About 2 weeks ago I tested out a late DME in my early car, along with a late AFM (this was to see if the DME was good). After this test, I put in my early DME/AFM back in and it started and drove fine for about a week, but now I have a confusing electrical problem.

The morning after putting my early DME back in, it wouldn't start. I installed a new DME relay and it fired right up. I should note that over the next few days, it would take more cranks than usual to fire. Couple days later, same no start problem, another relay did nothing this time. Cranking for about 15 seconds total started it, and when it got started, it ran fine across the RPM range and didn't die when driving around town.

Now it won't start, period, and I believe I'm looking at a wiring issue. Jumpering the relay changes nothing. I have tach bounce. I've tried 2 early DME units, both run the car just fine when it is running.

I have no voltage at fuel pump, but 12v on the coil. Even without the DME relay being plugged in, I have 12v at the coil. I've tried a different ignition switch and fiddling with the relay but nothing works.

I'm really busy (midterms week) and I'm only getting 30 mins here and there to work on it - but I believe my next step is to see if the DME is getting power. What's the best way to verify this? If I have tach bounce, is that a sign I have DME power?

 

Call me

 

Have you tried running the pump directly at the terminals with a 12v power source?

 

This. You need to first verify the pump is even good by jumping it off the battery with a pair of jumper cables. Only do it for a second just to make sure it works, and watch the polarity. If the pump is good, you need to figure out why there isnt power at the harness. With the DME relay jumpered, that should force power to the fuel pump independent of the DME or anything else getting power. At that point, if there is still no power than you have a wiring issue between the fuse block and the pump. There is a large ground on the driver side rear of the car, underneath the carpet. You should clean that ground and see if that helps. After that, you should grab some jumper cables and run 12vdc+ from the battery over to where the fuel pump harness terminates and see if there is continuity through the circuit back to battery - if you bring power over and test it between the harness ground. If so, then the issue is in the supply side of the fuel pump harness. This is more likely since the ground is short, ending in the rear hatch. If you are here, like I was with the N/A you can try and trace the 12vdc+ wire out of the fuse box and find where it breaks, or you can replace the whole lead and replace it from the fuseblock back like I did.

And just to be sure, you of course already checked to make sure the + and - leads of the harness weren't shorted to each other?

 

On early cars the ICV runs off the same circuit as the fuel pump. If it is getting stuck it might draw more current and choke the fuel pump. Try unplugging it and see what happens.

 

Does your car have an alarm installed.

 

Thanks for all the tips, finally had time today to work on it got her started with some weird findings along the way.

I modified an old DME relay with an internal jumper to eliminate variables there - and after some fiddling with the relay/fuse box connections I got a consistent 12v at the pump the second I hooked up the battery..

Turns out the fuel pump I replaced my old pump with was dead (it was basically new but a friend had borrowed it briefly...guess he owes me a new one), so I put the old one back on after verifying it was good on the bench. Installing this pump immediately built fuel pressure, I could actually hear it in the rail.

Naturally the car still wouldn't start... I had spark so that had to mean that the injectors weren't pulsing. I swapped in a different DME and after a few hesitant cranks it fired up.

Questions:

Is there any way that the injector pulsing circuit can go bad on a DME because of an outside source (like an electrical gremlin elsewhere in the car)? If so, I still have a problem somewhere - I don't want to be chewing up DMEs and crapping them out with bad FI circuits.

Could a bad battery/voltage regulator explain any of this intermittent no start stuff? It runs fine when it is running, and always cranks fine, but maybe the DME isn't able to work properly.

I had good spark (at the plugs) even with the "bad" DME, so something isn't adding up.

 

A short in your injector harness or just too low impedance injectors could definitely hurt the driver circuit. It does have a safety mechanism that should turn off the circuit if it is drawing too much current, so you may not actually be hurting DMEs. A good way to test that is to try running with one injector unplugged to lower the load on the driver circuit.

 

I'll try that next.

I just bought a new battery (previous one was from 2005) and it didn't change a thing. I can hear the engine firing, but just not catching. I am 99% sure the injectors are not staying open long enough for the first ~20 seconds of cranking... and once the engine heats up a little, it starts. When the engine is warm and I kill it, it restarts on the first crank of the starter.

I measured about 6 ohms between the engine's head and the ground terminal when the car was off. When running, it was about 15-20 ohms. Obviously I cleaned the main engine ground thoroughly, but it's still 1.3-1.5 ohms when off and 8-9 ohms running.

Are those in spec? I would think that the resistivity and geometry of the circuit would lend to < 0.5 ohm.

 

Okay just had a thought.

That EFI temp sensor in head acts as a thermistor between ground and the DME - I wonder if a bad earth circuit (or a problem between the sensor and the DME pin) is causing it to run too lean until it heats up a little - because the engine is trying to fire and simply not catching, which makes me think it's too lean. The only temperature-dependent components that affects AFR that I can think of are that sensor, and the temp sensor within the AFM by which air density is calculated.

I'll test it as per Clark's when all is cold in the engine bay.

 

Dear Hacker: So what was the final culprit besides the fuel pump circuit? No injector pulses??My '83 has been derelict for many years and I need to know more about the DME/fuel pump circuits/ ignition circuits before I try to start this thing once the engine comes back from the shop...Thanks, Tiger 03447

 

I'm still not sure exactly where the problem was, but it's gone for now (knock on wood). I took down the fusebox and cleaned up connections, and it was about an hour later that I remember it starting.

In hindsight, I'm pretty sure my multimeter was busted, it was giving readings I verified to be off by about 1.5 ohms later (it was reading high). I also believe the thread title was ill-chosen, I probably was getting 12v the whole time at the pump, the multimeter was also at fault.

 

Multimeters are not very accurate for measuring low impedances. Anything under 3 ohms should be considered a short. It takes more sophisticated equipment to measure low impedances. A better method for measuring low impedances is to apply a voltage and measure the voltage drop from end to end of the wire to see if you are getting excessive power loss.