Diagnosing starting issue

[SnowyOwl]

What I'm getting when I turn the key to "START" position:
- Voltage meter drop, electronics (clock, blower motor) go off
- No crank, no click, no voltage at the starter "S"-terminal/"wake-up"-terminal

What I'm getting when I pull a jumper cable from battery to the S-terminal, and connect the cable when key on RUN position:
- Car starts and runs fine

-> Looks like MLPS.
- However nothing changes with new sensor.
- Tested old sensor with resistance meter, it gives 0 ohm resistance on Neutral or Park position, inf on other positions. (I.e. it seems to work)
- Also ensured that the sensor is installed in the correct position
- Rewired the sensor connector, it had some corrosion, but not much. No help.
- Tested that 5 V is present on the MLPS's cable harness EEC module signal cable, when key is on RUN position.


There ain't too many places left to check. When the MLPS harness is disconnected, is it safe to try starting, and check if I get voltage on the ignition switch cable, or will I burn something in the EEC or so? That would help to rule out if it's the ignition lock not passing voltage through.

Any other places I should check? No OEM anti-theft in this car. There's some aftermarket system installed, which should be disabled. I'm assuming problems on there if the ignition lock appears ok, though.

 

[1997ThunderbirdLXV6]

What I'm getting when I turn the key to "START" position:
- Voltage meter drop.

What's this voltage drop about? I mean if the starter isn't engaging, plus the accessory circuit cuts off (which is normal in "Start"), then shouldn't voltage remain constant? Where are you measuring voltage?

 

[SnowyOwl]

What's this voltage drop about? I mean if the starter isn't engaging, plus the accessory circuit cuts off (which is normal in "Start"), then shouldn't voltage remain constant? Where are you measuring voltage?

In this case I'm "measuring" the voltage by just by staring at the voltage meter on the dashboard, and observing it going down as I turn the key. It's a good question why does it drop if no cranking takes place. Maybe it gets cut off among the accessory circuit?

 

[Grog6]

you have a bad connection, either at the battery or startwr. Or your battery is bad. Chech the voltage with a seperate voltmeter.
When you turn the key to start, the bad connection is dropping the voltage. Dash power is tapped off at the megafuse, under the fusebox under the hood. Start measuring at the battery, trying to start the car at each place; battery, starter, megafuse. one of those wires or connections are bad.

 

[SnowyOwl]

you have a bad connection, either at the battery or startwr. Or your battery is bad. Chech the voltage with a seperate voltmeter.
When you turn the key to start, the bad connection is dropping the voltage. Dash power is tapped off at the megafuse, under the fusebox under the hood. Start measuring at the battery, trying to start the car at each place; battery, starter, megafuse. one of those wires or connections are bad.

Sounds like a viable theory as well. Assuming this is the megafuse, I measured less than 0.04V potential difference between either end of the fuse and the battery positive terminal (when starting). I think that's still fine?

I'll try to measure from the starter soon too, but before that I'm going to craft a breakout for the MLPS sensor connector, to find out if the voltage ever gets there.

 

[SnowyOwl]

Combining the information from these two sheets (Page from EVTM and install instructions for my new MLPS) , when I start, I should get voltage to the MLPS sensor pin 5 (the EVTM pic) or in pin 1 (install instructions sheet pic).

Current should flow thru MLPS to the pin 8 (or 4), if the MLPS is fine.

 

[1997ThunderbirdLXV6]

Maybe it gets cut off among the accessory circuit?

More than likely.

 

[1997ThunderbirdLXV6]

Combining the information from these two sheets (Page from EVTM and install instructions for my new MLPS) , when I start, I should get voltage to the MLPS sensor pin 5 (the EVTM pic) or in pin 1 (install instructions sheet pic).

Current should flow thru MLPS to the pin 8 (or 4), if the MLPS is fine.

It's pins 5 and 8 per the workshop manual. Pins 1 and 4 are empty pins.

That aftermarket installation sheet, very confusingly, shows the connector pins flipped. You can tell by the pins relativ to the connector tab and guide:

 

[1997ThunderbirdLXV6]

the MLPS sensor connector, to find out if the voltage ever gets there.

That makes sense.

You should also check, with everything off, for continuity between pin 5 and the car's body, and between pin 8 and the car's body. Either one would suggest a short circuit somewhere.

 

[SnowyOwl]

It's pins 5 and 8 per the workshop manual. Pins 1 and 4 are empty pins.

That aftermarket installation sheet, very confusingly, shows the connector pins flipped. You can tell by the pins relativ to the connector tab and guide:

View attachment 12424

True lol, the whole aftermarket sheet is slightly confusing in nature Maybe it tries to depict the cable/harness connector pinout, while the workshop manual depicts the sensor's plug, if that makes any sense? Not sure.

