Speedometer issues 86 300zx gauge dash

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bukschott
Posts: 4
Joined: Wed Sep 29, 2010 1:36 am
Car: 1986 300zx non turbo stick, 5speed

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Recently purchased 1986 300zx. Good shape, everything works but speedo.
When I take off the needle will peg and stay there. Found out from the dealer that there are two possible cables. Speedo cable and cruise cable. Cruise does not work. Cruise cable goes to ungodly expensive ($555.00) Cruise Module.
I hope this isn't the problem as I will just drive with the tach.
Any suggestions on where to start troubleshooting will be greatly appreciated.


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Perrenial Badass
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Joined: Wed Jun 04, 2008 3:32 pm
Car: 1973 240Z, 1986 300ZX, 1987 300ZX, 1984 Corvette
Location: Florida
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I'd put my money on the speedo cable having popped out of the back of the cluster.

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evildky
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Joined: Sat Jan 31, 2004 9:23 pm
Car: 71 240ZT, 87 300ZXN/A-T, 06 350Z GT, Tundra TRD RW
Location: Louisville, KY
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cruise cable? this is a new one to me, the cuise works on vac and simple electronic's, there is no cable

being an 86 IIRC you have a cable that leads to the engine bay where it hooks to a remote electronic sender (this is one of those items that changed a few times, early cars have a cable fomr the trans to the cluster, then I think 86 only had the remote sender then the 87-89 had an electronic sender and an electronic cluster

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AZ-ZBum
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Joined: Mon Aug 04, 2003 9:48 am
Car: 05 Murano, 84 Z31 AE, 88 Z31 Vert
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Perrenial Badass wrote:I'd put my money on the speedo cable having popped out of the back of the cluster.
Can I take that bet? Please? Please can I? How much money are you willing to wager? :biggrin:

And the real answer is....

Transmission fluid is in the cluster and causing a faulty signal.

1984-1986 Analog dash Problem:
Cars have a bad speed indicated (usually reading 100+ while doing 20mph).
Odometer works normally.
Fluid leaking onto floor from under dash (usually on the clutch or gas pedal).

1987-1989 Analog dash problem:
Cars have a bad speed indicated (0mph or way high).

1984-1989 digital dash problem:
Cars have a bad speed indicated (0mph or way high) BUT STILL READING A NUMBER.

Reason:
What's happening is the seals in the pinion gear assembly are bad, so the transmission fluid seeps up the cable and into the cluster. The speed sensor is contaminated, so it sends out a false reading. That's why on the 84-86 analogs, the odometer works correctly, but the speedometer needle swings around like it's a** is on fire.

Briefly:
The only thing you have to replace is the two oil seals.
Everything else is just cleaning and reassembly.

Solution:
- Get the car up in the air.
- Find the speedometer pinion gear assembly at the right rear of the transmission.
- Remove speedometer cable from speedometer pinion gear assembly (if applicable).
- Replace BOTH oil seals (one internal, one external) of the pinion gear assembly.
- 84-86 analog only:
- - Drain speedometer cable of excess transmission fluid.
- - Remove instrument cluster (84-86 analog only).
- - Clean out transmission fluid from cluster.
- - Clean all fluid from all under dash components (steering rack, ducts, etc)
- 84-89 digital:
- - Drain speedometer cable of excess transmission fluid.
- - Clean out transmission fluid from speedometer sensor.
- 87-89 analog:
- - Clean out transmission fluid from speedometer sensor (attached to pinion gear assembly).

bukschott
Posts: 4
Joined: Wed Sep 29, 2010 1:36 am
Car: 1986 300zx non turbo stick, 5speed

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Thanks for the help guys. I'll check into it this weekend and get back to you so you can settle that bet.

bukschott
Posts: 4
Joined: Wed Sep 29, 2010 1:36 am
Car: 1986 300zx non turbo stick, 5speed

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Well fellas. I still haven't gotten to that speedo issue. It may be a little while before you can settle that bet. On Oct. 10 my son was riding with a friend and they were in a serious car wreck. He's been in the hospital since and my wife and I have been there with him. I do have some time to work on the car. He has a brain injury and will probably be out from behind the wheel for at least six months. Good news is, the docs expect him to make full recovery! Tough thing for a kid to go through in his senior year but day-by-day with lots of prayer and patience, he is getting better!

blanketsmoke
Posts: 1
Joined: Sun Jan 11, 2015 10:48 pm
Car: 300zx Z31 83
Cefiro A31 88

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Thanks AZ-ZBUM for the information on the transmission oil on the dash. I slowly watched the deterioration of my gauges over a number of months. I have taken the dash out and cleaned the oil off the back of the dash and throughout the dash. Will I need to drain the gearbox before removing the pinion gear and where do you suggest I get new seals?

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AZ-ZBum
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Joined: Mon Aug 04, 2003 9:48 am
Car: 05 Murano, 84 Z31 AE, 88 Z31 Vert
Location: Phoenix, Arizona
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No!!!! You don't need to replace the transmission oil for this job, but if it's really old oil, you may want to do it anyway
.

linkovich
Posts: 3
Joined: Sat Mar 07, 2015 4:22 pm

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I found this thread trying to figure out my speedometer problem and see it's still recently active, so I'm going to ask here to add to the discussion.

I have an 86 with digital dash, and having the same problem, sometimes reads 0mph and sometimes not. I've done what was described here to no avail so far.

It started by the speedometer reading 0mph about 40% of the time and would read and not read at random times while driving or between driving. No pattern. Odometer doesn't read when speedo doesn't read either. I took out the speedo cable and sure enough tranny fluid all over it and built up. Cleaned the end of the cable and put it back in. It didn't read 0mph for about a week, then went back to like 20% of the time. I took it in to my z specialist mechanic to fix something else and mentioned it to him to just look at and give a recommendation. He said he would bet it's the cable broken. Not sure what he did, looked like he just took it out and looked at it (said tranny fluid buildup) but again another week of perfect readings then back to normal. Then I removed and cleaned the entire cable, which had fluid all the way back to the sensor, and cleaned the pinion. The seal looked really clean and didn't have one to replace it with, so didn't. I only see ONE seal, where is the other one??? Put it all back together and it read 0mph ALWAYS!! Bought a used cable from 300zxpartsforyou.com (only place I could find one!), which looked in way worse shape than mine, installed it and still 0mph always. This time when I took out the cable, no tranny fluid buildup, very minor fluid at all.

So....what now? One question is are you supposed to lube the cable? If so, with what? Also....Could that be causing it or probably not and something else is wrong? Can I clean the sensor, could that cause problems? Where's the other seal in the pinion?

Next step I'm thinking is either clean and/or replace the sensor? The pinion looks like it's in great shape so I doubt it's that, but I will buy seals for it.

Thanks!

linkovich
Posts: 3
Joined: Sat Mar 07, 2015 4:22 pm

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I found where to buy new seals and it looks like there is only 1 seal for the 86?? Multiple places only had one seal and listed the years of the 2nd seal (inner seal) which started with 87s. Here's what the seal should look like for z31's (but maybe not for an 86?)..
http://www.thumper300zx.com/fixes/speed ... leseal.htm

Seadoorider68
Posts: 2
Joined: Sat Feb 10, 2018 9:36 am

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Where did you find the replacement seals? I have been looking everywhere for the last few months.

I have a 1987 Z31 with Digital Dash and i see fluid on the carpet under the cluster. I am sure it is the seals.

Thanks for the help.


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