VP 44 voltage issue

[webbrenda]

Hey guys. I have a 99 Dodge 2500 quad cab 4X4 5 speed. I was having an ongoing issue with WTS light either being delayed or not coming on at all and LP not cycling so I replaced ECM with a known good one and replaced ECM wiring harness with a new one I found at Firsttruckparts.   When I was troubleshooting LP not running( before I replaced ECM and harness)  I pulled the fuel system relay and got a P1689 code and truck wouldn’t start.    So now, after replacing those I still had code and no start.  After a lot of troubleshooting and wire checking I figured out on the fuel system relay that I have 12 volts to pin 86 from ECM until I plug the relay in and then it drops to about 1.8 volts.  I sanded and dielectric greased all the grounds. I finally ran a dedicated ground to pin 85. Same thing.   I have the plug unplugged from VP 44. I have checked resistance on wire from pin 86 to the ECM plug and it shows 0 resistance.  I’m lost at this point.  I don’t think it’s the ECM as it has 12 volts until relay is plugged in.  Any help would be greatly appreciated. 

[Tractorman]

2 hours ago, webbrenda said:

When I was troubleshooting LP not running( before I replaced ECM and harness)  I pulled the fuel system relay and got a P1689 code and truck wouldn’t start.    So now, after replacing those I still had code and no start.  After a lot of troubleshooting and wire checking I figured out on the fuel system relay that I have 12 volts to pin 86 from ECM until I plug the relay in and then it drops to about 1.8 volts.  I sanded and dielectric greased all the grounds. I finally ran a dedicated ground to pin 85. Same thing.   I have the plug unplugged from VP 44. I have checked resistance on wire from pin 86 to the ECM plug and it shows 0 resistance.

 

You are doing some good troubleshooting here.  Just for clarification (you probably already know this), the fuel pump relay is for the VP44 power supply, not the lift pump.

 

You are correct in that pin 86 should receive 12 volts from pin 35 on the ECM.  That fact that voltage at pin 86 falls to almost ground potential when you ground pin 85 tells me that there likely is a poor connection somewhere in the supply circuit.  Using a ohmmeter to check continuity is not always conclusive.  For example, 12 volts can be present under no electrical load, but can fall dramatically when a load is applied due to a poor connection.  A test with an ohmmeter may not show this.

 

The fuel pump relay coil gets supply voltage from pin 35 on the ECM and the ECM gets its supply voltage from pins 48 and 50 at the ECM connections.  Those connections get their supply voltage from Fuse #3 in the PDC (which also supplies voltage to pin 30 on the fuel pump relay).  There are splices along the way.

 

 John

 

 

 

Edited April 25, 2023 by Tractorman

[webbrenda]

Thanks for the reply.  I was in a bit of hurry when I posted and left out a couple things.  After the ECM and harness replacement the WTS and lift pump issue is fixed.  When I was first troubleshooting the lift pump issue before ECM replacement and before I studied and understood the wiring diagram I pulled the fuel system relay looking for issues.   I pulled the cover off the relay to check voltage because I couldn’t get my meter probes onto the spades. In doing so I accidentally shorted the relay and blew fuse 3.   Replaced it and was back to square one with LP and WTS.  So then I replaced ECM and harness.  Got done and the WTS comes on lift pump cycles. I primed filter and had fuel to VP. Tried to start and cranks but wouldn’t start.  Traced it down to no voltage at VP.  Then I went back to the relay and if I jump 30 to 87 I had voltage to VP and truck starts and runs fine.    That’s when I kept digging and figured out I only lose voltage to pin 86 when I plug relay in.   I never lose voltage to pin 30.   I will go check pins 48 and 50 with relay plugged in and see what I get.     To confirm what you said about resistance not always showing the problem, when I accidentally shorted the relay a bunch of other stuff started happening like ASD relay buzzing and no gauges and no bus on odometer which all lead to fuse 3 being bad.  I checked it probably 20 times with ohm meter and it checked good.  Finally swapped it with another and everything went back to the way it was.  Tore the fuse apart and you could see where it was “blown” and barely touching.    Thanks again and I’ll let you know what I find 

[Tractorman]

1 hour ago, webbrenda said:

 Tore the fuse apart and you could see where it was “blown” and barely touching.

 

You just gave an excellent example of how using an ohmmeter can lead you astray.

