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Wet Engine, won't downshift

karstic

karstic

NAXJA Forum User
Location
Milwaukee
To start off with, 97 with automatic transmission.

This morning I changed the oil and filter and air filter and decided to clean up the motor. I sprayed it down with Simple Green and with the motor running rinsed it with a hose, not a deluge just a light spray to rinse off the motor. While doing this the motor kicked up i idle to about 1000 rpm. I drove the Jeep afterward and the Check Engine Light came on and noticed the shift pattern was odd.

I drove onto the freeway and while drive up the onramp the motor upshifted fairly quick and I reached cruising speed (65-70mph). When I stomp the throttle the Jeep won't downshift out of Overdrive or doen into 3rd gear. It just slowly gains speed.

I can manually downshift into 3rd and the power is there.

I disconnected the battery and let it sit 10-15 minutes. The CEL is off, but the problem remains.

Any ideas?
 
Wet TPS.

Disconnect the TCU (or pull the fuse) and drive it shifting manually. If it shifts Ok, then the problem is electrical--TPS (major suspect here), TCU, solenoids, wiring harness.
 
karstic said:
Where is the tcu?
Click to expand...

Your 97 should be behind the glove box under the dash. The very late models were moved over to the knee panel about 2" from the steering column.
 
I had to drive the Jeep about 50 miles round trip. Going to my destination I had a few hiccups at cruising speed on the freeway but nothing major. On the way home it was horrible. Wouldn't shift into Overdrive, it would finally shift at about 4000rpm, weird shifts at surface street speeds.

Swapped in a new TPS and solved the problem.

I'm suprised the issues caused by the faulty TPS.

Is the TPS just a variable resistor?

Why does getting it wet cause such an issue?
 
Last edited:
karstic said:
I had to drive the Jeep about 50 miles round trip. Going to my destination I had a few hiccups at cruising speed on the freeway but nothing major. On the way home it was horrible. Wouldn't shift into Overdrive, it would finally shift at about 4000rpm, weird shifts at surface street speeds.

Swapped in a new TPS and solved the problem.:spin1:

I'm suprised the issues caused by the faulty TPS.

Is the TPS just a variable resistor? Si, senor

Why does getting it wet cause such an issue? water shorts the path through the resistor and sends the ECU some screwy values-- I don't undersyand why it won't dry out and work, either
Click to expand...
 
x2 on what Shorty said.

I also dont understand why it won't just dry out and work again. Although, with most electronics i have had better luck getting them to work again after being wet if i dont turn them on until they are completely dried out.

The reason it wouldn't downshift is because the TPS tells the TCU when you are mashing the pedal, and when to kick it down a gear. No detent (or kickdown) cable on the XJs like some other vehicles have.

Glad you got it worked out :cheers:
 
It'll dry out... after a month or two. Assuming it hasn't corroded internally by then, it should work great again! I'm not sure why exactly they didn't do a better job waterproofing those, really.
 
kastein said:
It'll dry out... after a month or two. Assuming it hasn't corroded internally by then, it should work great again! I'm not sure why exactly they didn't do a better job waterproofing those, really.
Click to expand...

Water resistant TPS sounds like a good project for a rainy day. I think I will try it for the Renix.
 
If I wanted to drop a hundred bucks or so on prototyping I'm fairly certain a 100% waterproof no-contact TPS could be built using a Honeywell HMC1510 (wheatstone bridge magnetoresistive angle sensor), an STMicroelectronics TS934 quad rail to rail micropower op-amp, and a voltage regulator or two... could encapsulate the entire module in 0.1" of silicone or epoxy and put a magnet on the end of the throttle cross shaft and it'd probably withstand a pressure washer. Might have to replace the op-amps with a small microcontroller with A/D and D/A capability to correct the nonlinearity of the angle sensor output though.
 
kastein said:
If I wanted to drop a hundred bucks or so on prototyping I'm fairly certain a 100% waterproof no-contact TPS could be built using a Honeywell HMC1510 (wheatstone bridge magnetoresistive angle sensor), an STMicroelectronics TS934 quad rail to rail micropower op-amp, and a voltage regulator or two... could encapsulate the entire module in 0.1" of silicone or epoxy and put a magnet on the end of the throttle cross shaft and it'd probably withstand a pressure washer. Might have to replace the op-amps with a small microcontroller with A/D and D/A capability to correct the nonlinearity of the angle sensor output though.
Click to expand...

You gave me a headache reading this! LOL! :dunce:
 
joe_peters said:
You gave me a headache reading this! LOL! :dunce:
Click to expand...
sorry... I'm too much of an EE nerd sometimes :spin1: Anyone have a table of throttle shaft angle measurements and the corresponding output voltages for a stock TPS though? I know it'll range from approx. 0 to 5 volts.
 
I'm by no means an EE nerd and definitely not a math wizard, but I know it's a linear function, so I'd assume if you could determine the closed angle and voltage and the WOT angle and voltage; some math should give you any variation in between. It's been twenty years since I've attempted and trig. or calc. so I'll defer to someone younger and smarter for the graph of values.
 
Anyone ever try to dry one out with WD-40?
 
Kejtar said:
That's why manual transmission rocks :D

Btw if you get CEL again and need the codes read, give me a holler: I got a reader.
Click to expand...

I got connections at CARB:smoker:

Actually my CEL hasn't gone out. I may try the battery reset trick tomorrow.
 
I've got an idea dude.

Does your oven have a "Warming mode?" If so try sticking the TPS in at about 180 for thirty minutes, see what happens.

Random Newcastle inspired thought...
 
GSequoia said:
I've got an idea dude.

Does your oven have a "Warming mode?" If so try sticking the TPS in at about 180 for thirty minutes, see what happens.

Random Newcastle inspired thought...
Click to expand...

Or try a hair dryer/heat gun.
 
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