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O/D lurching

J

jsark

CEG'er
Joined
Jul 1, 2002
Messages
52
I've acquired a peculiar ATX behavior recently. While in O/D, with "regular" application of the accelerator, the tranny repeatedly, subtly hesitates/lurches.

I've got two '98s, and the one with higher miles experienced the MLPS hard lurch behavior last year. Replacing the MLPS fixed that issue. So, when my other 98 began doing something similar, I leapt to the conclusion that it was the same issue.

So, I ran out to my local dealership and picked up an MLPS for $50 and swapped it out. No fix. Behavior still exists. I'm 95% certain the behavior occurs only while in O/D. If I depress the accelerator VERY slightly to gain speed, it won't occur. It's only during "regular" acceleration that it starts to stumble over itself.

Once the weather warms up a bit, I'll do a drain and fill, but let's assume for a moment it's not the fluid. Any ideas? Is there an O/D-specific sensor or something that I could try swapping out?

Apart from this behavior, tranny performs as advertised.
 
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When the VSS goes, it affects the OD. But that usually results in speedometer madness and "OD off" flashing. I suggest you check the VSS connector and wiring first.

Edit. Actually, check the fluid level and condition first.
 
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My 98 did that when I started having low-grade misfires.

O/D slightly lugs the engine and will cause the misfires to show first, generally.

If you have a scan tool that does Mode 6 data, check to see if the MISFIRECOUNTs are elevated for any cylinders.

Steve
 
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Thanks for the replies, gentlemen.

May as well start with the simple stuff first, so I'll probably do at least one drain and fill this weekend, and will report back with the results.
 
Progress update:

I did a triple drain-and-fill over the weekend, with no noticeable benefit. The bumping/lurching/hiccup still exists as it did prior to the fluid service.

I need to revise the symptom description to remove the O/D condition. I revised the thread title to reflect this. I was quick to suspect an O/D-related problem, because initial road troubleshooting suggested this.

Following the fluid service, I performed more thorough road testing and determined the bumping/hiccups occur whether O/D is engaged or disengaged.

Again, the bumping occurs primarily at a certain point of accelerator travel - what I would classify as the pedal zone for 'normal' acceleration. Hard application of the accelerator does not produce the problem, nor does feather-light application of the accelerator.

The bump never happens (as far as I can tell) below 40mph, but does occur as high as 80mph. So, to recap my progress so far:

1. MLPS swapped (no change)
2. Triple fluid drain-and-fill (no change)

Any further suggestions/guidance would be appreciated!
 
My guess is the TPS has a dead spot. This is why it's at part throttle. That's where they usually go bad. If you have access to an oscilloscope, test it and look for an anomaly in the graph, and if not, just go ahead and replace it. They are not that expensive.
 
Unfortunately, I don't possess the equipment or expertise to perform analysis or diagnostics of any sort.

Fortunately, I do possess the time, money and ability to swap out parts. Does anyone else have an opinion regarding mcgarvey's recommendation?
 
TPS is less than $30. Unless you have an oscilloscope and know how to use it, you can't test it. I'd put my money on this being your problem.
 
mcgarvey said:
TPS is less than $30. Unless you have an oscilloscope and know how to use it, you can't test it. I'd put my money on this being your problem.
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you can test it with an analog ohm meter as well. you just hook it up to the correct pins and slowly open the throttle plate. resistance should change with throttle position so you can watch the analog needle move. if at anytime it jumps to "infinity" then it has a dead spot and needs to be replaced.
 
Thanks, striker. Unfortunately, I don't have an analog meter.

For $30, I'm definitely willing to try the TPS. Hopefully, it's located somewhere easy to recognize. I'll check it out tonight when I get home.

Will post back results...
 
striker2 said:
you can test it with an analog ohm meter as well. you just hook it up to the correct pins and slowly open the throttle plate. resistance should change with throttle position so you can watch the analog needle move. if at anytime it jumps to "infinity" then it has a dead spot and needs to be replaced.
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Striker, I don't mean to be rude or anything, but have you ever actually tried to do this? Every dead spot on a TPS that I've ever seen has been so narrow that only a scope set to the slowest time range will show a definite "spike."

Like I said I'm not trying to be a jerk-- obviously you know what you're doing considering what you've done with the zetec, but I can say that in my years of working on cars for a living, I've never seen a bad TPS (or any potentiometer for that matter) that could show a fault using only an ohm-meter. Then again, it's been years since I've had any analog electrical testing device.

On a customer's car I would normally just throw a new TPS in and try it, but if it were my car (and my money for the part), I would certainly test it first. An oscilloscope is a tool with so many uses, if you ever come across a deal on a used one, definitely pick it up. As far as using analog multimeters on modern computer controlled electrical systems, you're asking to fry something with the current those things draw.
 
mcgarvey said:
Striker, I don't mean to be rude or anything, but have you ever actually tried to do this? Every dead spot on a TPS that I've ever seen has been so narrow that only a scope set to the slowest time range will show a definite "spike."

