Relay Size For Starter

[RRoller123]

While my engine and transmission are out, I am going to add a relay for the starter circuit, and also solder those endlessly problematic and needless bullet interconnections along the path.

What size relay have you folks used, who have added a relay to this circuit? Is a 30A relay enough? the burst amperage may be higher, but is it greater than 30A running?

Thanks.

Pete

[SteinOnkel]

I used a standard 30A automotive relay.

It's not really a starter relay, it's a relay for the starter solenoid. While the starter itself may pull more than 30A, the solenoid pulls <10.

On my VW I repurposed the choke lever to act as a theft-deterrent. It does not have a relay either (but the wiring is MUCH better so it doesn't need one) and when prototyping this I discovered its solenoid pulls only 8A. Been working fine for about two months now.

Cheers
Steiny

[RRoller123]

Excellent, you are right, it is only the solenoid draw that matters! I have some 30A relays already, so I am all set.

[18Fiatsandcounting]

I agree with what Steiny said. I have a 30 volt, 3 amp bench power supply that is "current limited" at a max of 3 amps (so as not to burn up the electronics). Set to 12 volts, it can almost pull in the starter solenoid when out on my workbench, and the solenoid will indeed then activate with just a gentle prod from me. That tells me the the starter solenoid likely draws somewhat more than that, but certainly well below 30 amps. 5 to 8 amps sounds entirely reasonable for a properly functioning solenoid, so you should be fine with a 30 amp relay.

You could also measure the resistance of the starter solenoid and then use Ohm's Law to estimate the approximate current. 12 volts = current times the resistance in ohms. So, if you measure 2 ohms, that would be about 6 amps.

-Bryan

[DieselSpider]

A relatively inexpensive universal relay works quite well and can usually be had for less than $10.

These Optronics are rated at 40 amps and cost about $5.50 at places like Advanced Auto Parts.

[SteinOnkel]

I bought a 20 pack for $5 of those relays at...ledsuperstore or some place online of that name.

You'd think that they are junk, but they've been great for years now.

Also, the spiders have really nice original Bosch relays. I repurposed the igniton mode relay (I think) for the starter. Can't think of a cleaner place to install it.

[spider2081]

I agree the Bosch Cube style relay is fine for installation between the ignition switch contacts and the starter solenoid.

I have made numerous tests on the starters and solenoids used on our cars.
The starter solenoid used on the Magneti Marelli draws a constant current of approximately 10 amps when not installed on a starter. When installed on a starter that is not installed on a car the solenoid draws in excess of 15 amps on my test bench. This is the solenoid only not the starter motor current. The current is higher because the solenoid is pulling in the Bendix drive of the uninstalled starter. One could assume the current would be higher if it were engaging the Bendix into a flywheel.
The pull-in current for a relay or solenoid far exceeds the steady state current draw required once the device has engaged or pulled-in. An old timers rule of thumb is the switch contacts for an inductive load should be rated at least twice that of it steady state current requirement.
Also the switch contacts of an inductive load are easily damaged when the switch contacts open (the device is turned off) an extremely high voltage arc occurs as the contacts open causing the contacts to burn. Hence one of t he reasons for the capacitor connected to the ignition points.
The Bosch style relays are available in various current ratings. I would choose a higher rated relay 40 or 45 amps. A higher current rating might increase the life of the added relay.
Final note Fiat used a #12 wire from the ignition switch to engage the starter solenoid. The wire size implies expected current draw would be 10 amps or greater.

[18Fiatsandcounting]

That makes sense, Spider2081, even if the current draw is quite a bit higher than in my experience. The starter I was working on was the original from my '69, so perhaps decades of functioning have loosened things up a bit, including the solenoid return spring, so that it doesn't draw as much juice these days.

Maybe one day when I'm super motivated I'll hook up a DC ammeter to a starter solenoid and measure the current directly.

-Bryan

[RRoller123]

Good stuff, will be getting to this project hopefully tomorrow, the weather here is supposed to reach the 50's!

[spider2081]

I cut the bundle wrap and separate the red wire with bullet connector C6 out of the bundle I pull the red wire back along the existing bundle, cut the bullet connector off and install a 1/4" spade connector. Push the spade connector on to terminal 85 of the relay. The length of the original red wire determines the relay mounting location. Fabricate a ground wire from terminal 86 of the relay to a good ground. I use one of the unused studs welded to the fender well. Fabricate a new red #12 wire to go from the relay terminal 87 to the spade connector on the starter solenoid. Remove the short red wire that has C6 bullet connector on one end and the spade connector on the other end. Its no longer needed. Install an inline 20 amp fuse with #12 red wire between the alternator output post and terminal 30 of the relay. Then when you put the engine and tranny back in, re install the battery hopefully the engine will start up after a couple cranks.
Enjoy your day

[RRoller123]

Burundi! Thanks, will follow that route. Will follow up on the blocks and transmissions sometime this week, just to see how things are moving along.

[tima01864]

I was looking over my electrical diagrams. There is a relay in the starter cicuit, Should another be added?

[RRoller123]

Can you post the diagram? That is the only way to be sure, but it sounds like they added a relay in the later models?

[tima01864]

I have not mastered the skill of posting photos, The diagram is in my electrical diagnosis troubleshooting manual. I searched Mirafiori and the manuals do point to a starter relay. The starter relay was only fitted to Automatic transmissions. Possible the relay is in the panel but not used for the manual transmission. Mirafiori archive covers years 79-82 and the manual I have covers 83-85

[SteinOnkel]

I made my spider a bit more beginner friendly today. There is a switch on the clutch pedal, but it is an emissions related thing. The pedal itself is only 4 weeks old and it comes with the plate for the switch, so that's nice.

The original switch is default closed, open when pedal is pressed. So that's no good. It was sticking anyways so I binned that and bought a brake light switch for a 1985 Jeep C6. Wired that into the starter relay ground circuit (Note: always always always switch ground in an automotive environment) and it works great.

An hour later my partner (the afore-mentioned beginner) drove the car to practice stick and of course she tried to start it in gear without pressing the clutch. Well, it doesn't jump forward anymore.

Worth considering, the switch cost around eight bucks.

Cheers
Steiny

P.S: 1978 manual definitely does not have a starter solenoid relay from the factor.