 

[1997ThunderbirdLXV6]

No OEM anti-theft in this car. There's some aftermarket system installed, which should be disabled.

This is a red flag to me!

My car used to have an aftermarket remote start system, which was installed/wired up in the most cavalier manner. It didn't malfunction per se, but it wasn't smart enough to tell when the engine had started; so it ran the starter either not long enough or too long. I removed that system and rewired everything back to stock.

 

[1997ThunderbirdLXV6]

Maybe it tries to depict the cable/harness connector pinout, while the workshop manual depicts the sensor's plug

No, because that would just change the order, i.e., 1-2-3-4 would become 4-3-2-1; but it wouldn't flip the pins between guide side and tab side.

That sheet is just wrong in my opinion.

 

[1997ThunderbirdLXV6]

Browsing around some older posts, I saw one where a guy had a similar problem. It turned out to be bad ground between starter and bellhousing, and he just solved it by loosening and re-fastening those bolts. Seems too easy, but doesn't hurt trying.

 

[SnowyOwl]

Test setup neat enough to make Mr. Henry Ford proud

 

[SnowyOwl]

Some measurements when turning the key to start:
- Battery: 12,4V
- MLPS pins 5 and 8: around 9V
- Starter solenoid/trigger voltage: around 6V
- Starter motor (the one that is hot at all times): Around 12V, same as battery

I'm slightly confused. Something seems to eat the voltage, probably both before and after the MLPS. The workshop that installed the jumper cable claimed that they got no voltage on the starter solenoid, but obviously there's at least some coming through.

Browsing around some older posts, I saw one where a guy had a similar problem. It turned out to be bad ground between starter and bellhousing, and he just solved it by loosening and re-fastening those bolts. Seems too easy, but doesn't hurt trying.

At least the battery terminal of the starter measures ~12V voltage both from the starter body and bell housing. I could try to pull a jumper cable from the starter body to battery negative to confirm this I think.

 

[SnowyOwl]

Gotta check how much I should get voltage to the MLPS. If it should give 12V I could try to pull a jumper cable from the battery to the MLPS->starter pin (pin number 5), and see if I get 9V at the starter.

Maybe some connector in the MLPS harness is going bad, and it substracts around 3V from the voltage passing through? Just thinking out loud here.

 

[SnowyOwl]

That makes sense.

You should also check, with everything off, for continuity between pin 5 and the car's body, and between pin 8 and the car's body. Either one would suggest a short circuit somewhere.

Something to check as well!

 

[SnowyOwl]

Electronics 101 fooled me again. I do get 12V to the MLPS, if the MLPS->Starter solenoid is _not_ connected. So until that its all fine. When I connect MLPS->starter cable (pin 5), I get 9V at the MLPS. So the voltage drain is somewhere after MLPS, before the starter.

 

[1997ThunderbirdLXV6]

Here's what I find odd about this:

You said in your first post that the solenoid  does engage when jumped to the battery, correct?

Yet at 6-9V at the solenoid, it does absolutely nothing at all? Not even a tiny attempt at a click? Is that normal? Anyone know?

Usually, a relay requires minimal power to engage. I'm not knowledgeable about this solenoid in particular though. But it leads me to think that something is wrong with the ground.

When you jumped the solenoid, did you connect the jumper cables (plural) to positive and ground at the solenoid, or did you only connect one cable to positive?

 

[SnowyOwl]

Here's what I find odd about this:

You said in your first post that the solenoid  does engage when jumped to the battery, correct?

Yet at 6-9V at the solenoid, it does absolutely nothing at all? Not even a tiny attempt at a click? Is that normal? Anyone know?

Usually, a relay requires minimal power to engage. I'm not knowledgeable about this solenoid in particular though. But it leads me to think that something is wrong with the ground.

When you jumped the solenoid, did you connect the jumper cables (plural) to positive and ground at the solenoid, or did you only connect one cable to positive?

Yes it engages with a direct jumper cable. When jumping, I've only connected the positive cable, so only one jumper cable. When starting without the jumper, at 6V on the solenoid I can hear (and feel) a zap, but the solenoid doesn't feel like its engaging anything, as it would cause a click (at least).

What is more interesting now is that I _do_ get 12V to the "real" starter solenoid cable, IF it is not connected to the starter! When I connect it, I get 6V on start! Direct jumper cable from the battery positive works as usual.

At the moment I cannot understand why the direct jumper cable from the battery works, while the "real" starter cable drops its voltage to 6V when starting.

