85"720 Posted February 8, 2020 Report Share Posted February 8, 2020 I have a 1985 and my speedometer is all out of wack. GPS says I'm going 65mph while my dash says 90mph ?? At any speed it's incorrect. For the most part it's like 15mph off then it starts to get worse the faster you go.. How would I go about fixing this? 1 Quote Link to comment
datzenmike Posted February 8, 2020 Report Share Posted February 8, 2020 To be 50% out isn't possible so the speedometer gauge is hooped. Get another from a wrecking yard or... https://ratsun.net/classifieds/category/9-datsun-parts/ ... an ad. 1 Quote Link to comment
Charlie69 Posted February 8, 2020 Report Share Posted February 8, 2020 Does the needle return to 0 mph at a stop? I replced a needle on the speedo in my king cab a while back and I did not relize until I had the cluster back in the dash that I put the needle on registering 10 mph at a stop. So my speedometer reads a steady 10 mph faster than I am actually going at any speed. Next time I pull out the dash I will fix it. Quote Link to comment
85"720 Posted February 8, 2020 Author Report Share Posted February 8, 2020 2 hours ago, Charlie69 said: Does the needle return to 0 mph at a stop? I replced a needle on the speedo in my king cab a while back and I did not relize until I had the cluster back in the dash that I put the needle on registering 10 mph at a stop. So my speedometer reads a steady 10 mph faster than I am actually going at any speed. Next time I pull out the dash I will fix Yes it does. I'm wanting to keep my cluster because of the odometer. Mine only has 140k miles on it. Would changing out my cable help out? 1 Quote Link to comment
datzenmike Posted February 8, 2020 Report Share Posted February 8, 2020 The cable is mechanically turned by the transmission. There's no way it can accidentally be turned faster than it should. Has the speedometer always been like this or did it just start acting up? The needle is on a spring that pulls it gently back to 0. The cable spins a magnet inside an aluminum cup that has the needle attached to. The spinning magnet induces an electrical current and it's own magnetic field within the aluminum and although they don't touch, the magnet pulls the cup around with it though at a reduced speed. If the magnet should rub against the aluminum it would mechanically pull the needle higher than it should. Take the gauge cluster out and apart. Try cleaning any obvious dust out. It's possible that the magnet and cup are sealed for the very reason of keeping dust out. You may be able to replace just the cup and magnet and keep the odometer. I've had a 620 truck speedometer apart but not the 720. I was trying to make it read higher by moving the magnet closer to the cup and spinning it with a drill. 1 Quote Link to comment
Charlie69 Posted February 8, 2020 Report Share Posted February 8, 2020 Get a good used speedometer and I believe you can swap odometers. Quote Link to comment
85"720 Posted February 8, 2020 Author Report Share Posted February 8, 2020 5 hours ago, datzenmike said: The cable is mechanically turned by the transmission. There's no way it can accidentally be turned faster than it should. Has the speedometer always been like this or did it just start acting up? The needle is on a spring that pulls it gently back to 0. The cable spins a magnet inside an aluminum cup that has the needle attached to. The spinning magnet induces an electrical current and it's own magnetic field within the aluminum and although they don't touch, the magnet pulls the cup around with it though at a reduced speed. If the magnet should rub against the aluminum it would mechanically pull the needle higher than it should. Take the gauge cluster out and apart. Try cleaning any obvious dust out. It's possible that the magnet and cup are sealed for the very reason of keeping dust out. You may be able to replace just the cup and magnet and keep the odometer. I've had a 620 truck speedometer apart but not the 720. I was trying to make it read higher by moving the magnet closer to the cup and spinning it with a drill. No it just got worse lately. It used to be 5 mph off throughout the whole gauge. I tried changing the cable but it wasn't the correct one. Is there a difference between the automatic and the standard transmission speedometer cables?? 1 Quote Link to comment
85"720 Posted February 8, 2020 Author Report Share Posted February 8, 2020 1 hour ago, Charlie69 said: Get a good used speedometer and I believe you can swap odometers. Hopefully I can. I'm looking for one. There's really no 720s where I live at. 1 Quote Link to comment
