Low voltage to fuel pump from CCRM

Cromulon · Jul 31, 2017

I am replacing my 1995 Mustang GT fuel pump. If I put 12 volts directly to the fuel pump through the connector on top of the tank, it doesn't turn. New Walbro 255 lph coming today. The dead fuel pump is original.

However, I am worried low voltage from the CCRM killed the fuel pump. I measure 9.6 volts coming out of CCRM on the wire connected to pin 5. Weird thing is there are 12 volts are coming in on the wire going to pin 11. This is with key on engine off.

I replaced the CCRM and still see 9.6 volts on the pin 5 wire just outside the CCRM plug. Same as old CCRM. Dang.

Using info from this Anatomy of CCRM (link below), I tested the old CCRM (out of the car) by putting 12 volts across pins 12 and 18, heard the relay click, and found near zero resistance across pins 5 an 11. So it seems there is no problem with the CCRM.

https://www.slideshare.net/mark0ls0n/anatomy-of-a-96-mustang-ccrm-r13

I am stumped. Next thing to check is some issue with the CCRM plug since I am probing the wires just outside the plug.

I am worried low voltage will kill my new pump. Maybe the fuel pump supply voltage will vary with engine load or something and there is actually no issue?

Any thoughts are greatly appreciated. Thanks.

white95 · Jul 31, 2017

Typically, I'd say a voltage drop across a set of contacts meant they're corroded or poorly made. Was the replacement CCRM In fact new or used? If all else fails, you can wire in your own relay and find a switched power source to tell the relay to energize.

Cromulon · Jul 31, 2017

The CCRM was new. Or I assume it was, I ordered it from RockAuto. Odd that both would have same voltage drop to 9.6v.

I'll pull the new one out tonight and test it. My assumption is no trouble will be found just like the old one. It is weird that it works fine outside of the car. I wonder if some other CCRM wire has a poor connection that is affecting the CCRM fuel pump feed. However, studying the CCRM schematic, I do not see a way for that to happen.

Agreed is looking like I'll have to run my own fused supply wire to the pump. Could even use the 9 volts already back there to switch a new relay.

I'd still love to find the actual problem. Will also be testing the CCRM plug tonight.

Thanks!

busta · Jul 31, 2017

Cromulon,

Would you be so kind as to take a picture or describe where I can find the connector that you used to provide a 12v source to the fuel pump? I have a '94 as well.

Just so you know, I also have about 9.5v out of the CCRM. Also measured the same voltage at the interia switch when I was diagnosing. I did try another CCRM as you did. I was actually thinking it could be caused by the fact that my car has not been running and the battery voltage may be stressed when turning the accessory on at this point.

Cromulon · Aug 1, 2017

Hi Busta - After dropping the tank to gain access, I connected 12v directly to the fuel pump connector on top of the tank. I made a 12v source with a 9volt battery and two AAs. The pump did not come on. In hindsight, making electrical connections that close to gasoline was dangerous. Not a good idea.

Interesting that you also have the same low voltage from CCRM.

Did a ton more work last night. Turns out the fuel pump had not failed!

The two fuel pump pins outside the tank were an open circuit. But, after removing the old pump from tank, I noticed varnish on the spade connectors on the fuel pump itself. I cleaned the pump connectors, hooked up 12v, and the pump ran and sounded ok! So the low voltage did not kill my pump. Instead, the deterioration of the spade connections on the fuel pump itself broke the circuit.

Went ahead and installed the new pump, and after a ton of tank wrangling, she rides again!

Let's see how long the new pump lasts. Fingers crossed on another 20 years!

ttocs · Aug 1, 2017

you tried to run it with 9v and two AA?! While the voltage is 12 volts if those batteries were FULL-FULL you would be lucky to pull 1/2 an amp. The stock fuel pump runs on a 30 amp fuse and while I have never tried this I would have to hazard a guess that with out the current its unlikely it moved the pump at all.

evilcw311 · Aug 1, 2017

I almost pissed myself laughing at the whole batteries attempt!!! Lol

Those are for devices that use only milliamps of current and can not discharge fast enough to make that kind of current.

While I do give ya a C+ for effort, I had to deduct tho for the batteries!!!

Sent from my iPhone using Tapatalk

Cromulon · Aug 1, 2017

Hahaha. The batteries totally worked!! Both fuel pumps spun. Good test.

I will give you it may not have spun if the pump had any load to deal with at all.

ttocs · Aug 1, 2017

evilcw311 said:
I almost pissed myself laughing at the whole batteries attempt!!! Lol

Those are for devices that use only milliamps of current and can not discharge fast enough to make that kind of current.

While I do give ya a C+ for effort, I had to deduct tho for the batteries!!!

Sent from my iPhone using Tapatalk

The image of a 9v with two AA's on the end of it stuck together is an image I will not be able to get out of my head for some time. But I don't mind it makes me grin every time I think about it....