Is it advantageous to fit a relay in my Dolomites starter circuit?

[Richard the old one]

I am aware that some mechanics fit a relay in the Dolomite starter circuit.

I do not know if this is to improve the starting of the car by getting more volts to the starter solenoid or to prevent the ignition switch contacts from burning out.

My question is what is the purpose of fitting the relay and is it advantageous?

[jikovron]

Your right, it reduces the amps handling of the ignition switch, I had an issue with a performance starter on a skoda estelle where the solenoid drew a tad too many amps so would fail to bridge the main contacts resulting in random none cranking.
I added a relay onto the back of the solenoid which instantly solved the problem due to having direct access to the battery effectively.
Its probably seemed unnecessary given the likely short duty cycle but dolomite wiring is supremely poorly specced imo.

[Carledo]

Not necessarily your problem, but being rather involved with a number of auto Sprints, I always fit a relay to the starter circuit, not so much for the benefit of the ignition switch, which is fairly robust, but to save the rare, fragile, inaccessible and horrifically expensive transmission inhibitor switch.

Really though, it's a no brainer, yes the system was adequate when the cars were new (being kind with "adequate") however the cars were designed with a lifespan of about 10 years max, most of the survivors have quadrupled that and still counting! Bearing that in mind as well as the increasing cost and rarity of model dedicated electrical components like ignition switches, surely it makes sense to future proof such things. Relays cost less than a fiver and even a semi competent amateur can fit one in a couple of hours with minimal tools. Why wouldn't you?

I've seen relays fitted to all manner of things that DIDN'T really need them, rear foglights (<4a) screenwashers (2a? and only in very short bursts) Horn (5a for maybe 2 secs max) and seen the same cars NOT fitted with headlight or starter relays with current draws in the 10s of amps. Doesn't make sense! I like to put one on the HRW too, but that's just me!

Steve

[tinweevil]

Yes, I did on my 1500HL and it did help solenoid throw & cranking. Knowing me there may well be, somewhere on this forum, before & after voltages.
Use a good quality relay. I bought a batch of ten average quality, two fitted up under the eyebrows for headlights lasted fine for years. In the engine bay on the ns turret (minimum current path) exposed to heat soak they only lasted ~ 6 months daily use.
(Disagree on the HRW, not enough switch cycles to cause an issue)

[Carledo]

Disagree on the HRW, not enough switch cycles to cause an issue.

I relay the HRW, not to save the switch (never lost one yet) but to help the elderly and barely adequate wiring. It also seems to me that if you use a long high current wire to feed it, you'll get heating of the wire in service which cuts down on heat in the window element. Mount the relay in the boot (easy on my cars, the battery is there already) and all the current goes to the window, so should be hotter and wiring protected too.

Steve

[payload]

Another consideration: I also fitted a relay for the starter, and I took the opportunity at the same time fitting a hidden switch in the start circuit, which gave me a simple immobiliser. Not the last word in security of course but could deter opportunist thieves.

Ian

[cleverusername]

The starter circuit already operates in a relay like way. When you turn on the ignition, you throw the starter solenoid and that activates the starter motor. I am not sure that I would want to add a relay, it seems like adding another point of potential failure.

[Richard the old one]

Over the 40+ years that I and my family have been driving 1500HL Dolomite I have had to change, at a guess three or four ignition switches so they are fairly reliable considering the miles we have driven over that period. However I was surprised how large the initial current taken must be when I looked up the resistance values of the Solenoid pull in coil. This is only 0.21 to 0.25 ohms and the hold in coil is only 0.9 to 1.1 ohms. Both of these windings are initially in the circuit until the solenoid plunger is pulled in and the pull in winding is shorted out. If you work out the figures using V = IR it works out at over 50amps. I accept it is for a very short time and then the circuit only has to supply the hold in current, which could be as low as 6 amps as the starter motor drags the battery voltage down. Based on a sample of relays that I have they seem to have an operating coil of about 60 ohms so this would reduce the current that the ignition switch has to handle to a maximum of 0.1 amps if the battery voltage drops to about 6volts.

The one question I have for people who have fitted a relay. Have you found that the normal 30 amp relay to be adequate or is there a heavy duty relay that you would recommend?