 

One good method for finding poor connections in a 12 volt circuit is to use a test light with an incandescent bulb.  The bulb will impose a small load in the circuit and the bulb will glow dimly when there is a voltage drop present.  Your "barely touching fuse" would have been much easier to find with the test light approach.

 

I am very interested to see what you find.

 

- John

 

 

 

 

[webbrenda]

I was just out messing with it again. Tried to test for voltage at pins 48 and 50 but without the ECM being plugged in it causes the No Bus and no power.  I’d rather not penetrate the insulation on the wires to test.  Will a non contact tester work or do you have a good way of testing those with everything plugged in?   
Thanks again for your help and input.   Several days messing with this and restless nights.   Been a mechanic and maintenance guy for 35 years and been stumped before but can’t remember a more frustrating problem as this.  

I guess what I’m asking is how would I use a test light in this situation to find the poor connection. 

[Great work!]

Is the ECU that's installed now present when the shorting with the test leads happened? Have you tried the old one again? Probably damaged the relay driver circuit in the ECU. There is a dirty fix for it should you need it.

 

Totally agree with John a test light is the way to go.

 

I'm taking the 5 th on wire piercing controversy. 

[webbrenda]

I haven’t tried the old ECM but I will.  I know nothing about the internals of the ECM but I’m not seeing how it can put out the needed 12 volts and drop when the relay is plugged in.  I wouldn’t think it would take much to just pull in the contact. I could see if there was an issue with the VP after it pulled in causing the voltage drop but I have the VP unplugged so all it’s trying to do is pull in the contact.      I really wanted to get everything back to original by doing all this but I’m starting to think maybe just using a key on signal from somewhere else.  Thoughts on that?  Will it cause any other issues?     Interested in the “ dirty fix”. 

[Great work!]

Dirty fix is tap brown/white wire going to the relay into the lt green/black wire that comes on with the key. Down side is you loose reverse polarity protection for the vp.

[Tractorman]

3 hours ago, webbrenda said:

I guess what I’m asking is how would I use a test light in this situation to find the poor connection. 

 

You would have everything connected so that the electrical system is fully operational.  You would find the most accessible test points along the circuit to be tested.  In your case the circuit will start at Fuse #3 (20 amp) in the PDC.  The next item downstream in the circuit is Joint Connector #2, also in the PDC.  One wire leaves Joint Connector #2 and goes to pin 30 of the fuel pump relay.  The other wire continues to Splice #161.  One wire leaves Splice #161 to an unknown location, the other wire continues to Splice #167.  Three wires leave Splice #167, one to an unknown location, and the other two terminate at pin 48 and 50 on the ECM.

 

You would have the ignition switched on and have your voltmeter hooked up to pin 86 on the fuel pump relay to continuously observe the low voltage you have mentioned.  With a test light clipped to the negative driver side battery terminal, you would begin probing any accessible place starting at Fuse #3.  If the light burns brightly, then you have eliminated that part of the circuit as a problem.  If along the way you suddenly find the light burning dimly, then you have located a general area of where the problem is.  As you proceed, you are not proving where the problem lies, but you are proving what is working properly - a process of elimination that will lead you to where the problem lies.

 

But, before doing all of that, have you verified that no pins have been pushed back into the wiring connector at the ECM?  If everything is good there, then try the following:

 

Disconnect the wiring connector from the ECM.  Using a jumper wire protected with a 1 amp fuse, jumper pin #36 (brown wire with a white tracer that leads to pin 86 on the fuel pump relay) in the wiring harness side (not the ECM side).  If the fuel pump relay activates and voltage stays high and the test light burns brightly on pin 86 of the fuel pump relay, then you know the wire is good between the ECM and the fuel pump relay.  If the voltage remains low and the test light burns dimly, then you know there is a wiring problem between the two location.

 

Please verify the pin numbers that I have given are correct and that my instructions make sense to you before you do any testing.

 

- John

Edited April 26, 2023 by Tractorman
'cuz

[webbrenda]

John 

Thanks a bunch for your time and excellent explanation.  Very much appreciated.  I will do this when I get home tonight and report back. I have read and used forums like this to help me over the years but have never contributed when I figure something out and you have inspired to take the time and share. 

