Hi, before I get too far I actually have a 1968 Mercury Montego, not a cougar. There’s not really a place for Montego owners, every forum I’ve found is either dead or mustang related. If someone can point me somewhere active for Montegos please do.
Anyhow, the car never had a working dash until I recently restored it, the circuit board was disintegrated. Now just about everything works but my fuel lamp and my gauge always seems to be off by a bit.
The lamp is always on IGN ON, shuts off when lights are turned on. Gauge position doesn’t matter. Car running or not, same behavior.
Turns off with ignition, so it’s not on a constant hot. Stays on if sending unit is unplugged, gauge goes to E.
Hoping someone might be able to give me some ideas, it’s the last thing I need for the dash to be “right.” May be worth mentioning I used LEDs for the illumination bulbs/turn signals and incandescents for the dummy lamps/high beam indicator.
Assuming your car has the same low fuel indicator lamp circuitry that Cougars have, you can isolate the problem by unplugging the 2-wire electrical connector at the fuel tank. If the light then goes off, the problem is the fuel sender inside the tank. If it is still on, the problem is in the car’s wiring or low fuel relay under the dash.
Thanks for the reply, I seem to have only one wire running to the sending unit. Unplugging it did not change the fuel lamp’s behavior but it killed the gauge and it went to E.
I’ll have to check for a relay, I didn’t see one in there but maybe I have one. Will refer to my service manual.
The Cougar low fuel light used a thermistor on the fuel sender that got hot lowering resistance when not submerged in fuel. When the resistance got low enough, the low fuel relay was energized turning on the low fuel light. But if you don’t have two wires to the sender, that must not be how your 68 Montego senses low fuel. Sorry I can’t be of more help.
No worries, that’s interesting to know. When I replaced the sending unit it seemed to be the variable resistor style with a float & sweeping arm that rode across resistive material, wherever the float level is, the more/less resistance there will be. Still not sure what tells the low fuel lamp to turn on though, I’m assuming the gauge itself controls it. Will find out for sure eventually.
The low fuel system gets it’s proof-out circuit (lamp lit when key is in crank) with a red wire attached to the STARTER side of the starter relay. I’ve seen many times that that wire is attached to the BATTERY side instead. If so, the lamp will always be lit.
Assuming the Montego used the same low fuel circuit as Cougars, that’s a great suggestion. Could be the missing second wire at the tank connector was left orphaned somewhere between under dash and trunk, and is shorting to ground.
That’s quite possible, I’ll have to look around. I still wonder why it’d be affected if the headlights are on/off. Something weird along the line.
When I removed the old sending unit, it was a one-wire style unit without the thermistor. Because of this, I replaced it with the same style but it very well could have had the thermistor style and the PO did away with the other wire somewhere. Great lead here.
My car does the same thing. The low fuel relay is sensitive enough to voltage, that the voltage drop from turning the lights on with engine off is enough to turn off my low fuel light. With engine running, assuming a good alternator and battery, you shouldn’t see any difference in operation with lights on or lights off.
Here is a 68 Montego NOS 2 wire fuel sender with the low fuel thermistor, so some 68 Montego’s must have had an optional low fuel warning. Was it only offered on the wagons?
I’ve found something interesting while I was rooting around. Also seems the one wire sending unit is correct, my car is the Montego MX convertible.
No relay found, but I was reading diagrams in my shop manual and found that only one wire is meant to run to the low fuel lamp itself, and it grounds to the cluster. A previous owner seems to have run a different two-wire socket to it, with one wire being the proper black-yellow stripe and the other a blue-red stripe or maybe brown, I can’t tell. I’m trying to see if from photos I took when the cluster was out, may have to pull it again and see.. I’m just curious as to why they added an extra wire. Seems I’ll have to use a proper single-wire pigtail with the metal socket so it grounds to the cluster. Then figure out what that other wire is for.. Previous owners are strange.
Some people will just start connecting wires together until they get the result that they think they want. Might be as simple as that. Montego MX convertible - very cool car!
Let’s hope! And thank you, she’s a rare one. ~3000 convertible Montegos made in 1968, she has the bucket seats, middle console w/floor shifter and A/C. Kind of wish i didn’t have that last one, just more things to fix.. maybe one day.
I’d love to add a cougar to the collection someday, beautiful cars. Can’t get over the vacuum operated headlights and overall sporty look, especially the interior. Can’t remember- did they have the “rolling” rear turn signal like the T-birds did?
Yes they did have the sequential taillights, although most of us have a lot invested in keeping them working that way! I’m with you on the AC - glad my 70 vert doesn’t have it. One less thing to work on, and we drive them with the top down all summer anyway.
Yours is a rare bird! Would like to see a pic of it.
