Please help/ no high idle

ihc4b · Feb 16, 2023

jab8283 said: ↑

Okay boss I have your answer right here. On Detroit Diesel equipped International conventionals that have Opt-Idle, you have a failed mercury switch inside the hood or the wire harness is damaged or unplugged. The purpose of this switch is to protect anyone who raises the hood of such equipped trucks from accidental engine starting during Opt-Idle operation. Raising the hood causes this mercury switch to open contacts preventing Opt-Idle system from starting truck if someone is working in engine compartment. When the hood is in closed position, the contacts in the mercury switch close again allowing Opt-Idle to function. This switch is located under the hood on the drivers side just above headlight assembly. This same mercury switch also affects your fast idle / PTO idle speed. When hood is in closed position, fast idle is active, but in open hood position, fast idle is disabled. Engine can be started manually while hood is open but it cannot activate fast idle circuit. If this switch is bad such as being hit during front end accident, the mercury will escape preventing closed contacts. Also if wiring is damaged or unplugged, this too will prevent fast idle circuits from activating. On my truck since I do not use the Opt-Idle system, I had it disabled in the Engines ECM so that it cannot function. Then I took the two wires going to the mercury switch and joined them together and taped it real good with black electrical tape and used wire ties to secure wires to left headlight wiring harness in case any future owners of my truck want to use that Opt-Idle system again. Now I can set fast idle with hood open or closed. That is your only issue that prevents fast idle on International Conventional Trucks. You fix that then you fixed your problem. If you have Opt-Idle that you do use, then replacing that mercury switch and checking wiring connections is all you need to do to fix problem. This system is designed to prevent Opt-Idle system from engaging while a mechanic is working on your truck so if you use Opt-Idle, keep that system in tack and do not by-pass it or else someone can be killed or badly injured. If you don't intend to use your Opt-Idle system, first disengage it in the Engines ECM computer, then join those two wires together out by your headlight assembly under the hood on the drivers side. That's it, fast idle returns.
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Where do these wires from the mercury switch go to? I have Jake's but no cruise or high idle. I bought this truck used, so trying to understand this truck. I have 2 wires coming out of the main bundle of wires going to the ecm firewall side, both are black in their own loom and nothing attached to them, no connector just clipped wires. Wondering if it might of had that mercury switch and now it's missing. 2000 9400i with 12.7 Detroit.

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