Serehill Harness - Circuit Amps

[Cliff]

I am assembling a Serehill harness for my headlights and have a few questions. I have a pretty good understanding of auto electrics but figured some of you might have a little more knowledge.

1. I cannot locate fuses for the high or low beam circuit, and the TSM says "the 20 amp circuit breakers are built into the switch." Is this true or am I reading it wrong? Seems pretty straight forward but looking at the switch details in the TSM no CB is obvious.

2. What would be the amps/current requirements of the new harness? If I want to go to HID or LED ights can the 20 amp circuit support that?

3. If a 30-40 amp CB and relays are recommended (as they are by the Serehill plan), am I risking damage to a circuit designed to handle 20 amps?

[Tripwire]

i would imagine the Mod significantly reduces the current from the switch which instead of carrying the full load now caries a 1/2 amp (if that) just to operate the relays which apply battery directly to the bulbs

yes the thermal breakers are inside the switch to the best of my limited knowledge it very common

[tgreese]

Regarding amps of the circuit, you can estimate how many amps you need from the wattage of the bulbs. Any aftermarket bulb will list a wattage of the low and high beams. the street legal H4s are 60 watts at high beam so 2 bulbs is 120 watts divided by the voltage is roughly 10 amps.

Picking a fuse or CB, a good rule of thumb is double the current and one step up, so I'd pick a 30 amp breaker for these bulbs. 20 amps seems ok. I assume the Serehill harness has relays and its own circuit breaker.

The main problem with the factory wiring is the small wire they use, and the number of switches in the current part. With the factory wire, there is a voltage drop due to resistance by the time the power reaches the bulb.

As Tripwire writes, adding relays lowers the load on the switches to just enough current to power the relays (about 250 mA). Then, the only time you'd open the headlight switch breaker is if you had a short between the switch and the relays. Power for the lights comes from the relays, not the switch.

Much more here if you want to read - http://earlycj5.com/xf_cj5/index.php...detail.142321/

[Cliff]

Thanks! The CJ5 link answered everything.

[nograin]

In case someone comes along and reads this thread, I'll give you my answers.

Originally Posted by Cliff

1. I cannot locate fuses for the high or low beam circuit, and the TSM says "the 20 amp circuit breakers are built into the switch." Is this true or am I reading it wrong? Seems pretty straight forward but looking at the switch details in the TSM no CB is obvious.

You are reading it correctly. The breaker is between the B1 terminal and the H terminal for the headlights.

2. What would be the amps/current requirements of the new harness? If I want to go to HID or LED ights can the 20 amp circuit support that?

The circuit protection should be based on the capacity of the circuit. There is no HID retrofit at this time that will provide the proper light pattern. LED lamps with proper pattern and testing are available from Petersen, Truk-lite, and JW Speaker. They should draw less current than any factory lamp. However watch for heat in theheadlight bucket as high temperatures will reduce capacity of wires and connectors.
As Tim has posted, the lamps themselves will at most require something in the order of 10-12 amps. The weakness in the circuit will be the wiring lengths and connections. IMO 20 amps is about as higher as I'd go for fuse or breaker with the connectors used for the lamps. If using 18 ga automotive wire, then a 15 amp breaker would be more appropriate. For 14 ga and up I'd be comfortable with 20 amp especialy if using heat and oil resistant wire in the engine bay.

3. If a 30-40 amp CB and relays are recommended (as they are by the Serehill plan), am I risking damage to a circuit designed to handle 20 amps?

Yes. It means if there is an overload for any reason, heat will cause the wire or connector to fail before the fuse or breaker opens.

[nograin]

With a '77, connect to the alternator output (unless you plan to run the headlights mostly with the engine off).
This shortens the distance the current has to flow going to the headlights.
It also reduces the chances of overloading the battery feed circuit.