Installing starter relay, 300SDL - PeachParts Mercedes-Benz Forum

cmac2012 · #1 05-19-2017, 10:30 PM

I'm putting in a reman starter and thought I'd look for some advice. This page has a good write-up:

http://www.gbcnet.com/mercedes/docs/OM603%20Starter%20Motor%20Removal%20and%20Installation.pdf

At the end he talks about the relay:

As a bonus, I added a relay to the wire coming from the ignition switch (the small one). Bosch makes a kit for this and my word of advice is to make sure there is a 15 amp fuse between the battery and the relay. The way it works is that rather than have the current to actuate the solenoid (and therefore the starter) go through the ignition switch and Outer Mongolia, the relay acts like a remote switch, giving the current far less distance to travel, and much less resistance. This significantly reduces the chance of the starter motor unexpectedly not starting, as well as providing more robust starts. The only thing the wiring from the ignition switch has to do is to switch on the relay.

Also speaks of a Bosch relay kit. Anybody done this or know if it's a good idea or not? I was under the impression that relays for this were standard.

kerry · #2 05-19-2017, 11:19 PM

Some people put the relay in 123's because of poor connections thru the ignition switch. There are pictures on here somewhere of someone's installation. Not a bad idea in my view.

Diseasel300 · #3 05-20-2017, 12:42 AM

It's awfully easy to over-complicate things for the sake of over-complication. The start contacts in the ignition switch were designed to handle the current to the starter motor solenoid. There are millions of cars of this vintage out there still reliably starting day after day with the same ignition switch they left the factory with. If the ignition switch gets to the point that it is worn on the start contacts, the rest of them are about ready to go too.

Just my own 2¢

cmac2012 · #4 05-20-2017, 12:49 AM

That sounds plausible.

Now I'm at my second, more serious issue, probably should start a new thread. The reman does indeed to be OEM - Spanish made motor, Bosch made solenoid, but the post on the reman, the smallest post, is quite a bit bigger than the original. My metric calipers are lost, my english measure the large post on the reman at 15/64ths - just under 6 mm.

The original is 9/64ths, or about 3.5 mm. Must be 4. It has a stamp of '15 A' near it. The reman has a stamp of 50. No "A" - I can assume it was implied. I bought it mail order, will likely need to ship it back.

funola · #5 05-20-2017, 10:48 AM

Those are terminal numbers on the wiring diagram, not amps. A starter solenoid draws around 7 to 8 amps, not 15 not 50. If the ring terminal does not fit the bigger stud, make it fit! Crimp a new terminal or cut it into a spade terminal (that's what I'd do).

Using a relay to fix a weak igition switch is a valid/ cheap/ inexpensive way, especially if the ignition switch has a history of being weak, or hard to replace, or unavailable.

cmac2012 · #6 05-20-2017, 12:56 PM

The seller said the same thing. I was skeptical that there would be that big a difference between solenoids. Would not be difficult to put a large connector on the wire. I'll look for a brass rather than alum so I can solder it.

Diseasel300 · #7 05-20-2017, 01:08 PM

Tin-plated copper is what you should use. Unless you're shopping at some specialty store, there are no aluminum terminals. Tin-plated is preferred to brass, it will oxidize/corrode less in time. Crimps are preferred to solder, especially in a vibrating environment such as the starter motor. If you use a proper crimper, the resultant connection is better secured and more vibration tolerant than solder.

cmac2012 · #8 05-20-2017, 02:39 PM

One sees crimped connections a lot. I had always thought it was mainly quicker. I recall some of the cheapo battery cables year ago with large crimped ends. Corrosion would find its way in. But I do have a good crimper. I would clean the wire end well. A dab of dielectric grease after would probably be good.