1990 Dodge, How to determine failure of MAP sensor, IAC, TPS or vehicle speed sensor???

[leroyd92]

Got a weird one....

1990 Ramcharger Tbi system.

Truck has a hesitation when you drive it ( normal ) but it does not when you hit the gas hard, or barely push the throttle. The hesitation is unnoticeable when it's fully warmed up.

Truck will start to die, when cold, backing up distance. Both with foot on the brake and not on the brake. When this happens, I'm not giving it any throttle, just in gear. If it doesn't die fully, the RPM is certainly affected by something while rolling.

I can start the truck and let it sit in the driveway for any length of time, and it has a constant RPM tone, no surging, no variance.

I can put the truck in gear and sit still, while warning up, and there's no variance.


Can anyone point me in the proper direction as to what this could be? The map sensor and IAC look to be perfectly aged to say they're from 1990. I have both on order and will keep them regardless since this is my off-road truck now. I'd like a general understanding of what to look for when the sensorS is/are going out. I believe I understand how all of these should act up but being a TBI, I'd like confirmation

 

[michaelluscher]

You ever get the injectors straightened out?
Do you have access to an OBD1 scan tool, or can you borrow one?
Any check engine/power loss lights on the dash?
Follow this procedure, and count the flashes

List of codes here
https://charm.li/Dodge and Ram/1990...rouble Code Descriptions/2 and 3 Digit Codes/

If you wanna consider using a scan tool on it, check for a black 6 pin connector as pictured (labeled 3)

OTTOMH, those symptoms sound like a coolant temperature sensor issue
There may be more than one, so the gauge on the dash may not be what the ECU sees

 

[dishdude]

Yep first thing is to do the keydance and check for codes, these older systems will set codes and not turn the MIL on.

 

[michaelluscher]

Yep first thing is to do the keydance and check for codes, these older systems will set codes and not turn the MIL on.

True
On the flip side, they can have screwy sensors that are feeding bad info, but no light
Only live data lets you catch it

Didn't I help someone here a few months back with a Shadow/A604 with a transmission issue, a vintage scan tool was the path to repair

Hopefully OP doesn't have to go that deep, or he'll be hunting on eBay for a primitive scan tool
Because I have too much free time on eBay, I'm also overstocked on MT2500s and a few OTC brand scan tools

 

[Chris142]

Yep first thing is to do the keydance and check for codes, these older systems will set codes and not turn the MIL on.

This. Turn the key on and off 3x. You will likely need an older brick style scanner. Different than today's obd2 as with the old ones you needed to know values and compare them to what yours is.

 

[leroyd92]

To my absolute amazement, I cannot get the truck to do the code sequence. It doesn't matter what I try or how fast I do it, I can't get anything to blink

 

[D60]

Truck has a hesitation when you drive it ( normal ) but it does not when you hit the gas hard, or barely push the throttle. The hesitation is unnoticeable when it's fully warmed up.

Truck will start to die, when cold, backing up distance. Both with foot on the brake and not on the brake. When this happens, I'm not giving it any throttle, just in gear. If it doesn't die fully, the RPM is certainly affected by something while rolling.

I can start the truck and let it sit in the driveway for any length of time, and it has a constant RPM tone, no surging, no variance.

I can put the truck in gear and sit still, while warning up, and there's no variance.


Can anyone point me in the proper direction as to what this could be? The map sensor and IAC look to be perfectly aged to say they're from 1990. I have both on order and will keep them regardless since this is my off-road truck now. I'd like a general understanding of what to look for when the sensorS is/are going out. I believe I understand how all of these should act up but being a TBI, I'd like confirmation

When my OBD1 351 did this, it was TPS.

Try backprobing and sweeping with analog VM

THAT SAID, it did it even when at operating temp.

I also just did a '99 Magnum with similar symptoms. Terrible idle and hesitation when cold, ran great when warm. Cleaning the TB and replacing the IAC cured it.

I think with SD the ECM believes the IAC is open as much as commanded, but can't tell because no MAF.

edit: except usually the IAC is after the MAF, so my theory falls apart. I suppose if these O2s aren't heated it takes longer for them to come into play also so maybe it can't figure out things aren't working as they should (incl IAC) for a longer period of time?

 

[wlk]

TPS but only get a Mopar oem. My 94 Dakota was having dropped rpms and stalling issues when stopped in gear but not in P at idle. Tried aftermarket IAC, TPS and pcm coolant sensor. Nothing, went to the junk yard, got some oem ones and replaced IAC, still did it, then TPS and that fixed it. No codes either

 

[leroyd92]

Borrowed my female friends snap on scanner,

Yeah, FEMALE!!!!- she's awesome.

