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7.3l Dual Alternator Problem & Solution

Problems with 7.3l Diesel Dual Alternator Auxiliary Drive Belt System

PROBLEM: During installation of a 2nd alternator system in a 2003 F350 for a Truck Camper nomad, we experienced a tremendous amount of belt slippage on the new alternator’s drive pulley. In the course of examining the problem, I determined that the arrangement of the standard driven components (water pump, steering pump, primary alternator and A/C compressor) were in the proper arrangement with the belt tensioner (see belt routing diagram for single alternator). However, placement of the second alternator in the previously slack portion of the belt, behind the belt tensioner, ‘unwinds’ the tensioner, creating slack over the 2nd alternator, causing the belt slipping.

EVALUATION: With the placement of the 2nd alternator on the lower passenger side, the correct sequencing of the driven equipment and idler is to move the tensioner to a smaller section of slack belt, between the 2nd alternator and crank pulley. However, that location is occupied by a fixed idler. Removing the fixed idler, double idler tensioner and replacing both with a single idler tensioner would be the best designed solution. Since redesigning Ford’s placement is a big change and one that likely Ford engineers likely chose not to accomplish, a different solution is necessary.

SOLUTION: The proximate cause of the slippage is the 2nd alternator tension operating to unwind the tensioner. If it possible to increase tensioner ‘winding’ force to be greater than the unwinding tension, the situation should be stable albeit at a higher overall belt tension. Tensioner force is created by rotating the tensioner CCW and to do that, the belt needs to be shortened. Specifically, from the 137.7” specified by Ford to 135.9”. The parts list below identifies a number of different manufacturer part numbers that will work.

RESULT: The alternator used was a 200A, large case 6G with an overdrive pulley. The OD pulley (58mm) creates even more belt tension with a given drive torque. Changing it to a standard diameter pulley (64mm) would reduce belt tension over the tensioner but was not needed. The belt change alone resulted in elimination of the slippage.


Technical Details

Parts and information for installing a second alternator on a 99-2003 7.3l Powerstroke. 

NOTE: Some of this information may be relevant to doing the same on an E series van but due to the different belt routing, an appropriate size (shorter) belt may also be needed as it suffers from the same design error (incorrectly placed tensioner) as the Superduty.

Alternator, 135A Large Case 6C3Z-10346-BA (This kit was originally designed for the 110A small case alternator but the more reliable 135A large case alternator is recommended. It MAY require spacer washers between the mounting ears and bracket to prevent the case from interfering with the bracket, depending on what revision bracket you have. You may also be able to perform minor grinding where it makes contact before you install the bracket. If there is alternator to bracket interference, the alternator rotor will grind on the stator.)

  • Idler Pulley F8TZ-8678A
  • Lower Alternator Bracket F8TZ-10A313-AA
  • 6 Bolts (3 alt, 3 bracket) N606068-S437
  • Bolt, M12x100mm (lower idler) (no PN)
  • Radiator Hose, upper F81Z-8260-CA, Motorcraft KM4351 (Not required but recommended. Allows serpentine belt to be replaced without removing radiator hose).
  • Tensioner F8UZ-6B209-CA (For reference, same as sgl alt. Does not need to be changed)
  • Belt Poly-V 8-Rib Effective Length 137.75″ Outside Length 138.5″. 
  • Goodyear 408-1377 
  • Gates K081377 

IF YOU USE THIS BELT, YOU WILL HAVE BELT SLIPPAGE. It is too long and allows the tensioner to operate at minimum tension which contributes to the design induced slippage problems. Belt tension created by operating the 2nd alternator will cause the tensioner to ‘unwind’, allowing slack, resulting in slippage. It is necessary to increase belt tension over the tensioner as much as possible to by ‘cocking’ the tensioner back further with a shorter belt. 

I RECOMMEND one of the belts below;
Belt: 135.9” Gates K081359 (‘heavy duty’ version has ‘HD’ at the end of the PN)

  • ACDelco K081359HD
  • ACDelco 8K1359
  • CARQUEST K081359HD
  • CARQUEST K081359
  • Continental Belt 4081360
  • Ford F8UZ 8620 EB
  • Sterling K081359
  • International Harvester (IHC) 1893843C1
  • Goodyear 1360K8MK
  • Goodyear 4081360
  • NAPA 25-081359HD
  • NAPA 25-081359
  • DewEze 742039

Wiring: At a minimum, you will need a plug for the alternator to connect it to your system, depending on how you want to control it. If you intend to connect it as designed by Ford, you will need to find the Ford wiring diagram which connects control lines to the ECU. Once wired, you will need to take it in to a dealer to have the ECU programmed to recognize and operate the 2nd alternator. I BELIEVE, that the ECU delays the operation of the alternator till the system voltage drops below 13.4V indicating the primary alternator is maxed out. For this reason, I do not operate under ECU control.

I recommend using an external switch, providing 12V to the Ignition (I) terminal to switch the alternator on when needed and after the engine has warmed up. Ford specifically inhibits the second alternator from operating during the glow plug cycle because contributing more power when they are operating raises system voltage to exceed their operating voltage (11V) and will quickly burn them out.

Alternator connector: Standard Motor Products HP3910, S-545, HP4210, S-821 (parts vary by connector and wiring color but fit the same.)

Doorman 645-136

CAUTION: I advise against using an ‘overdrive’ (smaller pulley 56mm in dia) pulley on the second alternator. A smaller pulley requires higher belt tension to provide the same power to the alternator. Since the second alternator’s belt tension unloads the tensioner, using a smaller pulley will possibly cause more tension and potentially more slippage. Using an overdrive pulley on the primary alternator is ok.


Single and Dual Alternator diagrams from the video are below.

Single Alternator & Serpentine Routing
7.3L Power Stroke Dual Alternator Serpentine Routing
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