eaglescout526 Posted October 19 Share Posted October 19 Like the title says, its getting back fed power. On the 84 XJ. Now its some, like maybe 6 volts at the most but not a full 12. Now I just want some working theories. I have a plan of how to get to the bottom of this and why this is happening. I only noticed this from two things. First, the fuel gauge is bouncy and sensitive. At first I thought the old gauge was bad as the light didnt really come on for low fuel. I swapped the gauge for a NOS one and that worked, light and all. Drove around, got some gas and noticed the float was stuck again. Great. Well its unstuck now and sensitive. And I mean sensitive. Like you can rock the whole XJ and it will start moving. Hell even just backing out with the trans not in gear and letting it roll back on its own and the gauge will start moving. Second, upon trying to figure out why this is, I removed the cluster fuse and found that the brake light was DIMMLY lit along with some of the gauges getting power when it should be dead. Which is how I noticed I am being back fed. Why? Not sure. This is when the engine is running. We all know what I have done to this XJ. TBI swap, System Sentry, dash to chassis ground per cruisers tips. But none of these I can find any correlation at all as to why this is. I could be losing my mind and the alt is back feeding the power to the cluster, but I would see the voltage gauge show some life with maybe 13v from charging. Nothin. ICM could be back feeding power to the tach, but not likely as those are pulses. But it is only about 6volts for the brake light being dimmly lit. I went on a whole rampage and pulled every fuse while leaving the cluster fuse out and the brake light didnt turn off. Is my sending unit back feeding power somehow? I could have two separate issues here for all I know. Just any theories one might come up with while having read this will be awesome. I dont think I will be tackling this today. Probably next weekend or after. Oh and Ω, I did pour over the wiring diagrams to see what it could be. I found nothin that stuck out to me. Link to comment Share on other sites More sharing options...
Eagle_SX4 Posted October 19 Share Posted October 19 What alternator are you using? If it is a GM 3 wire then that is probably your problem. I had the same issue with my Eagle and had to put a diode in line of the alternator wires. Here is a link to what I used to solve the problem. https://www.ebay.com/itm/383842448033 Link to comment Share on other sites More sharing options...
Eagle_SX4 Posted October 19 Share Posted October 19 Bouncy fuel gauge is normal on both my dad's eagle and mine I wouldn't worry about that very much. They are both very sensitive and shaking the cars does allow the fuel gauge to bounce. This happens even though I have a 94 XJ sending unit and my dad has the original. My theory is that in the early gauges there is not a capacitor that feeds the gauge and is just relying on the change in ohms and with the fuel sloshing in the tank the arm moves faster causing the gauge to jump up and down. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 19 Author Share Posted October 19 9 minutes ago, Eagle_SX4 said: What alternator are you using? Looks like a reman delco remy by ultima Ill try the diode and see what that does. Whats odd is my alt isnt doing exactly what it did to you with your Eagle and keep it running. 5 minutes ago, Eagle_SX4 said: Bouncy fuel gauge is normal on both my dad's eagle and mine I wouldn't worry about that very much. They are both very sensitive and shaking the cars does allow the fuel gauge to bounce. This happens even though I have a 94 XJ sending unit and my dad has the original. My theory is that in the early gauges there is not a capacitor that feeds the gauge and is just relying on the change in ohms and with the fuel sloshing in the tank the arm moves faster causing the gauge to jump up and down. Oh yeah I know that. I can do the same to my MJ and get the gauge to move. What bothers me is when I say it moves and is bouncy I mean it bounces and moves. Like it will move from where it rests after reading correctly to suddenly its at half tank when you slosh the tank or move like rolling backwards or driving forward. And when driving it will go down pretty damn close to E until you come to a stop and let it level out. This is what bugs me the most aside from the back feeding. It will bounce its way down or back up. Ive never seen a fuel gauge do this unless its the full speedo size matching gauge and even then that didnt move the way this one moves. I can plug another sending unit in and move the arm up and down and see the gauge behave normally with the engine off as thats the only way to really test the gauge functioning. Link to comment Share on other sites More sharing options...