Code 37. Transom sensor stuck open.


Guess I'll have to diag it myself

 

[zrxkawboy]

Code 37. Transom sensor stuck open.

Might need to move this to the boating forum.

 

[leroyd92]

I love my new phone

 

[michaelluscher]

37 PTU solenoid circuit An open or shorted condition detected in the torque converter part throttle unlock solenoid circuit. (Auto transmission only.)
https://charm.li/Dodge and Ram/1990...sts/DR-24 "PTU Solenoid Circuit"/Part 1 of 2/
https://charm.li/Dodge and Ram/1990.../DR-25 "PTU Solenoid Circuit" With Overdrive/

What transmission you have?

 

[dishdude]

Ignore 37, it's a bogus code. All the basics still apply here, check the timing before doing anything else. Most Chryslers of this era you unplug the coolant temp sensor and then you can check/set the base timing. Does anyone still have a timing light?

 

[leroyd92]

I crawled under the truck a time or 2 and remember the pan looking like the a727. It was not square like the other option

 

[leroyd92]

I'll check the timing, not sure what it's supposed to be but I have a manual. The hood says 6 i believe but that was an emissions recall

 

[D60]

The high end HF digi light is the high end Innova is the BluePoint (but SnapOn proper seems to have their own proprietary light, complete with the price tag you'd expect). FWIW

Can't a transom also be an old-school over-the-door window? Maybe this is an American Pickers code?

 

[mk378]

If you never see the CEL blink it could be because the bulb is out. Does it light up when you turn the key on but don't start?

Sensors like MAP and TPS should be voltage output and they can be tested with a voltmeter.

 

[michaelluscher]

I'll check the timing, not sure what it's supposed to be but I have a manual. The hood says 6 i believe but that was an emissions recall

You probably had the recall completed, with the new EGR back pressure transducer
Updated timing spec is 6° BTDC

In the timing procedure, it says
4. Disconnect coolant temperature sensor electrical connector. The CHECK ENGINE lamp on the instrument panel should illuminate once this sensor has been disconnected.

If that doesn't cause the CEL to come on, you might have other issues
If you have access to that Snap On scan tool again, you might be able to put it into timing mode like a DRB II would've

3. With the engine at operating temperature, connect DRBII diagnostic tool and access Basic Timing Mode in Special Functions section.

 

[leroyd92]

The CEL doesn't light up during crank, start, or turning the switch three times. It wouldn't shock me if some funny business was done to trick Nevada emissions or because something was wrong for so long the light burnt out. I'm trying to see if my Bosch scanner works with this because I can dedicate that scanner to working with this truck.

I can borrow that snap-on tool at any point so if need be I can do live data.

Once upon a time I had an F-150 that wouldn't get any signal to the O2 sensors until I drove it for about 4 minutes. Everybody on this website said that that wasn't possible and I had a body harness issue of some sort. I ended up videoing it with my phone and posting it on here. My issue ended up being the four fuel injectors were leaking and it was so rich that the sensors wouldn't switch back and forth. I changed out the injectors and I started getting signal on startup or shortly after. I did that on my Bosch scanner so I'm optimistic that I can use that for this truck but who knows. I would fork out a couple hundred bucks to buy a mt2500 scanner if I need to. This is going to be the truck we take out into the wilderness for days at a time

 

[michaelluscher]

The CEL doesn't light up during crank, start, or turning the switch three times. It wouldn't shock me if some funny business was done to trick Nevada emissions or because something was wrong for so long the light burnt out.

Sounds about right
Pre OBD2 hacks to get around emissions testing were much cruder, yet easier
I'd tear into the cluster looking for blown/removed bulbs

Once upon a time I had an F-150 that wouldn't get any signal to the O2 sensors until I drove it for about 4 minutes. Everybody on this website said that that wasn't possible and I had a body harness issue of some sort. I ended up videoing it with my phone and posting it on here. My issue ended up being the four fuel injectors were leaking and it was so rich that the sensors wouldn't switch back and forth.

That's quite a catch

I would fork out a couple hundred bucks to buy a mt2500 scanner if I need to

It wouldn't be a couple hundred bucks (less), but I am overstocked on various MT2500s and early OTC scan tools
Rather they go to someone who will use them