Like I said I'm not trying to be a jerk-- obviously you know what you're doing considering what you've done with the zetec, but I can say that in my years of working on cars for a living, I've never seen a bad TPS (or any potentiometer for that matter) that could show a fault using only an ohm-meter. Then again, it's been years since I've had any analog electrical testing device.

On a customer's car I would normally just throw a new TPS in and try it, but if it were my car (and my money for the part), I would certainly test it first. An oscilloscope is a tool with so many uses, if you ever come across a deal on a used one, definitely pick it up. As far as using analog multimeters on modern computer controlled electrical systems, you're asking to fry something with the current those things draw.
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i have actually done it and seen dead spots, you have to go really slow and pay extremely close attention to the needle. i went to UTI for auto tech and i couldnt agree more with you about getting an O-scope, so many uses its almost invaluable. as for using an analog meter on modern cars, your actually less likely to fry stuff because of how much resistance is actually built into it. a DVOM has very little resistance which is why they put fuses in them. that way you blow the fuse in the meter before you fry anything in the car.

also i agree that if its a customers car and i have a known good TPS i will more than likely just use it and if it fixes it great, if not i put the old TPS back in.

in this case, since its less than $30 its probably worth just replacing it anyways.
 
Okay, fellas. I picked up a TPS at Autozone for $25 on my way home. I won't have time tonight to install it, which actually gives me time to ask one more question.

I didn't see a how-to for TPS swap-out, so is there anything special I need to do, or is it a straightforward part-off/part-on job?

For instance, when swapping the MLPS, the ATX must be in neutral and the lines on the sensor body must be lined up. Anything like that for the TPS? I don't see anything noteworthy on the part itself.

Thanks!
 
OT, somewhat..

OT, somewhat..

As far as using analog multimeters on modern computer controlled electrical systems, you're asking to fry something with the current those things draw.
Click to expand...

The warning is correct, but for the wrong reason.

as for using an analog meter on modern cars, your actually less likely to fry stuff because of how much resistance is actually built into it. a DVOM has very little resistance which is why they put fuses in them. that way you blow the fuse in the meter before you fry anything in the car.
Click to expand...

You have it backwards.


Analog VOMs typically have an input impedance of 20K-50K ohms/volt for the Volts function. That means for every volt of electricity in the circuit under test, the connection of the meter is a parallel resistance path through which current will flow. In high current, low voltage applications, this additional loading is negligible and can be ignored. In a low current circuit, the VOM loading can substantially lower the voltage measured and increase the total circuit current (Kirchoff's Law).

DVMs typically are rated at 10 Megohm/volt input. You'll be hard pressed to find a circuit that it loads until you get to very high voltages (which would normally require the use of a high voltage divider probe anyway).

The analog meter usually has an advantage when looking for rapidly changing voltage or a "slow" transient. The sampling rate of the DVM is too slow to respond and will be more likely to miss the changing input.

On the resistance function, both types of meters generate a constant current that is fed into the circuit under test so that the meter can then measure the voltage drop (which is expressed as ohms).

DVMs typically use a much lower value of current, plus, they usually have two settings - a higher current diode check function which permits a total voltage greater than .7v to be generated in the unit under test and a lower current output that forces the output voltage to remain below semiconductor bias voltages.

Analog ohmmeters often are capable, on their higher range settings, of causing a substantial amount of voltage to be generated in the UUT. This is especially true if the meter uses a 9V battery for power. This higher voltage on the high ranges is also due to increased current being output by the meter. In general, the use of an analog ohmmeter in a digital or low current circuit is not advised due to risk of semiconductor junctions being blown by the excessive current or voltage.

Fuses are provided to protect the sensitive input section of a DVM from operator mistakes, like connecting the meter into a powered circuit while set in the ohmmeter function or from over-current conditions in the Amps setting or from connecting the meter to a voltage above its specified rating. Actual protection provided depends on the design of the particular meter. Consult the manual for details.

Class dismissed. Back under the hood of your cars.

Steve
 
The new TPS is installed. I'll put it through the ringer over the next day and report back with the results!
 
Update:

Unfortunately, the behavior still exists. However, I noticed yesterday before changing the TPS that the hiccups no longer existed at lower speeds. This leads to me believe (possibly incorrectly) that the fluid servicing had produced some effect.

As it stands now, I'm not sure the TPS swap accomplished anything, but the only speed at which I can consistently force the behavior is 70mph. I'm (so far) unable to reproduce the hiccups at any other speed - which is an improvement.

This condition is almost something I'm willing to live with, but by all means, if anyone has any additional ideas, pipe up!
 
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