datzenmike Posted February 8, 2020 Report Share Posted February 8, 2020 I don't believe so. Too long is OK too short simply won't fit. Sounds like the magnet is rubbing the cup inside forcing it to turn faster. Take it apart, it might be obvious dirt. 1 Quote Link to comment
wayno Posted February 8, 2020 Report Share Posted February 8, 2020 (edited) Can you take a photo of the back of your gauge cluster showing the speedo drive connection? Is this what your gauge looks like, message me of your interested on this cluster. Edited February 8, 2020 by wayno 1 Quote Link to comment
datzenmike Posted February 8, 2020 Report Share Posted February 8, 2020 From about '80 the cable clips onto the speedometer head. Previous ones had a collar that screws on. 1 Quote Link to comment
wayno Posted February 8, 2020 Report Share Posted February 8, 2020 (edited) In the early 80s there were two different kinds of speedo cable connections to the instrument cluster speedo drive, clip and twist type. Edited February 8, 2020 by wayno 2 Quote Link to comment
85"720 Posted February 8, 2020 Author Report Share Posted February 8, 2020 2 hours ago, wayno said: Can you take a photo of the back of your gauge cluster showing the speedo drive connection? Is this what your gauge looks like, message me of your interested on this cluster. Yes Mine looks exactly like that. The speedo connection I cant recall how it looks but i thought maybe they gave me a 86 speedo cable or one from a Manual transmission if it was different. 1 Quote Link to comment
85"720 Posted February 8, 2020 Author Report Share Posted February 8, 2020 1 hour ago, wayno said: In the early 80s there were two different kinds of speedo cable connections to the instrument cluster speedo drive, clip and twist type. I think mine is the twist type. I'm not 100% sure. Dont have the truck at the moment to check it out. 1 Quote Link to comment
wayno Posted February 9, 2020 Report Share Posted February 9, 2020 That last photo of my 2 gauges are of the square type, your 85 has round gauges so you need to look at yours, the round instrument cluster I have has the clip on type as you can see on my last post, it is also white plastic like yours likely is. 1 Quote Link to comment
Seven Two Oh No Posted February 15, 2020 Report Share Posted February 15, 2020 The speedo in my 85 would occasionally exhibit similar symptoms - would read correctly at lower speeds but fluctuate 10-15 mph higher at cruising speed. Changed the cable and the issue stopped. To Mike's point, I'm not sure why this was happening as the cable simply rotates based on trans output, but new cables are under $10 on Rockauto so worth ruling that out before hunting for a new gauge. 1 Quote Link to comment
datzenmike Posted February 16, 2020 Report Share Posted February 16, 2020 A jumping speedometer is easily explained. The cable is really a long tightly wound coil of wire. It can rub the outer sheath and snag momentarily. The cable will continue to turn storing up torsional energy. When it releases the cable has to speed up the slower section ahead of it and the needle jumps. Lubricating it can help but for $10 it's easy to replace. 1 Quote Link to comment
Draker Posted February 16, 2020 Report Share Posted February 16, 2020 I had this issue.. I lived it up. graphite will help if it’s not damaged. 1 Quote Link to comment
canon3383 Posted February 17, 2020 Report Share Posted February 17, 2020 Mine is 20 mph off at any speed, I might when it gets warmer remove the clear cover and take the needle and put it in the right position, but I know how to read it and confirmed the inaccuracy with my GPS. 1 Quote Link to comment
85"720 Posted February 19, 2020 Author Report Share Posted February 19, 2020 On 2/17/2020 at 7:10 AM, canon3383 said: Mine is 20 mph off at any speed, I might when it gets warmer remove the clear cover and take the needle and put it in the right position, but I know how to read it and confirmed the inaccuracy with my GPS. I thouggt the same thing. I tried doing that and it made it worse. 😓 1 Quote Link to comment
Charlie69 Posted February 19, 2020 Report Share Posted February 19, 2020 A freyed inner speedo cable may do this. Quote Link to comment
85"720 Posted February 26, 2020 Author Report Share Posted February 26, 2020 Update. Swaped cluster and everything working good. Speedometer works good too 1 Quote Link to comment
datzenmike Posted February 26, 2020 Report Share Posted February 26, 2020 Always keep old parts. 2 Quote Link to comment
85"720 Posted February 26, 2020 Author Report Share Posted February 26, 2020 12 hours ago, datzenmike said: Always keep old parts. +1 learned that from my dad. 1 Quote Link to comment
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