[webbrenda]

John

I just ran a jumper wire from pin 36 which is a brown with white tracer and didn’t have voltage to pin 86 at the fuel system relay.  In tracing the wire up I found that the brown/ white wire isn’t in the same location in the plug at the back of the engine at the fire wall.  This is a new wiring harness I purchased from Firsttruckparts.  Their part number is 3944847 but it says “ other oem part numbers and list 4 or 5 numbers.  One of which is 3934387 which I thought was the one I needed from looking at several websites.   All the connectors on this harness are correct and in the correct location.   Can you verify the correct part number I need?   There was no tag on my original harness.   I had ordered one off eBay several months ago and had tried it before this one and found it was wrong and missing some connectors and I’m assuming it’s for a 2000 or 2001 as they don’t have a crank position sensor I believe.   I have emailed Firsttruckparts but I’m sure they are closed today.    I would try to figure out how to remove the pins from the plug and correct this one but I’m sure that’s going to snowball.   Again, any help or suggestions are very much appreciated. 
 

[webbrenda]

I went through and all the wires on the ECM plug match color and position with the original. The issue is at the 2 plugs at the end of the harness that connect behind engine at the fire wall ( the 11 and 14 pin pin connectors).   Short of completely undoing all the wrap and tracing each wire I’m not sure what to do.  I do find that there is a solid blue where the brown/ white wire should be and the brown/ white wire is where the blue wire should be compared to origainal harness but there are extra wires on this harness compared to original.   Can’t seem to find a diagram that shows color wire on the 11 and 14 pin connectors.   

[Tractorman]

I am sorry, but I don't know how to help you acquire the correct part number for a wiring harness.  But, I would like to back track just a bit.  Are you using a FSM for a '99 year model truck?  If so, can you post a photo similar to the one below.  The diagram in the photo below is from my 2002 FSM.   The wire (BR/WT wire) from Pin 36 goes through Connector 125 (pin 8) and Connector 130 (pin 9) and arrives at pin 86 on the fuel pump relay.  The color of the wire stays the same from beginning to end.  What does your diagram show?

 

It does sound like you have a wiring harness issue.

 

- John

 

Edited April 30, 2023 by Tractorman
Correction - 2002 FSM (previously stated 2001 in error)

[webbrenda]

I was using a 99 wiring diagram that was posted on here.  The brown/white from pin 36 goes to the 11 pin connector at the fire wall and straight to relay. 

I can’t find anything that shows those 11 and 14 pin connectors for a 99. I found one for a 2000 but I haven’t been able to figure out what harnesses interchange or if any of them do.   From ones I’ve seen for sale it looks like the 98.5 and 99 were the same and 2000 and 2001 were the same.   I can’t imagine why the new one I bought has all the correct plugs and all the wires on the ECM plug are correct but it is different at these two plugs. 

[IBMobile]

The engine wire harness part numbers for model years 1998.5, 1999, 2000, and 2001 with manual transmissions are all different.

1998.5......05611436AA

1999..........05014177AA  supersedes to... 56045480AC

2000 up to 12/23/99.......56045319AD

2000 on and after 12/23/99.....56045319AE 

2001-2002  up to 1/7/02.......56045904AB

2001-2002 on and after 1/7/02........56045904AC

[Tractorman]

A big thank you to @IBMobilefor answering the question about wiring harnesses.

 

- John

[webbrenda]

Yes. Thank you.  Looks like that part number for 1999 is the other end of the harness that connects to PDC.   I was looking for the ECM harness.   Would you happen to have those part numbers?  

[Great work!]

Here is a photo of my 1999.

As you can see my wiring is transitioning for my new project. 

I hope this helps.

 

[webbrenda]

I know I’m not going to have enough information but here goes.   I found a blue wire and the brown/white wire in opposite positions of where they were in the old harness so I cut and spliced them to correspond to match original harness.  Now I have voltage to relay and truck starts and runs fine. CEL went out and I have no codes.   I thought I had it but noticed volt gauge was only showing 12 volts or less with new alternator.  Checked the big wire at back of the alternator and it’s putting out around 22 volts and blew the 140 amp fuse.   Checked the blue wire at back of the alternator and it’s shows 12 volts.     What would cause the alternator to be putting out 22 volts?  Voltage regulator correct?   I’m not knowledgeable on how the regulator works in the PCM nor do I understand how anything with what I did would cause it.  Help please and thank you. 

[IBMobile]

The connector on the left is C126 and the connector on the right is C125.  Below are the wire pin out diagrams for C125 and C126.They are the same for 1999 and 2001 in the FSM.