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Discussion Starter #1
I am not really a car person, but am trying to learn. I need to purchase some parts for my husband’s 1965 Mustang and need some advice about what parts I need. A little info about the car... According to the vin it had a build date of 7/23/65 It is a coupe, originally with a 6 cyl engine. I have other notes where I decoded the vin but am not 100% sure what they all mean. Somewhere along the way someone replaced the engine with an 8 cyl 289.

The Firewall to engine gauge feed wires have been chewed up by mice. Basically all of those wires are damaged a few inches away from where it connects at the firewall. How do we tell which one we have/need? In the descriptions it says with or without gauges, 2 or 3 speed heater, and also differentiates between 6 and 8 cyl. The car doesn’t have gauges. Which version do I need, or how can I tell?
 

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HI and you can get the oil wire and temp wire from any mustang store, or get them from a parting out mustang. I get my things from cjpp and they have an description of the part you are buying. Both 65 & 66, I believe, are the same wires for a 6 cyl or V8, the oil for the oil and temp for the temp. 65 have idiot lights for the oil and battery, 66 were all gauges. If you do not have a gauge for your oil and your engine has been updated, make sure, if you want your idiot light to work, you get the oil sending unit for a 65 (it is small and screws into the left drivers side engine). 66 had a bigger "bell shaped" sending unit in the same place for use of a gauge. I hope this helps. Good luck and if any other questions, just ask away.
 

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Discussion Starter #3 (Edited)
I’m going to clairify my question. So this is what I’m looking at getting. https://www.johnsmustang.com/1965-mustang-firewall-to-engine-gauge-feed-8-cyl-w-warning-lights-2-speed-heater-motor.

On our car the wires are chewed right at the point where they form a Y just after the plug. The car doesn’t start because of this... one of the wires goes to the starter, I’m not familiar enough to know where the rest go to besides the warning lights. I am guessing I need the 8 cyl, warning lights, 3 speed heater model because I read some posts on here that said later 65 manufactured cars have the 3 speed. Is there a way to tell by looking at this plug? I have a picture, but am not sure how to post it.

Editing to add - the car has an oil light and an alt light. The only gauges are for fuel and temp. I don’t want to change any of this, I just want to replace the damaged wiring so that hopefully the car will be able to start and run!
 

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That pic looks like it. I would do 2 things next. I would see if the spark plugs where getting spark. If not, go to your coil and see if you have a hot wire coming from your firewall to the positive side of coil (you could check the coil first as it was probably chewed up). You are right on the heater info, but I do not know about this wiring. Some ideas is to wait for another person to give into, or find a wiring diagram online.
 

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I forgot to mention that some of these mustang part stores have technical assistance. That and these site have been so much help! 😁
 

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So I did actually replace part of my engine wiring harness. To put things as simply as possible, working from firewall to engine parts, is the damage before or after that clip part of the connector? (plastic thing with 3 metal contacts sticking out) sorry if you gave that info already but it wasn't clear :) That is called a pigtail by some people I believe. Anyhow, if the chew is after the clip towards engine it's straight forward. Unclip and replace (those can be hell to get loose just fyi mine was!) Before the clip pigtail connector towards firewall it gets a bit more complicated. That might entail re-splicing or even going under the dash if it's bad enough. Hope not.

If said car is originally a 6-cyl (going to 8 is a very common swap) you would probably want to buy parts for that unless you know the instrument clusters was replaced too. Biggest difference is between the cars with a tachometer (RPM gauge) and non tachometer. Mine is the non-tach type. 1968. I'll bet you do not have a tach for a 65 coupe base 6-cyl.

Also, posting photos of the damage here, and anything else relevant always helps!

Also if that car is going back to the mousey place I'd spray bittering agent and or rodent repellents :) in the engine bay!
 

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Discussion Starter #7
I appreciate the help! The rest of the wiring I can see appears undamaged, though there are places where they have been cut/spliced, etc over the years. The mice and rats were climbing in and out through the firewall right where these wires are and they are all chewed up pretty bad. The car has been sitting for most of the last 9 years. It started fine about 5 years ago, but we had to park it in a barn where it wasn’t accessible. 4 years ago we needed to move it from there and it wouldn’t start, that’s when we discovered these chewed wires. I’m not sure if they were what was causing it to not start, or if my husband just saw the lack of any gauges and gave up at that point. The tires were pretty much gone at that point too, so we basically dragged it out and it has been in our driveway since. My husband hasn’t touched it because he thinks the entire thing needs rewired. I’m tired of it sitting and deteriorating further, so I thought I’d start with what is obvious and inexpensive and work my way from there.
 

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I appreciate the help! The rest of the wiring I can see appears undamaged, though there are places where they have been cut/spliced, etc over the years. The mice and rats were climbing in and out through the firewall right where these wires are and they are all chewed up pretty bad. The car has been sitting for most of the last 9 years. It started fine about 5 years ago, but we had to park it in a barn where it wasn’t accessible. 4 years ago we needed to move it from there and it wouldn’t start, that’s when we discovered these chewed wires. I’m not sure if they were what was causing it to not start, or if my husband just saw the lack of any gauges and gave up at that point. The tires were pretty much gone at that point too, so we basically dragged it out and it has been in our driveway since. My husband hasn’t touched it because he thinks the entire thing needs rewired. I’m tired of it sitting and deteriorating further, so I thought I’d start with what is obvious and inexpensive and work my way from there.
Man, if you were within an hour of Washington D.C. I LOVE starting up old "sat forever" cars. In fact videos proliferate of people going to junkyards hunting for and restarting cars that were sitting that are even older than ours.

