I received the 'dread' P1688 - but when reviewing the various threads, it is often accompanied by a number of other fault codes and usually the poster is stating that the engine won't run.

My CEL came on ....but other than a code that can be reset (engine off) - but returns when engine is running - the truck drives and the VP pumps.

I expect I will have to replace the VP.....but I don't want to rush and replace it if there might be something else to check and maybe something else might be the problem rather than the VP.

I did read discussions about alternator problems where a bad (shorted) diode could create an AC component on the DC - and 'fry' electronics. I checked - and had 14.10 VDC output, and the AC component was around 16 - 20 mV (or 0.016 Volts). So- that rules out alternator problems. 

Anything else to check? Is it unusual to throw a P1688 but the pump still keeps on keeping on? (Is a zombie...still operating but it doesn't know it is supposed to be dead??)

As said - I am not too worried about having to replace it, if absolutely necessary; I just replaced the injectors on my other 2002 truck - so I have some experience is much of the work to access the VP for removal, so I can do it more quickly. The affected truck is my 2x4, and the Raptor FRRP is working fine, with about 16 PSI always available as shown on the fuel pressure gauge.

Thanks for any help.

All I can say...

http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/59-obdii-error-codes/227-p1688-internal-fuel-injection-pump-failure

 

DTC P1688: INTERNAL FUEL INJECTION PUMP FAILURE

 

Monitor & Set Conditions

 

Fuel Pump Control Module (FPCM) is monitored with ignition on engine off, and with engine running. DTC may set if FPCM, PCM memory, programming or hardware error is detected.

Possible Causes

• Good Trip Counter Equals Zero
• Fuel Pump Control Module (FPCM)

Testing

 

1. Turn ignition on with engine off. Using DRBIII® scan tool, check for DTCs. If Good Trip Counter for DTC P1688 is displayed and not equal to zero, go to next step. If Good Trip Counter for DTC P1688 is displayed and equal to zero, replace FPCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article.

 

2. Ensure ignition is on with engine off. Using scan tool, check for DTCs and record FREEZE FRAME data. Try to duplicate conditions recorded in FREEZE FRAME data. Using scan tool, check for DTCs. If DTC P1688 returns, replace fuel injection pump. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. If DTC P1688 does not return, test is complete.