Cummins 4BT & Diesel Conversions Forums banner
21 - 22 of 22 Posts

·
Super Moderator
Joined
·
8,979 Posts
I was planning on putting anything with over 5 amps on a relay in my new design. An engineer at my work said I was being ridiculous, and needlessly adding more complexity than necessary.
After looking at factory fuse boxes in my other trucks, it's seems like only relays are used over 20 amps. The original Wagoneer had one relay for the starter. (it also was a rolling fire hazard).

The way I was designing this was going to need 12-14 relays. I think I'm taking this idea of keeping all high amps firewall forward might be a little stupid. I spent a couple hours trying to find a general consensus on when to use a relay, but couldn't find a good answer. Too many variables. What's the switch rated for? Is it a high amp spike start, then low amp run circuit? How far is the run?

Bearing in mind that I'm using all the 1972 original switches on the dash, what is a general, or rule of thumb amp load to be considered safe without employing a relay?
Depends how voltage sensitive your components are. For headlights and radiator fans IMO relays are a must as power output drops fast with voltage loss. For most other things it doesn't matter if it gets 11v instead of 13v.

One of my british vehicles had the headlights running from about 4m of cable through the fuse-box, steering colum switch and then back to the headlights. Relays made an enormous difference.

Using relays can also mean a bundle of smaller trigger wires to your switches instead of big fat ones. Which can help cable routing a lot. I have a table of wire size and max current that I use: But I can't find it right now!
 

·
Registered
Joined
·
1,774 Posts
...

Bearing in mind that I'm using all the 1972 original switches on the dash, what is a general, or rule of thumb amp load to be considered safe without employing a relay?
The original 1972 switches were designed to safely carry the amp load for the devices that they are connected to. After 50 years of wear, abuse and oxidation, it is a gamble. Often the plastic parts inside the switches get brittle.

An example from my 1962 Volvo PV544 - It currently has a 94 Amp Delco 12si alternator.

I use relays and individually fused circuits (i.e. one failure does not take out multiple electrical units) for:
Starter
Air conditioner
Air conditioner clutch
2 radiator fans (part of the AC condenser assembly)
2 fans on the auxiliary cooler
Electric pump for the auxiliary cooler
High beams
Low beams
Maybe a couple more... The wiring was done several years ago

And a standalone fuse block for switched power (takes the load off of the 1962 ignition switch)

A few weeks ago, I had no starter (no "click" from the starter and no voltage drop on the dash voltmeter). I quickly swapped 2 relay pigtails and confirmed that the relay was OK (it clicked on both relays). The trouble had to be between the starter relay and the solenoid on the starter. Quickly found the wire that fell of the starter solenoid. Since I was 80 miles from home - that was extra good news.

Russ
 
21 - 22 of 22 Posts
Top