99 Altima hesitation: I think I'm onto something!

In my *limited* experience, the semi-short is probably AT THE CONNECTOR ITSELF, where the wires join up with the connector pins.

You will notice that unless the connector is potted, and I bet $2 of your money that it isn't, some adjacent wires are shorting via the connector itself.

TAKE APART THE PLUG, and inspect it with a magnifying glass, especially the wires/solder-terminals around the wires you're concerned with.

The solution is to pot the wire/solder connections, or just bend them out of eachother's way.

Lg

By "potted" do you mean that each wire enters the connector in its own channel and doesn't share a common space with the other wires? If so then that's actually how the connector at the TPS is made. Also, the two wires are not adjacent at the TPS connector, they have a third wire between them, and there is no conductivity between the middle wire and the outer two.

The two wires ARE adjacent at the ECU connection however, but no amount of prodding or jiggling seems to produce any variation in the resistance... I would expect at least some change if they were making partial contact there. Then again, I would also expect some variation at some point as I've been wiggling the whole harness from the TPS to the ECU, but I haven't had anything definite.

I haven't been able to get either wire out of the ECU connector yet, it looks like I have to insert something in the terminal side (not the harness side) of the connector in the hole above where the pin from the ECU goes, and pry the locking tab down, but that hasn't worked, yet.

At this point, I'm seriously considering cutting one of the two wires at both ends, and jumping it with a new piece from the TPS to the ECU... the only issue there being that that becomes a weak spot in the connection and I would have to find a way to run a wire permanently.

Thanks for the reply, I'll keep at it...

It doesn't have to become a weak spot, it can become the strongest part of the harness. When I do such things myself, I take a piece of air hose of the right length, slit it on one side for the length, and

*tuck* the new wires into the rubber hose, then tape the hose closed along the incision that I made. A utility knife with a razor blade works best for this. Just make sure the hose/wire assy doesn't come into physical contact with hot parts, like parts of the exhaust system, and route -away- from electrically noisy things, like plug wires and coil packs.

Silicone rubber in a tube can seal up the plug itself and make it waterproof.

Lg

Good point. It's not a very high resistance short in relation to the resistance expected at WOT though... 1.2 Kohms vs 4, so it seems like a short of that level would have an effect.

I'll give that a try and see what I get.

Thanks for those suggestions, I will test all of those things and post back.

Hi Kevin,

I just read your other post RE: Testing automotive electronics, and realized you may have thought I was testing the TPS directly and seeing the short between two pins on the TPS itself. What I'm actually seeing is a short between two separate wires in the harness between the ECU and the TPS, with BOTH ENDS of said harness unplugged. So in a normal situation, there should be NO conductivity between the two wires.

I also measured the TPS resistance at the ECU with the TPS end of the harness connected, and the ECU connector disconnected, and this was where I was seeing about 1/3 of the expected resistance at WOT.

Well, actually it is between the signal ground wire and the reference voltage, but yes. In my original post I was thinking it was between the TPS signal & the signal ground, but that's not the case.

I've got the factory service manual and it doesn't show any other devices connected in parallel, at least not in the TPS section of the book. Maybe one of the overall schematics would tell a different story. If there are multiple devices sharing the same reference voltage signal I would expect they are tied together inside the ECU and not all joined somewhere inside the harness?

I'm 99% sure I have the right wires; I can test either at the TPS end or the ECU end and I get the exact same resistance between the supposedly unattached wires, and if I test from one end of either wire to the other end of the same wire, I get a hard short, so that would seem to indicate I have the right wires. I did think it was strange that it's not a hard short between the two wires though, and also strange that it does not vary at all, even with all the moving I did of the wiring harness. The most change I saw was maybe .03 Kohm and I couldn't ever be sure that was due to my fiddling with the wires or just random changes in connection strength.

I'm anxious to give that a try in the morning and hopefully learn for sure if there is a problem here or not.

Not at all, it just sounded as if you had misunderstood my original post so I wanted to clear that up.

And I appreciate that. As does the rest of the group I'm sure.

Anyway, I'll do some testing of the voltages in the morning and report back; thanks again for the input.

Now that I think about this, something's not adding up. Maybe I've got it backwards in my head after going over it so many times. But if the semi-short is between reference and ground, then theoretically the current should pass through to the TPS rather than cross the 1.2Kohm bridge, unless the resistance between the ref. voltage and signal ground is greater than

1.2K. Obviously a test of voltages will show the truth...

I fault find electronics for a living so here's my thoughts.........

You really need to find out if that cable has a straight run between ECU and TPS with no branches off to other sensors etc. There wouldn't normally be any such branch but who knows, all I know is that my Nissan Primera hasn't got any.

If it's a straight run of cable then the technique for finding where this

1K2 resistance is is to measure it while heating up the plugs and cable with a hot air gun. The reading will 99% of the time change when you've found the right spot.

There's a possibility the reading could be a protection diode inside a plug and may not appear on the circuit diagram, if that's the case you'll get a different reading with the meter leads reversed. Some meters have a diode range and you'll get a reading of about 0.6v one way and no reading the other.

Well, you were right! I tested the voltage on pin 23 and it seems to be fine, it's within spec and free of any dropouts or spikes from closed to WOT. I think what I was seeing in the resistance checks was something in the branch of the TPS wiring that goes to the transmission controller in the automatic models. The service manual has it in the schematic with a broken-line box around it for automatics, so I just discounted it right away, but I guess there is probably still something attached there maybe with a resistor installed to close the circuit. So thanks for your guidance Kevin, if you hadn't made me second guess myself, I probably would have cut the wires by now and tried to run new ones... and would have been pretty damn depressed when it made no difference. Luckily all I have to deal with instead is a little dose of humility, heheh.

