The Lotus Grand Tourers

Hi all,

Does anybody know what reading you should expect testing the distributor pick-up of a 1990 Excel SE with an ohmmeter? I am trying to work out whether mine needs replacing with a few tests before pushing the button. I've got no spark as you might have guessed! I can get a HT current from the coil secondary when I simulate the lifting of the ground that the AB14 does - so all good there. I have also tested the AB14 with a Lucas YWB135 tester and it passed. I've also tried removing the rev limiter which I was informed will stop a spark if it has failed, but that made no difference.

I am going to use the YWB135 to simulate the pick-up as 2 of the leads do create the AC current that would be seen by the AB14, so that will give me an in circuit test of the ignition amplifier that will confirm the HT current is getting to the distributor cap. If that passes the issue must be in the distributor. I should say the car had not been started for 14 years and the pick-up does have some grime on it with the cap off. As it has no moving parts I want to check it electrically before replacing it.

So, if anyone can help with what should be seen in terms of resistance across the pick-up terminals that would be great.

Duncs

LotusMonkey wrote: ↑

Sun Dec 26, 2021 23:50

Hi all,

Does anybody know what reading you should expect testing the distributor pick-up of a 1990 Excel SE with an ohmmeter?

~3200 Ohms

Marten wrote: ↑

Mon Dec 27, 2021 15:25

LotusMonkey wrote: ↑

Sun Dec 26, 2021 23:50

Hi all,

Does anybody know what reading you should expect testing the distributor pick-up of a 1990 Excel SE with an ohmmeter?

~3200 Ohms

Thanks Marten - I’ll treat anything else such as a short as problem isolated!! Duncs

Mine reads about 2700 ohms, and I have a spare one which reads about the same. I would be more concerned about open circuits than shorts. The pickup coil itself is fairly robust, but the two cables that travel to it get flexed every time the mechanical advance operates on the distributor. Unplug the black connector leading from the AB14 to the distributor, and connect your meter across its pins. This is where you should see the 2700 - 3200 ohms. Then remove the distributor cap and twist the rotor arm (it will only go one way) to activate the mechanical advance. The reading may vary fractionally as you move the rotor arm, but not more than a few ohms, and you should certainly not have an open circuit at any point. An open circuit would indicate a break in one of the cables.

While you have the multimeter connected in this fashion, try flexing the cable between the distributor and the black connector, these have been known to become brittle too.

barker_001 wrote: ↑

Mon Jan 03, 2022 21:43

While you have the multimeter connected in this fashion, try flexing the cable between the distributor and the black connector, these have been known to become brittle too.

Thanks - I got open across the terminals when I tried them and have tested the AB14 in circuit using the Lucas tester so I think we’re safe to say it is the pick up.

Thanks for the tips as I will check the resistance on the new one before I fit it in.

Duncs

Glad you managed to find it. It's a while since I bought my pickup, can't recall which of these two suppliers I had it from, but both have been very helpful over the years:
http://www.distributordoctor.com/
https://simonbbc.com/

Or SJ Sportscars now have them in stock too:
https://www.sjsportscars.com/parts-and- ... 2E0050.htm

barker_001 wrote: ↑

Wed Jan 05, 2022 20:35

Or SJ Sportscars now have them in stock too:
https://www.sjsportscars.com/parts-and- ... 2E0050.htm

Picked one up from SJ - so hopefully once fitted with the new timing belt it’ll be time to start getting some life out of the old girl!

So the saga continues!!! New pick-up fitted and measuring 3kohms. Cranking no spark, but I do get a spark at the coil feed when I use the Lucas tester to simulate the pick-up feed into the AB14. So, amplifier and coil circuit is fine and the problem still seems to be at the pick-up part of the circuit.

Continuity and resistance on the pick-up extension lead are fine. Voltage read at pick-up and at end of extension lead is the same - around 75-80mV when cranking. Does anyone know what this voltage should be? The Lucas tester seems to push out much nearer 900mV when used.

I am running out of parts to replace now and am starting to wonder about the distributor magnet?

Any wisdom welcome

Duncs

Duncs,
About 10 years ago I (or rather, LotusBits ) replaced my original 1992 SE ignition system with a mapped DTA ignition system.
I still have the original ignition parts in a box, in case any future owners wants to revert to the original.
You are welcome to borrow and try these parts (coil, distributor, AB14 etc) and see if you can determine which of yours is failing.
I live in Yateley, NE Hampshire, about 1 mile from the Surrey border.

Let me know,

Tony

tonypoll wrote: ↑

Fri Jan 14, 2022 18:43

Duncs,
About 10 years ago I (or rather, LotusBits ) replaced my original 1992 SE ignition system with a mapped DTA ignition system.
I still have the original ignition parts in a box, in case any future owners wants to revert to the original.
You are welcome to borrow and try these parts (coil, distributor, AB14 etc) and see if you can determine which of yours is failing.
I live in Yateley, NE Hampshire, about 1 mile from the Surrey border.

Let me know,

Tony

Hi Tony, that is really kind of you but I know the AB14 and coil to be good as I can get a spark using a Lucas tester with the AB14 in circuit (and the coil is new). It’s a new pick-up. I am certain the problem is something to do with the pick-up and magnet at this point. I’ll bare it in mind if things get silly, but I am hoping it is something to do with the pick-up induced AC voltage.

Duncs

For anyone interested I am going to keep adding to this thread as I learn. Don't want to fill people's inboxes, but given the I am learning bits in my research that don't appear to be elsewhere in the forum they may be useful to someone else.

The pick-up voltage varies according to RPM and can be anything form a 30mV up to 50V at higher RPM. The YWB135 Lucas Analyzer produces a 2.5v peak to peak AC Sine(ish) signal at a rate of 20ms. I know this as I put mine on my oscilloscope. If anyone is looking to simulate the pick-up in the future with a frequency generator, to test their amplifier that is the way to go

I have realised that my new high torque starter motor actually runs slower than the old starter did, and comparing it with a few videos on YouTube. I am starting to wonder if my battery may be part of the issue with the voltage drop across the starter meaning there is too little juice to run the ignition amplifier when cranking. Will run some voltage and CCA tests and see what gives. Engine bay earth brades have also been replaced, as the old one was half worn away from rubbing against a brake pipe!

Duncs

A weak battery has been know to do these things. It was my plugs that did it for me 5 years old

Posting an update as now have a spark - hooray! Turns out that it may well have been a problem with the starter motor drawing excessive current. I got a new 760CCA battery, and tested the grounds and voltage drop on positive and negative side at turnover right back to the battery, and losses were tiny. Starter had always seemed to turnover slowly, and was no different with the new battery. Replacing that starter, with another old one led to faster turnover and a spark all the way through to the plugs. Which means that the new pick-up I got was fine too. Exciting times, as have serviced the carbs already so it is time for a new accelerator cable and to start seriously thinking getting her turning over, pending cam belt change.