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Help With Jake Brake


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Hello, Tried this about 2 weeks ago but go no response, so thought I would try it again since I got a little more info. I have a 1997 Mack RD 688 dump truck with E7 350. The guy I got it from said he disconnected it. He told me that he disconnected 2 wires from behind the pump. I have never owned a truck with a Jake Brake so I was wondering if anyone might know what he is talking about. Any help would be appreciated.

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Ok, I found the switch on the fuel pump that the guy was talking about and there are not wires going to it. I dont see any terminals on it, just 3 holes in the back, bottom, middle and top. So, does anyone know what wires are suppose to go there???

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Ok, I found the switch on the fuel pump that the guy was talking about and there are not wires going to it. I dont see any terminals on it, just 3 holes in the back, bottom, middle and top. So, does anyone know what wires are suppose to go there???

OK there should be some wires tied up some where for it one gos to the center and on one side of that switch when you find them!

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Here is a link to the Jacobs Vehicle Systems installation guide for your engine brake. http://www.jakebrake.com/service/pdf2/018061C.pdf Page 10-13 shows the wiring system for the engine brake (both mechanical & VMAC). Your truck probably had the wiring there already, but this should give you an idea of what the circuit looks like. Since you found a switch on the pump you should be looking at the non-VMAC wiring. I can't remember the color of the factory installed wires coming from the cab to the pump switch, but I think they're brown. Hope this helps.

"Mebbe I'm too ugly and stupid to give up!"

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Do you think it is 2 wires encased in brown. If so, I saw a plug with 2 wires that is encased in brown casing wired tied up on the pump side of the firewall. That is the only thing I see on that side that is not pluged into something. I will look at the diagram you linked to. The only thing that may make me believe they are not the wires is the plug I am talking about has round holes and everything that I have looked at from Jacobs seems to have blades on the switch. Hopefully I can figure it out from what you sent. Tried to follow wires from switch on dash but they all seem to go into the main wire harness.

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I think those should be your wires, but its been a while since I've looked at one. If I remember correctly there are two wires in separate weatherpack connectors on the right side of the cab bulkhead about halfway down the branch harness for the hood. You will need a jumper harness to connect from the cab wires to the pump. Check to see if the wires have voltage when conditions for brake operation are met (per the installation manual linked earlier), then see if voltage is gone when the clutch switch is open.

"Mebbe I'm too ugly and stupid to give up!"

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Ok maybe you could humor me one more time. Sorry for the step by step. I found the wire, he had it wired tied up, hard to see. But I took the tester, and it is hot with the clutch out. When it is depressed, it cuts power off so it looks like the clutch switch is working. Also, when switch to jake brake is in either low or high, it has power with clutch out and no power with clutch depressed. So, my question is, I see where the diagram shows that 1 wire I found goes to pump switch, but it seems as though all the wires are in place to the switch in dash, where it shows another wire from the pump switch is suppose to go. Am I missing something here???

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Sorry for any confusion. The wiring shown in the Jake installation manual is for a truck that was not built with the Jake harness integrated. I don't happen to have a wiring diagram for your year model handy, I do have one for a CH 1993-1994 with factory wiring for Jakes which should be set up about the same as on the RD with the biggest difference being wire numbers (which you probably can't read anyway due to fading). They may or may not be the same, I'm not sure. I have removed the extra wiring shown on the diagram so all you see is the engine brake wiring. The wire numbered 22-A-.8 is the first wire in teh circuit, it goes from the fuse to the jake switch and the relays. When the jake switch is turned on low power goes to pin 85 on the low relay, it goes to pin 85 on both relays in high position. Power goes through the relay and exits on pin 86 and goes to the clutch switch, form the clutch switch to the fuel pump switch, from the pump switch to ground. when this happens the relays (either the low if in low position or both if in high position) switch internally from pin 87a to pin 87 allowing power to flow from pin 30 through the relay and out to the jake solenoids. Some or all of this circuit may still be intact on your truck, you might want to check the jake housings on the left side to see if there are any wires connected to them and so on.

Factory Jake Wiring.pdf

"Mebbe I'm too ugly and stupid to give up!"

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Thanks for that but I am not, by far, an electrical wiz. My switch has 3 holes in the back of it. first is marked "Common" 2nd "Norm Open" 3rd "Norm Closed". I was assuming that the hot wire I found goes to one of them and then a ground comes off another. But I dont know what goes where. So if I cannot figure it out I will just have to get someone to wire it for me.

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Ok this is where I am stuck. Maybe you could tell me if I am just a derlick or what. Electrical is definetly not my strong suit. But, it seems as though I have figured out the wiring, or maybe not.

I took the hot wire I found that is attached to the clutch switch and connected that to the common on the fuel pump micro switch. I took a ground and hooked that to the Norm Closed terminal on the switch. All seems to be working like it is suppose to. I have not re mounted the switch yet, but when I have the Jake dash switch on either low or high, and I physically push in the micro switch, the Jake is disengaged. When I let off the switch, within about 1 second the Jake engages. But my question is now, when I physically let off the micro switch like when the acclerator arm would do, and rev the engine up, it sounds like the Jake is engaged as I am reving it up and I know that cant be right. So is there something that I did wrong, or something that is not working? Thought I had figured it out but does not seem like it.

Jeff

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For some reason the switch is working backwards,the engine brake should only be engaged when the throttle lever is in the idle position,maybe the switch is bad or the wires are hooked up wrong,try switching the wires and see what happens,but in the mean time do not rev the engine with the engine brake engaged,maybe that's why he had it disconnected.

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