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Andy, Truck Diagnostic Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 1648
Experience:  International and Isuzu trucks , International, Maxxforce, Cummins and Isuzu engines, Diagnostic and electrical troubleshooting specialist , Hvac and truck /trailer refrigeration troubleshooter
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1999 Peterbilt 379, Left front turn signal won't blink with

Customer Question

1999 Peterbilt 379, Left front turn signal won't blink with out headlights or clearance lights on and then it is very dim. With out the headlights on the rear turn signals blink normal. With left turn signal blinking with headlights on, it causes the brake lights to flash like a Railroad Crossing light also when one side is bright, the opposite side reverse light lights up. I have check all grounds and replaced some and no change. It has been narrowed down to the cab. I have tried to locate an "MFTS Module" inside the dash but only found ABS Box. Any ideas?
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Andy replied 1 year ago.

Hi there . My name is ***** ***** Would a wiring diagram help you ? If so , can we start with the full vin number please?

Customer: replied 1 year ago.
I have Schematic P94-6023 and SK25762. I have also disconnected everything from the firewall and checked each section separately, with no change.
Expert:  Andy replied 1 year ago.

Have we tried a turn signal switch ?

Expert:  Andy replied 1 year ago.

Since it likely has headlight dimmer built into it as well , this may be the problem. I will look up the diagram I have for that unit.

Customer: replied 1 year ago.
I have not tried a replacement turn signal switch, however I have checked for power by probing the wires from the firewall connectors on the outside of the cab back to the turn signal switch. The turn signal wires have power when selected and blink. I may have mentioned this already, but also there is power (12v) on the green wire to the light (left front turn signal) and it blinks. I even tried a jumper ground and no change. The only change that happens is when a test light is grounded and probing the left turn signal power wire, with the headlights on and the turn signal blinking very dim, when the wire is probed the light blinks bright like normal but the running light is not on until the blinker is off.
Expert:  Andy replied 1 year ago.

Here is the diagrams for exterior lamps for that truck . It is 2 pages long and I blew it up so that you could see it better . In doing , I had to split the images in halves .

Customer: replied 1 year ago.
it will be later this week before I get back to the truck. Will let you know if I find anything.
Expert:  Andy replied 1 year ago.

Ok sounds good thanks

Expert:  Andy replied 1 year ago.

Please let me know if you need anything else. Thanks

Expert:  Andy replied 1 year ago.

Hi there . Looking for an update ! Any luck ?

Customer: replied 1 year ago.
I tried a new turn signal switch today with no change in the results.
Expert:  Andy replied 1 year ago.

Do you have SKype ? Perhaps we can continue this on there and diagnose this together during an audio conversation .

Customer: replied 1 year ago.
I do not have Skype. Sorry for the time delays in my response. I work third shift Quality Assurance for a local defense plant and right now we're there 7 days a week. The truck I'm fixing up is located 40 miles away. Hope to get back out there this week if work and time allows.
Customer: replied 1 year ago.
Here's where I'm at now. I found the power wire (#8) to the left front running light to be grounded. After disconnecting the hood harness at the firewall, found two #41 wires and a #42 wire also showing to be grounded. Next trip out will pull new wires.
Expert:  Andy replied 1 year ago.

Ok let me know how it goes and if you need more help . By the way , Skype is free to download . It is a smalll download that you install , then create an account . Once done , you could search for me and if needed we can work together that way .

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