Eagle_SX4 Posted October 19 Share Posted October 19 That looks like the same alternator I am using. The common issue with those alternators is back fed voltage. There are several threads out there on the subject. The answer is always to add a diode. My alternator was back feeding about 3-4 volts keeping the auto shutdown relay triggered until that 3-4 volts was enough to keep the relay triggered. Not sure how the 2.5 EFI is wired to shut down but it must be different than the H.O. 4.0. Which is why your symptoms are different than mine. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 19 Author Share Posted October 19 8 minutes ago, Eagle_SX4 said: Not sure how the 2.5 EFI Once power is off at the ECU it will shut everything down. 86 TBI used the same alt but 87-90 used a different one that has 4 wire plugging into it. One is for the gauge itself, the other is a splice of both battery and to a fusible link. I can see the tan wire providing some sort of back feed but this looks pretty basic with no issue. This is from the 86 electrical manual that has TBI with the above alternator. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 19 Author Share Posted October 19 Ok here’s a 50second clip of what it does. I backed out of the garage a little, pulled back in and recorded the gauge basically going from full to half a tank and twitching it’s way back. Just for context what it is doing. I have never seen this before. Link to comment Share on other sites More sharing options...
Eagle_SX4 Posted October 19 Share Posted October 19 That is strange maybe check the ohms on the sending while this is happening. See if the problem is with the sending unit. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 19 Author Share Posted October 19 28 minutes ago, Eagle_SX4 said: That is strange maybe check the ohms on the sending while this is happening. See if the problem is with the sending unit. Uh huh!! and good call! Ill have to be quick on that one. Whats odd is the sending unit is from an 86 2.5L XJW that looks otherwise completely pristine. Link to comment Share on other sites More sharing options...
89 MJ Posted October 19 Share Posted October 19 3 hours ago, Eagle_SX4 said: Bouncy fuel gauge is normal on both my dad's eagle and mine I wouldn't worry about that very much. They are both very sensitive and shaking the cars does allow the fuel gauge to bounce. You took the words right out of my mouth. I was going to suggest that these mid 80s AMC senders must just be very sensitive. Link to comment Share on other sites More sharing options...
Ωhm Posted October 19 Share Posted October 19 4 hours ago, eaglescout526 said: I removed the cluster fuse and found that the brake light was DIMMLY lit along with some of the gauges getting power when it should be dead. Which is how I noticed I am being back fed. Wonder if low voltage (6vdc) is making for fuel gauge needle not holding true. Might take full battery voltage for fuel gauge to stay more locked in on reading. 2¢ Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 19 Author Share Posted October 19 4 minutes ago, Ωhm said: Wonder if low voltage (6vdc) is making for fuel gauge needle not holding true. Might take full battery voltage for fuel gauge to stay more locked in on reading. 2¢ My thought too. When I get that resistor, I hope that solves the problem. While doing what Eagle_SX4 suggested I found something very very interesting. I didnt even actually start it up I just tested the sending unit first. It reads true at 75Ωs right now for an almost full tank. Connect it to the harness and that is instantly cut in half to 40Ωs. Now probing the two wires that B and C on the connector to the chassis side, I read .006VDC I went and added a ground from the fuel filter mount to the SU. No change. Harness side reads 57Ωs. Plug the two together and you have 40Ωs when reading the full tank. So I can only suspect one thing, 84-86 clusters may not be as plug n play as we may think with Renix. Im going to look in the 86 manual of the cluster and see. But that residual voltage is concerning. I should take a pic of the 84-86 style fuel gauge back side. Link to comment Share on other sites More sharing options...
eaglescout526 Posted October 20 Author Share Posted October 20 Ok did some more digging. I think I know why there is such an Ωhm difference on harness vs sending unit. Looks like beginning in 86 the cluster became known as "type 2" which has a solid state module for the fuel gauge. Probably thus resulting it working in conjunction with the new FSU vs the old one being just the float and not having a pump. So to probably fix my issue would be to swap to a 86 blue cluster and I shouldnt have any issue with the reading of the fuel level. Now the type 1's have a white housing, blue film to dim the lights and the fuel gauge has a printed paper circuit with resistors on it. Link to comment Share on other sites More sharing options...
eaglescout526 Posted November 21 Author Share Posted November 21 I want to get back to this. @Eagle_SX4 I got the diode and installed it. Now everything acts normal. Even upon shut down it doesnt act like it wants to keep going. And my ipod doesnt constantly restart upon shut down or starting. But this didnt fix my fuel guage. HOWEVER! I yanked out the sending unit and found the float to be on the fritz! Well changed it and got one hell of a surprise to be low on gas! Hahaha. Thanks for the help! Link to comment Share on other sites More sharing options...
Eagle_SX4 Posted November 21 Share Posted November 21 Glad that the diode fixed your back feed issue. Sometimes the sending units can just fail with no rhyme or reason glad you got it sorted. Link to comment Share on other sites More sharing options...
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