve the mpg (21-21, thats 320 miles to a tank).

I would have said thats really good and now your getting 380 a tank? oh boy.

I dont even get 200 miles from a tank, £15 got me 41 miles this week

OK a new or used AMM is on the list of things for me to get, 33Mpg is a fantasy figure for me on the 900c, that would be like Christmas come early

[kemosaabey]

Originally Posted by ShadowWorks

I would have said thats really good and now your getting 380 a tank? oh boy.

I dont even get 200 miles from a tank, £15 got me 41 miles this week

OK a new or used AMM is on the list of things for me to get, 33Mpg is a fantasy figure for me on the 900c, that would be like Christmas come early

thats bad - I'm moaning when I get 50 miles to the tenner...

The 9000 was the best mileage I ever got from a saab - 33 mpg!

AMM's can actually go lean as they age and respond slower....especially if the burn off function has failed and the hotwire gets coated....

Worn meter is less crucial on a lambda car - unless it's way out as the sensor takes over once the car is warm...

[nbell]

I've got another AMM from a 93 coming, so going to compare that with mine. Also must get round to having the timing and mixture checked, I don't have the tools for that.

I did wonder, if the tank is meant to be 68 litres, why I could only squeeze in 55-odd when the petrol light had come on. Either the reserve is v.generous or the fuel gauge is in need of adjustment.

[900t]

On a 93, the reserve is something like 8.5 litres, plus there is always that "extra expansion space" too.

[nbell]

Originally Posted by 900t

On a 93, the reserve is something like 8.5 litres, plus there is always that "extra expansion space" too.

Ahhh, thats bigger that I thought, I would get 50-odd miles after seeing the petrol low light come on. Useful, no need for me to sweat so much now when I see it on the motorway