[Richard the old one]

Over the 40+ years that I and my family have been driving 1500HL Dolomite I have had to change, at a guess three or four ignition switches so they are fairly reliable considering the miles we have driven over that period. However I was surprised how large the initial current taken must be when I looked up the resistance values of the Solenoid pull in coil. This is only 0.21 to 0.25 ohms and the hold in coil is only 0.9 to 1.1 ohms. Both of these windings are initially in the circuit until the solenoid plunger is pulled in and the pull in winding is shorted out. If you work out the figures using V = IR it works out at over 50amps. I accept it is for a very short time and then the circuit only has to supply the hold in current, which could be as low as 6 amps as the starter motor drags the battery voltage down. Based on a sample of relays that I have they seem to have an operating coil of about 60 ohms so this would reduce the current that the ignition switch has to handle to a maximum of 0.1 amps if the battery voltage drops to about 6volts.

The one question I have for people who have fitted a relay. Have you found that the normal 30 amp relay to be adequate or is there a heavy duty relay that you would recommend?

[Carledo]

Over the 40+ years that I and my family have been driving 1500HL Dolomite I have had to change, at a guess three or four ignition switches so they are fairly reliable considering the miles we have driven over that period. However I was surprised how large the initial current taken must be when I looked up the resistance values of the Solenoid pull in coil. This is only 0.21 to 0.25 ohms and the hold in coil is only 0.9 to 1.1 ohms. Both of these windings are initially in the circuit until the solenoid plunger is pulled in and the pull in winding is shorted out. If you work out the figures using V = IR it works out at over 50amps. I accept it is for a very short time and then the circuit only has to supply the hold in current, which could be as low as 6 amps as the starter motor drags the battery voltage down. Based on a sample of relays that I have they seem to have an operating coil of about 60 ohms so this would reduce the current that the ignition switch has to handle to a maximum of 0.1 amps if the battery voltage drops to about 6volts.

The one question I have for people who have fitted a relay. Have you found that the normal 30 amp relay to be adequate or is there a heavy duty relay that you would recommend?

I tend to use a 40a relay, the ones I buy are only a few pence more than the 30a version and more is better, right? I do have a slight leaning towards Victorian engineering standards!

Steve

[sprint95m]

My question is what is the purpose of fitting the relay and is it advantageous?

The Rover P6 3500 came with a relay as standard.

You can buy a kit (from specialists) to incorporate a relay into a Triumph Stag starter circuit.
This is done to prevent the dreaded click (from the starter) when you try to start the engine.
Given that slant four engined Dolomites also suffer from this I am thinking copying this idea is worthwhile,
however,
some time ago Jeroen stated that such is a bodge but, as I remember, did not say why.



Ian.

[MIG Wielder]

The MGB also was supplied with a high current relay from day 1 . I would say that this was a starter circuit that had been properly engineered. To re-install; a relay that was omitted to save initial cost to give a good engineering solution, can never be a bodge.

The MGB starter relay is a type Lucas 6RA; and this is one case that I'd try and obtain a genuine O.E. part. There are some on E-bay. There has been a recent article in the MG mag; on the short-lived copies of this Lucas 6RA part. The sample that failed in less than 2 years was dismantled and it was found that the contacts were poorly aligned.

I used to have a source of modern high current relays ( Richard is quite correct, the initial current is >50A ) and I have used these.
But they do need the large yellow crimp terminals to fit the high current contacts.
I recommend these.

https://uk.rs-online.com/web/p/non-latc ... s/8113097/

Tony.

[Bumpa]

I used the exact same relay from rs-online as Tony on my 1850HL to stop the dreaded click-click from the starter circuit. I wired it with a 70 amp fuse which looks so substantial you just can't imagine the current that would blow it.

[Richard the old one]

May thanks for all your comments and advice. I have ordered a batch of the relays that you have recommended to fit both my dolomites and son in laws.

[soe8m]

however,
some time ago Jeroen stated that such is a bodge but, as I remember, did not say why.



Ian.

It's like putting a bucket under a leaky tap instead of repairing the tap....

Jeroen