'88 TPS code.

[mnewman]

Hi all. New to me '88 2wd 5spd. Chasing down the CEL and a nasty hesitation.

Ran the codes and got:

21 - ECT sensor out of range (sensor is missing due to mechanical gauge; I will be fixing this)
23 - TPS sensor out of range/ set to high
67 - Neutral fault

So I went and tested with the volt meter... Path to ground is good, I get 4.96 volts on the power supply circuit and 1.46 volts in the idle position on the reference circuit. So from what I can see the TPS is working. Is the 1.46 v at idle too much? Are there any known connections this signal passes through that commonly goes bad? I've got the Ford trouble shooting guide on it way but I'm impatient. Any suggestions?

 

[Dirtman]

Should be under 1v at idle so yes too high.

Also with no ECT your engine is going to run like crap because it's always going to be in "choke" mode. The ECT sensor is not for the gauge it's for the computer to adjust the fuel/air ratio.

 

[mnewman]

I figured as much with the ECT. Exhaust smells rich so its definitely dumping extra fuel.

 

[mnewman]

So I had another TPS. I replaced it and got a similar voltage on the reference wire. Any ideas? 1.46 & 1.51. In looking at the truck it looks the previous owner replaced the TPS. It's clean and the tabs that hold the plug on are snapped off.

I see the ECT shares the same ground but other wise I don't see how it being unplugged could effect the voltage.

I thought maybe the ground had something to do with it but I'm getting 1-2 ohms resistance to ground at the TPS plug.

 

[Shran]

The TPS is adjustable, or at least most of them are. You rotate it while watching the voltage and tighten down the screws when you get it where it should be.

 

[mnewman]

I'm not sure how... The screw holes aren't slotted so the unit goes onto the throttle body in a fixed position. I'll take another look when I get home.

 

[Shran]

That's totally possible. So if that's the case, can you move the throttle stop screw so that the TPS voltage drops?

 

[mnewman]

I'll feel pretty dumb if that's all it takes. I'll check.

 

[Shran]

No need to feel dumb. Let us know what you find. I would not be surprised if a previous owner messed with it, if they were smart enough to remove the ECT, who knows what else was "fixed." We'll work through it.

 

[Paulos]

Yeah, the TPS isn't adjustable on yours. And adjusting the throttle screw is not a good idea, as it's set at the factory. It will mess with your idle RPM and you'll have to go through the trouble of resetting it (not as easy as it sounds). I would check the connectors at your ECT and TPS, disconnect the battery for 15-20 minutes to clear codes in the ECM, put your trans in neutral (for code 67), connect the battery, and pull codes again.

I've attached a chart for voltages, etc, but I've found it's better to check the ECT for resistance instead of voltage. Sometimes a bad sensor can affect other sensors, so fix the most obvious issues first, then clear codes and try again. If you still get a TPS code, it's either in the connector/wiring or the TPS (probably the TPS).

 

[mnewman]

My plan is to get the ECT installed and basically start over on trouble shooting the TPS. I just haven't had time to get back out and mess with it.
I also need to try and figure out if it's got a thermostat in it before I drain the system to install the ECT. I've never actually run it long enough to see if it gets up to O/T. I'll report back when I've got more info.

 

[mnewman]

I had some time this weekend and got back on the truck. I swapped the manual temp gauge into the dash sender spot since the sensor is dead. Got the ECT installed. Cleared codes and went back at it. Before I even started the truck I checked the TPS. Still 1.55 volts.

Fired it up and instead of a high idle then dropping it held about 1900 rpms. To me it looks like the throttle stop screw had been messed with so I backed it off a bit. Idle is now down to 700ish. Voltage on the TPS is now 1.41.

Idle isn't very smooth. I think it's missing. If I tap the gas the hesitation is still there but not nearly as bad but it wants to die. If it doesn't outright stall when coming down to idle it loads up and starts missing/surging.

I ran the codes and got a 23 and 67 (yes, the truck was in neutral). I know I need to find the procedure to reset the throttle stop (idle screw) but I am at a loss on how to get the TPS voltage in range. I took it off and tried clocking it several ways but it always ends up in the same spot. Maybe I'm missing something.

Thoughts?

 

[RonD]

Remove TPS, plug it in, turn on the key and test voltage on center wire, should be 0.0-0.7v, under 1 volt, if not its bad
Use a screwdriver and turn it to make sure it is "down" all the way and then turn it "up" all the way, should go above 4.50volts

Read here about TPS: http://www.fuelinjectedford.com/page30.html

On some TPS units you may have to make the bolt holes longer/slotted so you can adjust them


The screw on the throttle linkage is called the anti-diesel screw, it does a similar action as an "idle screw" BUT...............a fuel injected engine can't use an idle screw because there are no Jets to meter air:fuel mix and no Choke Plate cam to set higher idle when engine is cold, so Ford renamed it an anti-diesel screw, to avoid people trying to use it as an idle screw, lol.
All fuel injected engines use an air valve to set idle RPMs, Ford calls theirs the IAC(idle air control) Valve, computer can control idle RPMs +/-3rpms so very precise control using these


To set anti-diesel screw back to factory settings you need to warm the engine up fully
Then let engine idle and unplug the IAC Valve
RPMs should drop to 500 or so, engine barely running, if this happens then no need to adjust screw
If idle does not drop then you can try unscrewing the screw to see if RPMs start to drop, if not you have a bad IAC Valve or Vacuum leak

If RPMs do start to drop keep turning until engine is barely running, its set
Plug back in the IAC Valve and RPMs should go back up to 650-750rpms, warm engine target idle for V6, 650 manual trans, 750 for automatic


And just a general Heads up, always unhook 1 battery cable when changing engine parts or sensors and controls
The reason for this is safety of course but ALSO to reset computer, when computer reboots after at least 5min with no voltage it will "re-learn" all the sensor and control voltages
So if you just swap a sensor without this reboot, computer still uses the old data so nothing may change, no problems solved

 

[mnewman]

Thanks for the reply Ron. I did test the TPS with it not physically bolted to the throttle body. It was around .5 volts and ramps up to just under 5 volts so I feel the TPS is good. I'm wondering if I'm installing the TPS wrong. At one point I backed off the anti-diesel screw to let throttle close all the way and the TPS voltage was still above 1 volt.

Thanks for the info on the anti-diesel screw. I will go through that and see where I'm at with the TPS with it set correctly. Before I set the anti-diesel I'm going to take a look at the vacuum lines again.

 

[RonD]

You will need to elongate the holes so you can adjust the TPS down to .9volts, simple as that