WICruiser

My 85 has had speedo issues for a while, basically takes a number of miles to begin working and then is sluggish (both going up and down) for a while before it settles in to working. Once fully "warmed up" the speedo seems to work OK but reads slightly low. Seems to be worse in colder ambient temperatures and after sitting longer periods of time.

Last summer the odometer quit working. It was right after I reset the trip odometer so I assumed a bad gear (although gears were changed not to long ago).

Ran diagnostics on the speedo circuit and voltage at either the connector in the spare tire area or at the fuse panel only reads 8 volts. The voltage does switch as the rear tire is rotated, just never gets up to battery voltage.

Pulled the cluster and the odometer gears all look good, and rotate freely when the odometer drive motor is removed.

Checked voltage at the various terminals of the edge connectors that feed the instruments and find 12 volts at all of them.

Grounds for the instrument cluster have been cleaned.

Previously I replaced the instrument cluster film circuit and confirmed continuity between the 5 different rivets that tie into the speedo pins and the 2 screws that tie into the trip reset and everything seems to check out good.

Any ideas of additional tests or diagnostics I should run? I was thinking I may send the speedo out for testing but not sure if that is a good idea and if so where the best place to send it would be. Any thoughts or recommendations appreciated.

ROG100

Roger,
Exactly the same on one of my 82 cars. Tried (Sean tried) most of the normal tests. Seems to happen in colder weather or when sat for a period of time.
I think it is speedorheumatitis - I will remove it and send it to PaloAlto for a refresh.
Roger

SeanR

This is odd. Rog and I were just talking about this very issue over the phone a few minutes ago.

WICruiser

Thanks - I will check into PaloAlto.

Any other thoughts greatly appreciated.

WICruiser

I should have probably stated my tach is also sluggish. No idea if the sluggish problems are related but they both seemed to appear about the same time. However, the odometer problem was more recent and abrupt leading to the gear failure expectation but no gear failure found.

Ian S4

I have the datasheet for the device used in the speedo/odometer, but it's 600kb so I can't upload it. If you want it, please PM me and I'll e-mail it to you.

I suggest measuring the value of the resistor shown in the top-right of Fig 1 in the datasheet. It should be about 33 ohms. A while ago my own speedo/odometer died, and the culprit was a shorted capacitor (top-right again) which fried the resistor. I replaced both.

WICruiser

I sent the cluster out an Palo Alto confirmed that the speedo and tach function correctly. They provided confirmation of the contact points for battery voltage, speedometer input, etc. so I am back to trouble shooting the car.

I have re-confirmed the there is 12 volts to the correct terminal on the connector, and ground terminal on the connector has good continuity to ground.

Any ideas would be greatly appreciated. I don't like the thought of putting the cluster back again only to have the same problem continue.

WallyP

Put 12 volts and ground on it and use an accurate voltmeter to check for voltage drop across each connection. Don't depend upon continuity to find the problem.

928mac

think of it this way.
if all the little tiny wire hairs were broken except 1, you would still be showing 12volts (BTW it should be battery voltage, or more then 12)
But with all those broken wires you would not get enough amperage (flow) to light a large bulb.

I think you should look harder for a poor connection. Its hard to find sometimes, and I know that poor dash connections can suck.

WICruiser

OK, thanks for the input. I will check all of the connections along the path between the cluster and the sensor as well as the cluster voltage supply and ground paths

WICruiser

OK - finally got back to this over the weekend.

I confirmed full battery voltage (12.4 v) at the connections on the top of the relay/fuse panel but I only see 11.7 v at best on the feed side of the instrument cluster fuse, and I am down to 10.4 at the baterry voltage connector for the instrument cluster. Bottom line is I think I need to pull my panel and give a good cleaning.

I tried removing a few of the wiring connectors (A, B, C, etc.) along the bottom of the panel and they seem to release from the top but not the bottom. Is there a trick to getting the connectors to release, or are they just tight from being installed for 25 years?

I seen several posts regarding cleaning the panel connections and will do some searching but if anyone has a preferred method please point me in the right direction.

