amberwood Posted August 14, 2009 Share Posted August 14, 2009 Just joined up having recently purchasing my first semi. The first gremlin has just turned up in the electrical system. The 10A fuse on the engine brake pops after a single application. I was hoping to source a wiring diagram, or even better tap into someone who has seen this problem before.DTR Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/ Share on other sites More sharing options...
vanscottbuilders Posted August 14, 2009 Share Posted August 14, 2009 DTR,I would suggest that you look around this site, or go online to E-Bayto find and purchase a Service Manual for the R688 right away.It's a practice I have used for years for all of my vehicles, and it has served me pretty well.Reprints and original manuals in all conditions are readily available for most later Macks,and are pretty reasonably priced.I know my early Mack R model Service Manual has wiring diagrams.I'm just curious - how are you going to use the truck?Paul Van Scott Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39093 Share on other sites More sharing options...
amberwood Posted August 15, 2009 Author Share Posted August 15, 2009 DTR,I would suggest that you look around this site, or go online to E-Bayto find and purchase a Service Manual for the R688 right away.It's a practice I have used for years for all of my vehicles, and it has served me pretty well.Reprints and original manuals in all conditions are readily available for most later Macks,and are pretty reasonably priced.I know my early Mack R model Service Manual has wiring diagrams.I'm just curious - how are you going to use the truck?Paul Van ScottI have a specialty sawmilling business, and the mack is set up with PTO etc to tow a 40' rigid log skel with a midmount Loglift crane. I try to check ebay regulary but no luck on the right manual as yet.DTR Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39113 Share on other sites More sharing options...
Rob Posted August 15, 2009 Share Posted August 15, 2009 Just joined up having recently purchasing my first semi. The first gremlin has just turned up in the electrical system. The 10A fuse on the engine brake pops after a single application. I was hoping to source a wiring diagram, or even better tap into someone who has seen this problem before.DTRChances are that you have a shorted solenoid, or a pinched, cut/chaffed wire in the harness. If it were mine the first thing to do would be to unplug both cylinder heads from the harness and drive the truck. If it still blows the fuse, the solenoids and/or associated wiring to the heads can be eliminated for now. If the fuse holds, then plug only one cylinder head back in and drive the truck again. If the fuse still holds, unplug the one cylinder head, and plug the other one in. Drive the truck again and discern if the fuse still blows. If it still blows I would unplug, or unfasten the wiring harness from the switch in the dash that enables the system and see if the fuse holds. If the fuse now holds, get the multimeter out and "ring out" the wiring to find a direct short to ground by reading the meter, and following the wiring, (by hand most times) to find the problem.By doing these functions you are isolating where the problem lies.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39115 Share on other sites More sharing options...
amberwood Posted August 16, 2009 Author Share Posted August 16, 2009 Chances are that you have a shorted solenoid, or a pinched, cut/chaffed wire in the harness. If it were mine the first thing to do would be to unplug both cylinder heads from the harness and drive the truck. If it still blows the fuse, the solenoids and/or associated wiring to the heads can be eliminated for now. If the fuse holds, then plug only one cylinder head back in and drive the truck again. If the fuse still holds, unplug the one cylinder head, and plug the other one in. Drive the truck again and discern if the fuse still blows. If it still blows I would unplug, or unfasten the wiring harness from the switch in the dash that enables the system and see if the fuse holds. If the fuse now holds, get the multimeter out and "ring out" the wiring to find a direct short to ground by reading the meter, and following the wiring, (by hand most times) to find the problem.By doing these functions you are isolating where the problem lies.Robthanks rob. i will give it a go first chance and see where the problem lies.DTR Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39140 Share on other sites More sharing options...
Rob Posted August 16, 2009 Share Posted August 16, 2009 thanks rob. i will give it a go first chance and see where the problem lies.DTRKeep in mind there are many components to make the system work. There is the switch in the dash, the switch on the clutch pedal, the fuel pedal, sometimes a speed sensor in the system, and of course the switch in/on the injection pump or linkage. Most any of these can fail internally from use and cause a short to ground. By you mentioning that the fuse does not blow until you go to use the engine brake is the first "clue" the the system is giving.Does the fuse blow if you select the engine brake on while the truck is not moving? How about if the engine is not started, and the engine brake is selected? This can rule out about 1/2 of the system right off if the fuse does not blow. I would start the engine, run it at about 1800 rpm, select engine brake, get off the throttle and let the engine come back to idle, then "rev" the engine up and down a few times and then check the fuse again. The speed sensor portion of the system should be alright if the fuse holds doing this as the brake should not work unless the truck is going a few mph.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39147 Share on other sites More sharing options...
HK Trucking Posted August 16, 2009 Share Posted August 16, 2009 Hi Rob.I see his truck is a 1990 R688, so it undoubtedly has a mechanical engine, and could have either a Dynatard or a Jake Brake.The Dynatard has a ground out contact inside the governor which closes the engine brake circuit whenever the rack moves to the "no fuel" position by grounding out a control relay which was usually mounted under the right side of the dashboard on those trucks. That is why the Dynatard cuts out once the engine returns to idle speed - when the governor opens the rack to idle position, the contacts move apart, the ground is broken, and the relay shuts off the Dynatard. The Dynatard had the 2 position dash switch (on/off) only.With a Jake Brake of that vintage, there was a 3 position dash switch (off, 1 & 2), a switch on the clutch pedal, and a switch on the throttle arm at the inj. pumpA Jake Brake on a mechanical engine will kill the engine if it is activated with the trans in neutral and the clutch pedal not depressed, that's why they used the clutch pedal switch, figuring that once the braking cycle is complete, the driver will depress the clutch pedal as the engine nears idle speed. I have heard some drivers describe "soiled underwear moments" when they were Jake braking approaching a turn, and missed a shift or whatever and then they let off the throttle with the trans in neutral and..... yup, the Jake brake kills the engine and there goes the power steering while making the turn!!!There were no speed sensors in either the Dynatard or Jake Brake mechanical engine setups.On the electronic engines, in addition to the dashboard on/off switch, and the clutch pedal switch, the Jake brake is controlled thru the ECM, which cuts off the engine brake when the RPM's drop below about 850.Otherwise,your troubleshooting tips are right on target, couldn't have said it any better. . Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39158 Share on other sites More sharing options...
