Bad Ground/Power Connection; PCM?


DNGR_RNGR

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Hi guys,

New member here, long time member over at FTE. I have searched for hours here on this forum and need some advice.

1994 Ranger 4.0L/5spd 4x4

I took my truck to my mechanic last week to have the clutch changed in it. Since we couldn't get it in the shop right away, I wanted to diagnose a problem I had been having.

The truck would fire right up every time and ran very good 85% of the time; the other 15% of the time it ran crappy. Did this intermitently. The CEL doesn't work, so I wanted to have it scanned.

I have cleaned the MAF, installed a new IAC and TPS.

The day we tried to scan the computer, something got screwed up and the truck wouldn't start right. It seemed like the PCM was getting a command that the truck was at WOT. The engine would instantly flood upon trying to start. The only way we could get it to start was to unplug the fuel pump relay and then plug it back in once it started. Once running it would idle at ~2500-3000 rpm.

While my mechanic was working on another vehicle I pulled the intakes and VC's off and changed the gaskets. Buttoned everything up and got the clutch changed.

Now the truck will start with the fuel pump relay plugged in but only if it is cold and you have to crank for quite some time. Once the engine is warm and you shut it down, it doesn't want to start again without cranking it for a VERY long time. Always getting spark. It idles better and reving the throttle indicates the engine is quite responsive.

We checked the fuel pressure at the rail and it is fine. Holds pressure after the truck is shut down. So we believe the FPR to be fine, no gas in the vacuum line.

The computer is spitting out a pile of codes. 113; 118; 121; 341; 522; 539; 542; 558. The 118 code for the ECT can't be right because I changed it for good measure while I had the lower intake off.

Then we took a test light and hooked it to the positive side battery terminal and put the probe into the ground wires (back probing) for the TPS, ACT, and the DPFE sensors. Doing this would barely light the test light. So we were starting to consider a weak ground in or going to the PCM. We have also considered the PCM to be bad. We have swapped relays around to no avail.

I know this was a long post and I probably forgot some details, but this truck really has us baffled. Some of the codes don't make any sense. There was a code for the AC/defrost being on during the test-it wasn't. Another code for the vehicle not being in park/neutral-it was.

ANy ideas? Thanks for any help you guys can lend. Great site!!
 
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DNGR_RNGR

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UPDATE

I spoke w/ my mechanic over the phone a while ago. He took a jumper wire from the negative battery terminal to the ground wire on the TPS. Verified he had a good ground there and tried to start the truck. Truck fired right up and ran smoothy.

What are would be the best ground locations to check for loose fitment? The ground on the fender next to the PCM looked good to me last night, but I didn't test it.

Any thoughts?
 

RobbieD

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There should be multiple battery negative cables or wires- not just only a single one to the fenderwell or other body part. There should also be a ground cable to the engine itself (which is really sounding suspect here), and also very likely there's another grounding cable to the frame, too. My '94 2.3L has (stock) the triple-point grounding as described from the battery's negative post. One to the body; one to the engine; and one to the frame.
 

DNGR_RNGR

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Thanks RobbieD!

My mechanic just called to tell me that it is running and what he did. He took a new wire and ran it from the fender and spliced it into the ground wire that feeds the various sensors (IIRC, it's gray). Took the truck for a drive and it runs much better. Pulled the mountain behind his shop (not a small mountain) at 70 mph in 4th gear. Idle is good and the driveabilty issues I had seem to be gone.

I will check those grounds out once I have it home. Are you suspecting the ground strap running from the engine block to cowl? I suspected that last night, but it was very late and I was up for 16+ hours.

Thanks agian!
 

RobbieD

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I don't know if there is a "factory" ground strap from the cowl (or other point) to the block. It certainly wouldn't hurt anything to have on, though.

I'll check the EVTM tonight, for the 4L version, but if it's like my 4-banger the design of the system is that the engine, body and frame are all three directly grounded to the battery negative post. Just a guess, but it sounds like on your truck the battery-to-engine cable may be loose or simply bad. On mine, if I recall, from the battery negative post there is one ground cable to the inner fenderwell (grounding the body), and then a second cable which goes through a terminal bolted to the frame rail, then continues to the engine block (grounding the frame and engine). In effect, the truck's three main components are all direct-grounded to the battery.

On a lot of old-school electrical systems the engine was grounded directly to the battery negative posts, and then supplemental grounding straps connected the engine (electrically, or in a grounding way) to the body, chassis and frame; as in "daisy chaining" the grounds versus direct grounding electrically the vehicles' main components.


Added later: Yep; the EVTM shows two more additional grounds in addition to the ones already described. There's a pair going through a round plug, that grounds a lot of engine management stuff. Sounds like you were weak on this circuit, as your mechanic jumpered it and the engine's running better. Might just be corrosion in that plug, but at least you're on the right track.
 
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