Having a problem with overdrive working intermittently(92 3.0L, auto trans).
Seems to be related to throttle position, as if I feather the throttle, the trans seems to go into od. Vacuum related maybe?
Possibly a vacuum leak? Checked the vacuum tree on the aft of the manifold, and all the plugs seem to be in order. One more thing; removed one of the plugs(off the tree) and found it wet......Trans fluid? Modulator valve leaking?
How does the modulator interract with the overdrive, if at all?
Thanks in advance for your insights.

 

I don't think that a faulty vacuum modulator can cause lack of 3-4 shift, and especially without other symptoms like high, hard shift points.

The A4LD from late 1988 to when it went "-E" will not shift to OD unless the ECM grounds one of the three wires leading to the solenoids, on the lower/front left side of the transmission. If you want the details, see this thread.

There are also issues internal to the trans that will prevent OD, but you said that varying the foot feed affects your symptom.

[later]
[smacking hand on forehead]

Any chance that you also get a "Check Engine" light at the same time? If so, go directy to the Throttle Position Sensor and replace it. A dead spot develops where the sensor wiper rides most (ie at cruise position) and when the sensor hits that spot, the ECM thinks you just floored the accel and drops out OD. That happened to me several years ago, I put in a $4 junkyard TPS and it never happened again. Miserable two weeks living with it though.

On the 3.0l, the TPS is non-adjustable so you can't screw that up, but the screws retaining it to the throttle body are both locktited in and Posidrive head, so a normal Phillips screwdriver will just round them out. Use small vice grips on the heads instead, or a Dremel. Buy (3mm?) allen head replacement bolts at the hardware store to reinstall. That much I *do* recall.

Regards,
Al S.

 

I would say that your modulator is gone. Tranny fluid at the tree is a tale tell sign tha it is toaste. I would also change your oil (now mixed with tranny fluid.)


If it does not correct the problem, then i would suggest the TPS. And the easiest way to get it out is with a drill and an easy out #1 (also called a screw extractor) and replace the screws with hex heads from the hardware store.


Also I would check your tranny fluid level. The A4LD is VERY PICKY when it comes to fluid level.

 

You mentioned that you pulled the vacuum lines off the vacuum tree and one was "wet". Is it wet with transmission fluid or fuel? You can tell by smell.

If it's fuel, then you probably have a bad diaphram in the fuel pressure regulator. You should also check your oil for fuel contamination. Fairly obvious fuel smell from oil on dipstick.

If it's transmission fluid, then your modulator has a bad diaphram. You should also check your oil for transmission fluid contamination. Not as easy to determine, but you may be able to tell by the color of the oil on the dipstick.

In either case I would recommend changing the oil and oil filter.

Since you are experiencing problems with the transmission going into overdrive it's probably transmission fluid at the vacuum tree. These transmissions are prone to exhibiting various problems when the fluid level is not just right. Since you seem to have a bad modulator, I'd change that first and replenish the fluid level. Check to see if that fixes your od problem.

Just my 2cents.

Ron

 

Had the same type of problem with my 90 4.0 a few years back, it seemed to be going in and out of OD. After a transmission shop told me it was a torque convertor and replaced it did we find out that the fuel filter was plugged.

 

Any chance that you also get a "Check Engine" light at the same time? If so, go directy to the Throttle Position Sensor and replace it. A dead spot develops where the sensor wiper rides most (ie at cruise position) and when the sensor hits that spot, the ECM thinks you just floored the accel and drops out OD. That happened to me several years ago, I put in a $4 junkyard TPS and it never happened again. Miserable two weeks living with it though.


I get this behaviour as well... exactly. I just changed the fuel filter (It was due,and dirty) but only saw a small improvement, and engine lite still coming on. Where is the TPS Sensor??? Also, would/could this cause an MAF code to come up?? I was told by mechanic it was coming up and to clean it. How to clean it? Carb cleaner??

I also had this problem about a year ago, and it was diagnosed as spark plugs. Changing them fixed the problem, but here we are, back again. I recently had another problem (coolant leak) but fixed/stopped that, and am confident there is no coolant in oil, so I don't see it being related.