Rob Posted August 17, 2009 Share Posted August 17, 2009 Hi Rob.I see his truck is a 1990 R688, so it undoubtedly has a mechanical engine, and could have either a Dynatard or a Jake Brake.The Dynatard has a ground out contact inside the governor which closes the engine brake circuit whenever the rack moves to the "no fuel" position by grounding out a control relay which was usually mounted under the right side of the dashboard on those trucks. That is why the Dynatard cuts out once the engine returns to idle speed - when the governor opens the rack to idle position, the contacts move apart, the ground is broken, and the relay shuts off the Dynatard. The Dynatard had the 2 position dash switch (on/off) only.With a Jake Brake of that vintage, there was a 3 position dash switch (off, 1 & 2), a switch on the clutch pedal, and a switch on the throttle arm at the inj. pumpA Jake Brake on a mechanical engine will kill the engine if it is activated with the trans in neutral and the clutch pedal not depressed, that's why they used the clutch pedal switch, figuring that once the braking cycle is complete, the driver will depress the clutch pedal as the engine nears idle speed. I have heard some drivers describe "soiled underwear moments" when they were Jake braking approaching a turn, and missed a shift or whatever and then they let off the throttle with the trans in neutral and..... yup, the Jake brake kills the engine and there goes the power steering while making the turn!!!There were no speed sensors in either the Dynatard or Jake Brake mechanical engine setups.On the electronic engines, in addition to the dashboard on/off switch, and the clutch pedal switch, the Jake brake is controlled thru the ECM, which cuts off the engine brake when the RPM's drop below about 850.Otherwise,your troubleshooting tips are right on target, couldn't have said it any better. .Hi Herb, I've never actually owned, (nor really worked on) anything with a "Jacobs" engine brake. The only one I've driven was a newer tractor and a VSS did control it, along with the speedometer, cruise control, hydraulic p/s boost pressure, and transmission shift points. I had assumed most vehicles with this engine brake did use some sort of speed, or throttle position sensor to shut the system down when the engine was idling. That is news to me that they would inherently shut the engine down with operator error.A mis adjusted "Dynatard" engine brake will kill the engine at idle also as has been the case with all three of mine!Learn something every day.Thanks,RobProlly see you on Thursday night after school lets out. Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39171 Share on other sites More sharing options...
amberwood Posted August 17, 2009 Author Share Posted August 17, 2009 Keep in mind there are many components to make the system work. There is the switch in the dash, the switch on the clutch pedal, the fuel pedal, sometimes a speed sensor in the system, and of course the switch in/on the injection pump or linkage. Most any of these can fail internally from use and cause a short to ground. By you mentioning that the fuse does not blow until you go to use the engine brake is the first "clue" the the system is giving.Does the fuse blow if you select the engine brake on while the truck is not moving? How about if the engine is not started, and the engine brake is selected? This can rule out about 1/2 of the system right off if the fuse does not blow. I would start the engine, run it at about 1800 rpm, select engine brake, get off the throttle and let the engine come back to idle, then "rev" the engine up and down a few times and then check the fuse again. The speed sensor portion of the system should be alright if the fuse holds doing this as the brake should not work unless the truck is going a few mph.RobRob. The fuse does not blow until the exhaust brake is activated, and actually it is the second application that the fuse fails on. It appears that a previous owner has fitted a 10A spade fuse directly behind the dash switch, this might be a reoccuring issue. Answering some of the other queries raise, yes it is a late mechanical engine, all clockwork and relays. i am bring the truck back to the yard tomorrow to repair the crane and will get the mutimeter out to check the potential problem switches.thanksDTR Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39185 Share on other sites More sharing options...
David Posted August 17, 2009 Share Posted August 17, 2009 I have full color copies of a mid 80's Mack conventional wiring diagram. Send $10 and a SASE to POB 97784, Raleigh, NC 27624. Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39213 Share on other sites More sharing options...
amberwood Posted August 18, 2009 Author Share Posted August 18, 2009 I have full color copies of a mid 80's Mack conventional wiring diagram. Send $10 and a SASE to POB 97784, Raleigh, NC 27624.OK, all fixed. It appears that there was a loose wire on the throttle microswitch. thank all for the helpDTR Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39232 Share on other sites More sharing options...
Rob Posted August 18, 2009 Share Posted August 18, 2009 OK, all fixed. It appears that there was a loose wire on the throttle microswitch. thank all for the helpDTR"Surge", or "inrush current" was probably your culprit as the relay engaged, then dropped out in quick succession.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-39253 Share on other sites More sharing options...
Paulie Posted August 26, 2022 Share Posted August 26, 2022 I’d like a copy of the wiring diagram. I have a 1985 Mack R688T Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-507494 Share on other sites More sharing options...
Joey Mack Posted August 26, 2022 Share Posted August 26, 2022 Which diagram? What system? Quote Link to comment https://www.bigmacktrucks.com/topic/10340-wiring-diagram-for-r688-1990/#findComment-507495 Share on other sites More sharing options...
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