WICruiser

Update - I have removed and cleaned the CE Panel. A little scary but once you have done it is not so bad. The connections really looked fairly decent. I was tempted to try and remove the rear wiring connections to clean those but they are layered such that you would really need to disconnect them all and given the general appearance it did not seem worth the potential problems.

I used a write up I found by search that describes the sensor circuit but modified the terminal numbers to match up with my '85 wiring diagram (and car - which was a nice plus). I can trace the circuit from the cluster connection to the spare tire well and back without finding any glaring problems. So I deciede the take the cluster out of the equation by jumping the voltage supply to the speedo (B13) to the sensor feed (B14).

With the jumper in place I get battery voltage at the spare tire connector. If I leave everything else the same, but plug int he cluster I get 8 volts at the spare tire connector.

Palo-Alto says the cluster tests fine but it sure seems to be the source of the problem.

The cluster curcuit foil is new and the traces from the edge connector to the rivets for the speedo all check good. I have also tightened up the terminals in the edge connector.

Any idea how best to check the connections internal to the cluster, e.g. between the pins on the foil and the meter itself? I am about out of ideas and could use some insight before I just give up and put it back together. The weather has been so nice I would really rather be driving it than working on it.

IrishLegend

My odometer quit working yesterday. :-(

WICruiser

If only the odometer quits it is typically a gear problem. Not all that unussual or hard to fix, just time consuming to get that deep into the instrument cluster.

westija

Not sure but the speedo issue sounds similar to this issue described in Pirtle's (lubrication). Cold temp would surely make it worse, I think. Hope this helps


From: Daryl Reece [dsr14@Cornell.edu]
Sent: Monday, July 29, 2002 9:21 PM
To: 928
Subject: [928] Re: Speedometer sticks until I hit 40 mph

> -----Original Message-----
> From: Constantine [mailto:dicongo@earthlink.net]
> Sent: 29 July 2002 11:47
> To: 928
> Subject: [928] Re: Speedometer sticks until I hit 40 mph
>
>
> Hi Carlos,
>
> The "Fonzie" manuever might be fun to do but can also jar something else
> and/or cause a fire in the pod. There have been instances of people
> messing with their dimmer wheel under the pod and starting a fire due to
> old brittle wiring sparking. I had my instrument cluster tested and my
> odometer fixed by a concern in Cali for about $120.00. Chump change when
> compared to car fire damage repair bills.
>
> Best to you,
> Constantine
>
> > My 86.5 does the same thing. The other day, my mechanic was
> test driving
> > it, me as passenger. Demonstrated a temporary fix: whack the
> pod on the
> > right side. Wakes the needle up every time!
> >
> > He said the gauge is just showing its age and could be rebuilt, if I
> > wanted to bother. Personally, I like the "Fonzie" manuever!
> >
> > Carlos Hernandez
> > 86.5 928 S 5-Speed, 170K Miles
> > Gran Prix White / Black Leather
>
>-----Original Message-----
>From: Jon Holdsworth [mailto:jon.holdsworth1@virgin.net]
>Sent: Monday, July 29, 2002 4:30 PM
>To: 928
>Subject: [928] Re: Speedometer sticks until I hit 40 mph
>
>
>My 88 SE does that too sometimes. I always put it down to the sensor in the
>transmission dying. (Like the ABS sensors that I think have died causing ABS
>warnings)
>
>Cant see that failed sensors would be cured by hitting the dash though.
>Strangely my other car (GM) has the same speedo problem but its a mechanical
>speedo with a cable. Sounds like it is the mechanicals in the instrument
>pod. On air reasoning !
>
>Jon
>Black SE
>
>
I just tore the speedo apart to fix the odometer so I had a decent look at
what's inside mine. I think this sticking behavior has to be lack of
lubrication in the speedometer's needle bearing. There is a coil behind the
speedometer that generates a magnetic field, which in turn generates a
torque proportional to the current traveling through the gauge coil. When
you get to 40, there must be enough torque to overcome the friction in the
bearing and pop off the bottom. I oiled my bearings with some light weight
oil (3 in 1 or sewing machine oil are adequate). The speedo bearing
lubrication would dry out over 20 years, just like any other lubricated
bearing. This stickiness would be jarred loose by the whack on the side.