Electricity. How the fuck does it work anyway?

 

[1997ThunderbirdLXV6]

Yes it engages with a direct jumper cable. When jumping, I've only connected the positive cable, so only one jumper cable. When starting without the jumper, at 6V on the solenoid I can hear (and feel) a zap,

Ok, then ground is fine I guess.

 

[SnowyOwl]

Snippet from Chatgpt's advice:

Diagnosis: High Resistance in Control Circuit

The key symptom is that the control wire shows 12V only when disconnected, and drops significantly under load (when connected). This suggests:

There is a high resistance somewhere in the path between the ignition switch and the starter solenoid.

Under no load (open circuit), a voltmeter reads full voltage (12V).

But when connected to the solenoid (which draws current), the poor connection can't deliver enough current, and voltage sags (6V is not enough to activate the solenoid).

-------

I guess this makes sense after all? My multimeter will show 12V and it will not close the circuit, so that the "high resistance bad connector" would give itself in. Back to square number one I guess.

 

[massacre]

Does the MLPS show the proper resistance throughout the range of motion? If that checks out, make sure it's pinned correctly.
On my car, i don't use a starter relay just the solenoid on the starter. Red/blue wire from MLPS goes straight to starter solenoid and it starts and runs fine.
Not sure what type of solenoid is on your starter, but you could try going straight to the solenoid just as a test?

 

[dDUBb]

I found a fried wire between my dash / transmission on my old 94 Tbird ..

Voltage doesn't mean anything; an amp test is more indicative of circuit condition. A single strand of wire will carry 12 volts but won't start your engine.

 

[1997ThunderbirdLXV6]

Side note regarding my comments on pins earlier:

In 1997, it's indeed pins 1 and 4. Yours being a 94, this shouldn't apply, but it may explain the discrepancy with the aftermarket MLPS paperwork. Also note a slightly different connector shape.

 

[SnowyOwl]

Getting there. I fed 12V from the battery to the MLPS sensor input, and I do get a crank. So it seems the line from MLPS to starter is fine, it's borken somewhere before MLPS.

 

[1997ThunderbirdLXV6]

, make sure it's pinned correctly.

Agreed. Check the wire colors against the manual for each pin. Example: R/LB = red/light blue, etc.

I found a fried wire between my dash / transmission on my old 94 Tbird ..

Voltage doesn't mean anything; an amp test is more indicative of circuit condition. A single strand of wire will carry 12 volts but won't start your engine.

With this in mind, I think you'll have to examine the entire path of the wiring. In 97, that would be the path shown below; 94 may vary.

 

[1997ThunderbirdLXV6]

At the risk of repeating myself, if you suspect the broken wire ahead of the MLPS, then again, the red flag in my opinion is the aftermarket alarm/anti theft you mentioned earlier. Those things are generally installed between ignition switch and MLPS.

Bypass that thing, or remove it altogether.

 

[Grog6]

Will the car start and run if you turn the key on, and jump the starter solenoid to the 12v terminal? If so, it's something in that wire that starts at the ignition, goes thru the mlps, to the starter. You'll have to trace it out. I had a crap anti-theft alarm in the tbird when I got it, and it caused all kind of problems. If they cut the wire in question and added a relay, like mine, that would be your problem. find the relay, and fix the wiring back to factory. Or put a pushbutton rated for at least 5 amps in a wire directly from battery + to the solenoid, and use that to start the car.

 

[SnowyOwl]

Getting there. I fed 12V from the battery to the MLPS sensor input, and I do get a crank. So it seems the line from MLPS to starter is fine, it's borken somewhere before MLPS.

Well fuck me sideways, this was a one-timer. Also tried feeding direct 12V to the MLPS out cable, and got nothing but ~7.2V at the starter solenoid when starting.

I guess it's time to start tracing wires. And while on that, say goodbye to the aftermarket anti-theft as well. I have zero idea how that is installed, but there are some anomalies visible:
- The cable of the starter solenoid connector is black:
According to my copy of EVTM, it should be R/LB = red with light blue stripes. This means that the starter relay cable has been changed, possibly when installing the anti-theft? -> Some parts of the system could be between the MLPS and starter.
- I have this strange brown box in my trunk, no Ford badges on its sides:
Anyone know if this is i. e. the key remote receiver, or something aftermarket? Looks strange to me.
- Some suspicious cables in the engine bay:
That's a pair cable coming from somewhere, and their heads taped like this.

And yes this blinky thing in the "coin holder", in the cabin, this is where I'll be starting the removal job I guess:

Thx for the advice this far, everyone! Let's see if I can find time to continue this thing next week.