Fuel pump ground

[bruscreen]

On the two days so far that I've been able to get my Delorean running, I've noticed the fuel pump works sporadically. I jumpered the RPM relay today and got a closer look. There are 12 volts present at the wires going to the brand-new DMCH pump, until the pump is plugged in, then nothing. So I ran a wire from the pump directly to ground at the battery, and the pump worked. I'm thinking of making a ground cable to run to the master cylinder (where there is good ground) but it feels like this should not be necessary, at least in the long run. Has anyone seen this before and know a better fix?

[Ron]

You will defeat the inertia switch (a safety feature) if you do that....

Check out this thread (especially page 2)...

http://dmctalk.org/showthread.php?1163-Melted-number-seven-fuse-Fuel-pump

[WelmoedJ]

+1

Grounding to the pump is essential for it's proper operation.
Cutting the ground is essential to cutting power to the pump in case of a roll-over of the car.
The inertia switch is there to stop the pump in this unhappy event.

The inertia switch is likely the culprit.
If the contacts are good, you best replace the current ground wire from the inertia switch to a better (frame) ground.
I have followed Bitsyncmaster's idea on this and used a 10 AWG (approx 2 feet in length) from IS to one of the mounting bolts of the shifter.
This not only stabilized the pump's functioning but also got the voltage (on the scale) up by .1 Volt.

You also should check the wire connectors in the lead to the pump, near the pump and the eyelets on the pump.
Once you have doine that, the pump will function without irregularities.

[nkemp]

Read the thread: http://dmctalk.org/showthread.php?1360-Ground-Bus

The thread may (or may not) be appropriate to your issues. I too had what I believe were ground related pump intermittent operation. More than once it got towed or pulled home. It would run fine then die. Later it would fire up without problem. I hate self fixing problems.

I installed a new ground bus and it seems to have resolved the problem. I noticed a big improvement in the voltage reading on the dash gauge as well. Likely did not improve the actual alternator voltage output but did improve the voltage at the front of the car. The ground cluster straight up from the gas pedal is the culprit. It is also that ground cluster that grounds the pump. It is not a terribly difficult job to do. I found the needed wire by buying cheap jumper cables of the right size and separating out one wire. Now the car is ground to the buss which connects directly to the battery.

While you are at it, check the related fuse (# 7 I think). Make sure it has not overheated. If the fuse panel is distorted or the fuse contacts are dark then you have contact problems. For that matter, this might be worth replacing even if no current problem.

Others have disagreed with me on this point but I'll mention it here again. Over the years of troubleshooting various electrical things at home and work, I've seen situations where digital voltage meters will display the desired voltage. As soon as you plug in the load it disappears. DVM's are very sensitive and are capable of measuring what I'll call "phantom voltages". Old style "meters" do not have this problem because of the load the put on the circuit. The trouble is that there is no current to support the voltage. This can happen when you have bad contacts such as the toasted fuse terminals where just a bit gets through the problem area.

The D's fuse terminals are very light duty. If you have toasted fuse terminals, one quick and easy way to resolve it to get behind the fuse panel, remove the terminals, install heavier duty spade connectors and use a remote fuse holder (from local auto store). Insert the remote fuse holder through the fuse panel holes and the fuse now "flies" above the old spot but you have MUCH better connections.

Note that I'm not disagreeing with the inertia switch being an issue (for starters, make sure the switch has not "tripped" for some reason). Verify that the switch is not the problem before checking the other issues.

BTW ... I don't like grounding to the frame... too many problems on the D. One ground problem I had was the main ground at the trailing arm. Although the ground was torqued tight, it failed. As soon as I moved the bolt the slightest, the car came to life. Corrosion had built up in the threads or between the ground and the frame sufficient to block current flow. I diagnosed this when I found that the SS body panels were hot (12Vdc as measured directly to battery ground).

All said & done I'm a believer in the new ground bus.

[EdR5150]

My pump completely stopped working due to a bad ground. I ran a short, thick ground wire from the pump to a bolt on the passenger side horn bracket. I did bypass the inertia switch, however I do plan to replace the original ground someday. If I ever get into a front end collision, hopefully it will be strong enough to break off the horn bracket, and shut off the pump.

J/K! That logic is flawed...

[Ron]

Others have disagreed with me on this point but I'll mention it here again. Over the years of troubleshooting various electrical things at home and work, I've seen situations where digital voltage meters will display the desired voltage. As soon as you plug in the load it disappears. DVM's are very sensitive and are capable of measuring what I'll call "phantom voltages". Old style "meters" do not have this problem because of the load the put on the circuit. The trouble is that there is no current to support the voltage. This can happen when you have bad contacts such as the toasted fuse terminals where just a bit gets through the problem area.
+1
I agree with this except the old meters can mislead you as well. For example, if you have a bad connection (power or ground) they can handle the 'power' the meter supplies and sends through it to test with for continuity/resistance, and good enough to pass enough from the battery/alternator to drive the meter while checking voltage. But, as said, when you put a load on it, it can't handle the amperage. An ice-pick probe is usually a good solution, but if in doubt, I use a high-beam headlamp, fused (or whatever is handy to match the expected max load of the circuit).

[bruscreen]

Thanks guys, lots of good info there! My main problem was not knowing where the ground wire went, so knowing where to look is a big help. I actually found info on the inertia switch last night, between the time I posted and when the moderators kindly moved this post where it belongs. Haven't made it back to the barn to check the switch yet, but will post back what I find.

[bruscreen]

Found the inertia switch, it's in the down position but when I took it off, there's no continuity through it at all and the indicator won't stay up if I pull on it. Looking at the contacts, there's melted plastic between two of them. No white mark either so it's the original unit.

All adds up to "time for a new switch".

[Ron]

Hey you're good at math. :)

[nkemp]

Don't you like it when it is simple! Now make it a harder job by getting a case of the "mizewells" and improve the ground circuit and check the fuse.

Mizewells are the reason I spend more time under my car than in it.

Nick