Author Topic: I'm stoked, and thanks to Cmaya! (Read 525 times)

Midlife · « **on:** February, 25, 2011, 06:09:06 PM »

I received a 1967 harness to restore, and it included a very rare convenience control panel (seat belt, door ajar, hand brake, and low fuel lights). It was missing a sub-harness that connected this panel to the main harness. Documentation was poor, and a resistor wire is part of this sub-harness. I couldn't create the sub-harness without a template to determine the resistor wire value and length of wires. Fortunately, this customer knew someone else who donated his sub-harness to use as a template. The resistor wire was broken or burned up. I consulted the MPC, using a known part number for the low fuel relay, which used the resistor wire. From there, I found other Ford vehicles that used that relay, and consulted the schematics for them. One schematic had a part number for the resistor wire, and found back in the MPC two values: 3.5" and 22 ohms, and 6.5" and 45 ohms. The actual resistor wire was 4" long.

Using the crappy diagrams, I was able to reverse engineer how the relay worked, but needed to determine what was going on in the tank to verify the value of the correct resistor wire. These resistor wires are hard to reverse engineer, because their resistance increases with current flow, so one has to know what's going on throughout the entire circuit to determine the resistance at the proper usage. There is no documentation as to how the low fuel sensor works in Ford's documentation, so I needed access to one.

Suddenly, it dawned on me that Craig calibrates fuel sensors and tanks. Aha! A quick phone call to Craig to find out if he had one of these sensors...no. Within 4 hours, he managed to borrow two, shoot me pictures which confirmed my suspicions of how they worked, and will shortly be making precise measurements of resistance of the low fuel sensor itself. It turns out it is a thermistor, which varies as a function of temperature. Huh? You measure fuel capacity by temperature? No quite, but when the thermistor is no longer surrounded by fuel, it's temperature will rise and cause resistance to change (either up or down, depending upon design). I should know all the details shortly. Thanks, Craig! I owe ya one or two!

B67FSTB · « **Reply #1 on:** February, 26, 2011, 01:20:59 AM »

NTC or PTC resistors won't do the job ?
Just curious about the outcome.

Yep , dudes like CMAYNA don't come very often.

RARE breed .

beach pony · « **Reply #2 on:** February, 26, 2011, 04:24:27 AM »

Bruno is right. Those of us who have had the pleasure to know Craig, know what a great guy he is. He really is one of the best around! Here's to ya buddy!

cmayna · « **Reply #3 on:** February, 26, 2011, 06:53:57 AM »

Here's a pic of the two sending units I have borrowed from a local Mustang shop. 68 at top and 67 at the bottom. In both units, the red wire is connected to a thermistor. By definition this is how a thermistor works:

"When the thermistor is surrounded by fuel, it stays cool and has high resistance so the light stays off. When the fuel level drops below the thermistor, the device warms up and it's resistance decreases. This allows enough current to flow through the light for it to illuminate."

I'm guessing that the thermistor activates the low fuel light when the fuel gets down to 2-3 gallons.

Later today, after I finish my pedicure and take my afternoon nap, I might get some energy to do some testing for Mid.

nymustang · « **Reply #4 on:** February, 26, 2011, 07:43:47 AM »

That's a pretty cool feature. How many mustangs actually came with that?

Midlife · « **Reply #5 on:** February, 26, 2011, 10:05:09 AM »

Quote from: nymustang on February, 26, 2011, 07:43:47 AM

That's a pretty cool feature. How many mustangs actually came with that?

In 1967, only 6138 out of 472,207. In 1968, only 4888 out of 317,411. Something like 1.5%. These were standard on Cougars, though.

gtscode · « **Reply #6 on:** February, 27, 2011, 05:35:44 AM »

Quote from: beach pony on February, 26, 2011, 04:24:27 AM

Bruno is right. Those of us who have had the pleasure to know Craig, know what a great guy he is. He really is one of the best around! Here's to ya buddy!

Couldn't agree more. He is a Great guy who is very willing to help out! Very lucky to know him.

Heap64 · « **Reply #7 on:** February, 27, 2011, 07:41:43 AM »

Wow, the title of this thread had me thinking something totally different was going with you too

buening · « **Reply #8 on:** February, 28, 2011, 12:37:15 PM »

Hey Mid, if you need a Convenience Group harness for the 70 cars to use as a template let me know. I have one in my basement that I haven't installed yet. I'm not sure how hard it'd be to reproduce if you wanted to. If you ever need to borrow it let me know

Midlife · « **Reply #9 on:** February, 28, 2011, 03:12:02 PM »

Hmmm...didn't even know they were offered in 1970! It turns out the wire lengths are much different between 1967 and 1968, and I only had a 1968 template to use for both 1967 applications. If you send it in, I'll refurbish it for free in exchange for doing some measurements on it.

buening · « **Reply #10 on:** February, 28, 2011, 03:31:48 PM »

IIRC its in pretty good shape except the convertible breaker wire, but I'd have to take another peek at it. This has the connectors for the seat back solenoids, round belts and park warning light in lower dash, head light warning buzzer, low fuel, convertible top breaker, and a slew of other things I can't remember off the top of my head. I'll try to drag it out and snap some pics. I may take you up on the offer. This came from a loaded convertible with factory tach. I haven't ever seen another harness for sale on Ebay, so I know its very hard to come by.

Midlife · « **Reply #11 on:** March, 05, 2011, 02:22:36 PM »

An update. I bought a low fuel sensor from NPD, and used a length of resistance wire equivalent to 30 ohms. I hooked everything up, and the low fuel light lit up when it shouldn't. I did a bit of trouble-shooting, and burned up the low fuel sensor. It appears that the resistance of the sensor went to 20+ Megaohms, so I obviously burned it up. There's very little left for me to do but to give up. I thought I could reverse engineer this thing, but it appears there is also an internal resistor within the relay itself. With three unknowns (resistance under load), there's no hope to reverse engineer it.