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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3362
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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I have a 1999 gmc 7500, 3126 cat engine, Vehicle has no

Customer Question

I have a 1999 gmc 7500, 3126 cat engine, Vehicle has no start signal at starter, good voltage to and out of ignition switch.
Ignition switch and starter are new.
I need a diagram of where this wiring runs and what else it may go through. its yellow at ignition and brown at starter, I'm an automotive guy and GM is suppose to be purple!!! I'm lost
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Daniel Wilson replied 1 year ago.

Hi my name is ***** ***** or manual transmission? what make and model transmission? could be a clutch switch or neutral safety. Let me know

Customer: replied 1 year ago.
manual, and the only switch I can find on pedal I have bypassed via jumper. still no signal,
Expert:  Daniel Wilson replied 1 year ago.

Does the check engine light come on and cycle with the key on?

Customer: replied 1 year ago.
it does, and vehicle runs fine when you "jump Start" the starter with a remote switch. is there a secondary clutch switch , if so it would have to be way up high like behind the instrument cluster.
Expert:  Daniel Wilson replied 1 year ago.

Click on this link and see PDF page 2 for the diagram. check the wiring for the intake heater and fuse. Also check the inline fuse/breaker and harness between the battery box and ECM. The schematic on the engine side will help.


Expert:  Daniel Wilson replied 1 year ago.

There should be a PTO/ park brake switch ... see if the engine PTO control works and the RPM can be raised with the cruise switches

Customer: replied 1 year ago.
it can
Customer: replied 1 year ago.
fast idle and pto are non factory on this chassis
Customer: replied 1 year ago.
intake heater comes on as it should also, all fuses and circuit breakers check ok,
Expert:  Daniel Wilson replied 1 year ago.

Thanks for the update. Are there any fault codes logged in the ECM? So you do not have cruise set/resume in the dash to pull flash codes? If not is there a diagnostic switch?

Expert:  Daniel Wilson replied 1 year ago.

What is the complete model and serial number prefix of the engine?

Customer: replied 1 year ago.
No flash code, just blink slow steady. Ready function I do believe. The more loom I pull apart the more broken wires I'm finding
Esn #7as64300
Core AR #1373531
Expert:  Daniel Wilson replied 1 year ago.

thanks for that. Where in the harness? Engine or chassis/cab?

Customer: replied 1 year ago.
I just wanted a diagram for a 99 c7500 and got a diagram for a bus. I appreciate the try but I don't need the help offered. Thanks.
Expert:  Daniel Wilson replied 1 year ago.

The engine schematic is the same whether it is in a bus, pete, international, kenworth, ford, or GM. The chassis connectors all plug into the OEM connector. I have the engine schematic and that is why I asked for the prefix so I can be sure you get the right prefix diagram for your serial number.

Expert:  Daniel Wilson replied 1 year ago.

I will opt out since you dont need the help.

Customer: replied 1 year ago.
Ok thank you
Expert:  Jeremy replied 1 year ago.

My name is ***** ***** I will assist you with your question today.

I'm an independent contractor and I don't get paid until a rating has been made. No rate, site gets it all.

We can go back and forth as needed to help resolve the issue you are having. Follow ups are free after ratings, so rate in confidence!

CLICK HERE for the diagram. It is yellow coming off the ignition switch.


You want a 5 star service and I want to provide a 5 star service. Please consider that I don't know your skill level as well as I can't see,

touch, smell, hear the vehicle/machine in question. Please provide as much detail as possible and I will provide as much back.

Once an answer is provided, you should see a rating screen. Please rate my personal service to you. The ratings are my personal feedback,

they are not the site feedback.

Thank you for allowing me to assist you through Just Answer!


Expert:  Jeremy replied 1 year ago.

If you are trying to find out if you can bypass everything and run straight to the starter, on a manual, yes you can. On automatic, no you can't. Automatic runs through the PCM for the neutral safety switch on it.

Sound like a big job with the harness on this one if you are finding more and more bad wires as you go along. Usually on a issue like that, I pull the whole dash out and order a new loom which will have all the plug ends marked.

Customer: replied 1 year ago.
lots of broken loom and wiring on vehicle side. Thank you for your time.
Expert:  Jeremy replied 1 year ago.

Ouch. Jobs like that get pretty rough.

That diagram I posted is tight off of the Chevy website so it should match up to your truck and what you have in it.

Expert:  Jeremy replied 1 year ago.

I see you viewed this and this is still showing as an open question due to no positive (3 stars or more) rating.

Were you not pleased with my service to you or is there a rating issue? Please let me know if you still need assistance with this, if there is a rating issue, and/or you need more time to respond.

I like to make sure all my customers are satisfied since you did pay a deposit for me to assist you, whether I’m paid by a positive rating feedback or not.

We can continue where we left off or start over if needed. Also, if you need diagrams, instructions, etc please don’t hesitate to ask.