Well, for parts I'd direct you to CJ Pony Parts and NPD! CJ has one of the best search matrices and NPD is the king of having things in stock and their search is pretty good too. Easier if you know what you need with more precision. They are also much easier to get to answer the phones ;) Ask me how I know. Both are good suppliers all in all. You'll probably find what you need in stock between the two. But it sounds like you may have some really bad damage deep inside the dash. If it's just unplug & replace or a re-splice outside the dash/firewall go for it. But if you're going to get into under-dash wiring damage your husband is probably right. It's a re-do. It's hard even with the dash taken apart and exposed. Some items you can access in an assembled dash but it's painful (definitely harder to invert my spine under that thing at 41 than 21 - go figure).
 

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Discussion Starter #9
Thanks, I will check those out. I want to go ahead and replace this particular set of wires, since we’ll need them even if we do end up needing to replace the dash loom too. Any advice/resources on how to go about restarting it properly? I’m thinking after this long we probably want to start with a new battery.
 

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With a 7/23/65 scheduled build date you SHOULD have a 3-speed heater blower so you'd want the gauge feed harness for a V8, with warning lamps (no gauges) and 3-speed blower. The 2-speed heater switch has OFF in the middle, low to the left and high to the right. The 3-speed switch has OFF to the left, then low-med-high as you go right. The other difference is that with the 2-speed blower there is NO blower motor resistor stuck into the heater box like there is with the 3-speed.
 

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Discussion Starter #11 (Edited)
With a 7/23/65 scheduled build date you SHOULD have a 3-speed heater blower so you'd want the gauge feed harness for a V8, with warning lamps (no gauges) and 3-speed blower. The 2-speed heater switch has OFF in the middle, low to the left and high to the right. The 3-speed switch has OFF to the left, then low-med-high as you go right. The other difference is that with the 2-speed blower there is NO blower motor resistor stuck into the heater box like there is with the 3-speed.
Thank you. I wasn’t clear that the damage is in the engine compartment, so it does appear straightforward. I’m thinking the clip will be difficult to disconnect, as it looks like it has been painted over. I kept thinking that the heater wires were something under the dash I wouldn’t be able to see, but I looked again tracing the wires from the pigtail and saw that it has the two wires, so it is the three speed heater. I’m conflicted on whether I should get the 6 cyl or 8 cyl version though.
 

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I would go with the 8 cylinder version of the harness. It will have the correct length of wires for your engine and routing to the appropriate temp and oil pressure sensors, and to monitor the charging system.
 

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Thank you. I wasn’t clear that the damage is in the engine compartment, so it does appear straightforward. I’m thinking the clip will be difficult to disconnect, as it looks like it has been painted over. I kept thinking that the heater wires were something under the dash I wouldn’t be able to see, but I looked again tracing the wires from the pigtail and saw that it has the two wires, so it is the three speed heater. I’m conflicted on whether I should get the 6 cyl or 8 cyl version though.
Like woodchuck says, check the heater switch and or look on the blower housing under the dash where the glove box is. If it is the 3 speed you will see a red resistor block mounted on the case. Both 2 and 3 speed blowers have 2 wires. the power is run through resistors to change the speed. On the 2 speed the resistor is mounted on the switch and on the 3 speed its mounted on the blower/heater core housing.
 

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This is the resistor you are looking for. The coiled resistance wires will be inside the blower housing so that the flowing air can cool them. All you will see from the outside is the tabs that the connector attaches to.
 

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Thank you. I wasn’t clear that the damage is in the engine compartment, so it does appear straightforward. I’m thinking the clip will be difficult to disconnect, as it looks like it has been painted over. I kept thinking that the heater wires were something under the dash I wouldn’t be able to see, but I looked again tracing the wires from the pigtail and saw that it has the two wires, so it is the three speed heater. I’m conflicted on whether I should get the 6 cyl or 8 cyl version though.
You'll want the 8 cylinder harness if you have a V8 installed as the wires that go to the ignition coil and sending units will be the correct lengths for their V8 location.
 

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Thank you. I wasn’t clear that the damage is in the engine compartment, so it does appear straightforward. I’m thinking the clip will be difficult to disconnect, as it looks like it has been painted over. I kept thinking that the heater wires were something under the dash I wouldn’t be able to see, but I looked again tracing the wires from the pigtail and saw that it has the two wires, so it is the three speed heater. I’m conflicted on whether I should get the 6 cyl or 8 cyl version though.
You need the 8 cylinder harness. The only difference is the length of the wires. The 8 cyl harness will fit your 289 perfectly.

To explain the options a bit:
The 65 Mustang originally came with the Falcon dash which had a large flat speedometer and warning lights. Mid year Ford offered the GT option was offered that came with a dash that had several gauges instead of lights. This became standard in 66 on all Mustangs.
The heater was also upgraded part way through the year going from the 2 speed to 3 speed.
The shape of 6 cylinder and 8 cylinder engines is very different so the wiring harness has to be different to reach the sensors on each engine.
Early (64 1/2) cars had generators, true 65s had alternators

So assuming you have a 3 speed heater and you car is not a G, you need:

65 Engine feed harness for a V-8, with 3 speed fan and warning lights and an alternator.

from NPD the part is:

https://www.npdlink.com/product/wire-assy-engine-gauge-feed-from-firewall-firewall/101925?backurl=search/products?search_terms=engine%2Bfeed&top_parent=200001&year=1965&year=1965

I recommend buying from NPD. They have the most stuff in stock, ship fast, by far the best customer service and if there is any issue, they will swap out the part or take returns no question asked.
 
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