Need help with diagnosing possible electric problem

[Mikey's 68 RT]

hello, I have a 1970 rt charger. All is stock accept for electric fuel pump. At night when I have my lights on and am at a stop light idling or slow down close to a stop my dash lights start to dim and the idle almost starts to die as if I'm loosing power or the electronic fuel pump is taking to much power and when my lights are on (especially when heater is on or wipers running and lights) it barely has enough power to pump the fuel. As soon as I step on the gas and go, my dash light brighten up to normal power and fuel pump is functions at normal power. Day time with out lights/ heater/ or wipers, it's not that noticable. I Installed a alternator and other parts but it hasn't helped. I thought maybe my electrical want set up right with my electrical fuel pump or maybe something isn't grounded? Any ideas for diagnosis based off of these symptoms? Thank you.

 

[mmissile]

What amp rating is your alternator?

 

[super-bee_ski]

Usually flickering or dimming lights points toward your Alternator.

Simply put:
1.You state that you are at idle, slow RPMs, again, alternator produces more power through RPM (to a safe cut-off)
2. If you are at idle and slowly bring up RPM will lights stabilize?

So, with the addition of a electric fuel pump it might be that your alternator does not put enough AMPS out.

Now, bad news, AMPS can only be checked "in circuit"
Did you replace with a 60 AMP alternator or an 80 AMP?

Also, did you change pulleys?
If pulleys are mis matched you will not get enough RPM, but yes, it points to that in my opinion.
More RPM equates to more power, idle (LOW RPM) equates to dimming.

 

[Daves69]

I'd be checking grounds and connections. Load test your battery. What is your idle rpm?

 

[jimt1971rr]

My roadrunner has the same issue and I know it is due to the amperage output of the alternator. I know I need a higher amp unit but just really can't afford one right now.

 

[Mike67]

Do you have a relay on your fuel pump? If not and your running it through your fuse block it could cause the higher amp draw to lower the voltage at the fuse block. Voltage is inversely proportionate to current...

 

[funknut]

what fuel pump do you have?

When you say you installed an alternator 'and other parts' what other parts did you add/change?

 

[Mikey's 68 RT]

Do you have a relay on your fuel pump? If not and your running it through your fuse block it could cause the higher amp draw to lower the voltage at the fuse block. Voltage is inversely proportionate to current...

I think this will be my first elimination. I don't have relay and proper wire kit installed I wired it directly to my fuse box accessory 10amp fuse. Would this diagnosis still match the symptoms of my lights dimming and windshield wipers slowing down and car idle slows and almost dies when I stop in drive stop light or brake to turn? As soon as I put on the gas and go forward all power is restored and everything functions again like it should. Thank you for your help.

 

[440+6]

I had a similar problem at idle 750 rpms, I had a local alternator shop press on a smaller pulley a few years back and all has been perfect since.
60 amp alt and if I remember correctly went from 3 inch to 2.5 inch pulley

 

[funknut]

I think this will be my first elimination. I don't have relay and proper wire kit installed I wired it directly to my fuse box accessory 10amp fuse. Would this diagnosis still match the symptoms of my lights dimming and windshield wipers slowing down and car idle slows and almost dies when I stop in drive stop light or brake to turn? As soon as I put on the gas and go forward all power is restored and everything functions again like it should. Thank you for your help.

The fuel pump shouldn't be drawing more than 3-5 amps... But yes, everything you mention sounds like your alt just isn't putting out enough juice at idle. You need either a higher rated alt, or you could get a smaller pulley on your existing alternator to spin it faster.

 

[Dennis H]

OR, it is simply a characteristic. I recently got rid of the electric pump after 2 failures. The mechanical Mopar Hi-Po fuel pump works good, reliable, and quiet. No noticable change in performance. What transmission? I like a nice low lope of about 700-750 rpm idle TKO 5.

 

[Mike67]

I think this will be my first elimination. I don't have relay and proper wire kit installed I wired it directly to my fuse box accessory 10amp fuse. Would this diagnosis still match the symptoms of my lights dimming and windshield wipers slowing down and car idle slows and almost dies when I stop in drive stop light or brake to turn? As soon as I put on the gas and go forward all power is restored and everything functions again like it should. Thank you for your help.

I can't say unequivocally that is your problem but it could be contributing, you can test it by temporarily running a wire strait from the battery to the pump to test the theory.....
I would check your voltage at the battery, then check at your fuse block with it running & compare. You don't need a $40 relay kit either. You can buy a $4.- 30 amp relay anywhere, use the switched hot circuit that is feeding the pump to control it. Run a #10 wire from either the starter relay or directly from the battery to one of the contacts on the relay the other contact to the pump. The larger wire is able to carry the load better and isolates it from the fuse block. Also put an inline fuse on the #10 closest to the source you will use..
I thought I read you were having ignition issues so I deleted the part of my earlier reply...

 

[Shorthorse]

The fuel pump shouldn't be drawing more than 3-5 amps... But yes, everything you mention sounds like your alt just isn't putting out enough juice at idle. You need either a higher rated alt, or you could get a smaller pulley on your existing alternator to spin it faster.

I agree. Th OP said everything else is affected when in use (lights, wipers, fan), the more draw he adds the more the inability of the charging system to handle the draw at idle. The fuel pump is not drawing enough current to be the sole cause. A relay for the pump would not do any good because if the pump motor draws, lets say 5 amps, it draws that regardless of how it is connected. Like funknut said, a higher rated alternator or a smaller pulley.
You didn't mention what your idle RPM was and what amp alternator you have. Also, have you checked your voltage regulator?

 

[Mikey's 68 RT]

So I put in a 100 amp one wire alternator and tested my battery to make sure it's getting fully charged. I originally had a 65amp alternator that was wired with the 2 field wire connection. So during test drive all went well till I pulled up to a light and put my blinker on. I could hear the fuel pump slowing every time my blinker flashed. The fuel pump went hi to low to high to low. So if it isn't my alternator, would it be my battery is not enough Volts? Its putting out 12 volts. Or would this go back to a grounding problem? I haven't driven and tested the alternator with my lights on and wipers or heater. Thank you all for your help.

 

[68gtxman]

Get rid of your old mechanical voltage regulator and put in a modern electronic VR.

 

[Mikey's 68 RT]

With the one wire hook up alternator, it has a built in regulator so it's not even hooked up to my stock regulator. Is there something else that runs through my stock regulator still that woukd be causing my problem?

 

[Mike67]

For a test come straight off of the battery to your fuel pump If you don't want to put it on a relay just yet. I still say you have to much load on your fuse block with everything running. Imo, and I'm nobody here, that the oe feed to fuse panel is undersized for everything you are running through it. If you want to check how many amps it's pulling with everything on you can use a dc setting on a clamp on meter or get in series with a flat scale meter from the point it gets power....just my $.02

 

[funknut]

With the one wire hook up alternator, it has a built in regulator so it's not even hooked up to my stock regulator. Is there something else that runs through my stock regulator still that woukd be causing my problem?

If it is internally regulated you will need to remove the stock regulator.

Someone can double-check me here, but I believe all you need to do is remove the blue wire from the regulator and insulate it, then remove the regulator and green field wire.

 

[Mike67]

You should have about 12.4- 12.7 volts at your battery...if it's less you could have a bad cell...

 

[Mikey's 68 RT]

Battery's good it new. Please tell me more about how to disconnect my stock regulator since my new alternator has a regulator attached to it and I am not using my field connections due to its a one wire hookup alternator.