Regards,

Now that you have resolved that issue.......................... What was the original problem you were trying to diagnose. Maybe we can figure that out.

Well, since you asked... :) what I'm dealing with are 3 or 4 different symptoms that I think (and hope) are related to the same underlying issue. They are:

1. The car wants to rev fast when starting, warm or cold. This morning it was about 10 deg. C and when I started it revved up to 2k which I think is a bit high. That's not so bad on its own, but until it warms up, the car wants to rev to 2k or so every time I come to a stop and put in the clutch. Also, when I start it warm it still wants to rev to 2k, and usually revs that high, comes down to 1500, and then often revs back up once or twice, or just hangs, before coming down to idle.

As an aside to the above, this car has a fast idle cam that opens the throttle as the coolant temp decreases. I had that disabled for a few weeks and the over-revving when cold was gone. It still ran a little fast until it warmed up, but the highest I ever saw was about 1500 rpm. I'm sure this would not be a good solution in the winter though. Maybe I could adjust the FIC to open the throttle less, but as far as I can tell, it and the throttle plate are both adjusted properly.

1. Warm or cold, the ECU seems to want to rev the engine (open the IAC valve) whenever I touch the gas pedal. If I'm stopped and in neutral, I can ease into the pedal until the revs reach about 1000 RPM, but at some point slightly above those revs, the engine will speed up to 2000 RPM without my opening the throttle any further. It won't return to idle until I release the throttle.
2. More noticeable when cold, but also occuring when warm, is an abrupt jolt when I ease into or out of the throttle at crusing speed. This is obviously more noticeable in lower gears, but it seems to be most apparent around 2,000 RPM. I suspect the same behavior I see in the above point is what's causing this.
3. Last, the car has a slight hesitation off idle, when I step into the gas quickly. The tach will visibly dip for a split second before anything happens. This is most apparent when fully warmed up, and, even more so when having just started the car after it's been sitting for a few minutes and is still warm, eg, stopping to get gas and then restarting.

Here's what I've done to try and diagnose:

Tested TPS, OK Tested coolant temp sensor, OK Tested intake air temp sensor, OK Tested O2 sensor via onboard test in ECU (blinked LED about 10 times in 10 seconds at 2000 rpm) Tested MAF sensor output, seems OK. Tested fuel pressure, OK Tested absolute pressure (vacuum) sensor, OK. Cleaned EGR valve and passageways, also disconnected vacuum line to EGR valve, no change. Cleaned throttle body and plate.

I've put in new plugs, wires, dist. cap & rotor, and air filter recently. I've had the dealer do their "regular" injector cleaning. I've run 1/2 bottle of Lucas upper cylinder lubricant. Smoothed out acceleration quite a bit, but no change in the above symptoms.

I think that's about it.

I think I'm to the point now where it's going to take a tech with a CONSULT scanner to get a look at what the ECU is doing when all this is happening to get to the bottom of it.. but as always I'm willing to try anything anyone can suggest.

Vacuum leak. Check all Vac hoses but especially intake manifold. Do propane test.

Lg

I've done what I could in this respect. Taped off the intake tube and blew into the idle bypass pipe, everything held pressure. All the vac lines I can find are fine, and I tried shooting TB cleaner around the intake manifold runners where they bolt to the head, with no change in idle. I guess it's not uncommon to have a leaking manifold gasket in this car, but it doesn't *seem* like I have one.

Restricted air filter fuel pump or pressure regulator defective pull the PVC valve to be sure it is working correctly

X>pull the PVC valve to be sure it is working correctly PCV valve ( positive crankase ventilation valve )

PCV is the only thing I haven't checked, because apparently the valve is buried up underneath the intake manifold, and I've heard from a few different people that they've never seen one go bad in a Nissan. Not that that's any reason not to check mine, but that combined with its location has made it a lower priority. I'll keep it in mind though as more things are elminated.

location location location. I know exactly what you mean. OTOH, it is a $3 US part, and it is probably a good idea to _replace_ it if you're going to the trouble of digging the old one out.

You want a *nice vacuum* there when the engine is running, sucking fumes out of the crank case. Just put your hand over the end of it when you pull it out of the engine port, and you should feel a nice vacuum. Then *jiggle it* to see if the weight is free to move back and forth, and doesn't *stick.* Or like I said, just replace it. If it has never been replaced before, for the price, this should be done anyhow.

Lg

Like others suggested, look into replacing the PCV. Also, is the idle screw missing? On the throttle body, there should be a screw in place to stop the cable actuated butterfly valve from resting at the dead stop position, so to speak. When I bought my Sentra from a used dealer, that screw was missing, and acceleration off of idle was always always abrupt. Replacing that fixed it.

Always do the simple things before you go Einstein on the electronics, I always say:)

CD

I would have to do some digging through the manual, but I think your car is also equipped with a throttle or idle tracking switch. That is another switch besides the TPS. It has contacts that tell the ECM when the throttle is all the way closed and all the way open. If you have a repair manual, try looking up the test procedures for that switch. I think it may even be adjustable. I do know that your symptoms are similar to other vehicles I have worked on that did have problems with the idle tracking switch. I have also seen similar symptoms from faulty gear selector and clutch switches. These are the ones that tell the ECM you are about to load or unload the engine. I'm not sure if your car is equipped with those switches, but if it is, then they need to be checked. BTW. All those switches will not normally set a DTC when they malfunction because the computer just accepts whatever position they indicate as fact.

As other have replied, vacuum or air induction leaks will also cause similar symptoms. Check brake booster, PCV, air duct to filter, etc Don't guess you have a smoke machine?

I'll be in training all this coming week so I may not get to post until late evenings. Hope this will give you some more stuff to consider though.