My tach is suddenly very unstable. Will sometimes peg at nearly 7000 rpm when I downshift then settle down to the correct RPM but bounce around 100 rpm or so.

Really strange thing is that it only happens with the headlights on. Turn them off and all is good. Anyone have an idea where I should start trouleshooting?

THanks

Craig
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Craig 73ES

Took the fuse box out and cleaned and gooped all contacts no change on the tach. One thing I noticed..when I turn the ignition key to the position theta runs the fuel pump the tach needle jumps once and drops back down,

Is the tach getting a pulse thru that circuit somehow? Could there be an ignition switch component to my troubles?

Still hunting!

Craig
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Craig 73ES

Quite by accident (again!) I discovered that my Tach gets jumpier when the radio interference capacitor in disconnected from the coil. Reconnect it and the tach quiets down although still a bit unstable.



My question is...can I add something that may have even more cushioning capability and keep the tach from jumping. A better capacitor? Some other device that will cushion the pulses from the coil?

THanks

Craig

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Craig 73ES

I bought a new radio suppression capacitor and decided to just add it as an extra one and not remove the original. With both of them in there tach is now very quiet. Possible my main trouble was internal to the tach..just age..and I have externally dampened the signal. At any rate I will continue to trouble shoot electrically as I believe the Swedish Embassy trouble shooting article is on the mark. Next thing will be to replace the voltage stabilizer with new electronic unit and see if that helps.

Thanks to all for the help

Craig
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Craig 73ES

Just as an update mounting 2 new radio noise suppression capacitors at the coil replacing the old original one has kept my tach stable and steady for months now.The effectiveness was verified when one of my new connector crimps pulled out and tach went crazy! I know its just a workaround but as a band aid it has worked great...very cheap which always gets me excited!




Have fun!

Craig
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Craig 73ES

Craig;

Congratulations...there's not much arguing with success...but I would love to have that car and an oscilloscope with which to take some waveforms and get a full understanding of the way adding these filter capacitors have corrected the issue. There is a good learning opportunity here!

I suggest, in general terms, adding those caps has quieted down the powerbus enough so that filtering inside tach (which is likely decreased due to old electrolytic capacitor drying up) is now adequate to keep the tach calmed down.

..."radio noise suppression capacitors at the coil replacing the old original one"...exactly what kind of capacitors did you use and where did you connect the two additional caps? Please give details!

Cheers

Used standard radio suppression units bought on Amazon.com ( not vehicle specific). Connection was identical to the original except I used qty 2...the bracket on the capacitor bolted on with the coil attachment bolt for ground and the wire from each unit attached to the coil positive connection.

I would also like to have the car scoped but without you standing there I would have no idea what to look for!

Have fun

Craig

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Craig 73ES

Craig;

Understood...so you essentially cleaned up the positive powerline to Ignition Coil as I suspected, which probably cleans up the entire EMI/RF spectrum of the powerbus. Without seeing it specifically, I expect it also served to clean up the power going into the tach which wasn't so good at filtering internally any more...I'll call it settling down exited tach operation by external filtering. I still suspect a higher than normal resistance in the power path to tach somewhere along the line which allows a higher than typical noise-floor (Reference: http://www.sw-em.com/electical_circuit_interaction.htm )...so if symptoms return in the future, we will need to revisit the problem and make some precision voltage measurements under load.

Again, this is somewhat of a bandaid fix and I expect it doesn't really address the root cause, but a fix is a fix!

Well done, and we've all learned something!

Check your headlight grounds?

If them grounds are in the buckets - in front fenders in later Volvos - try running a temporary wire to see if that works - much easier than cleaning everything.

If it is the grounds, clean all you can find.

Here's an update to the "jumpy tach" troubleshooting. I decided to isolate the tach from everything else so I ran seperate ground and power to it from the same 5 amp fuse. The brake warning light, voltage stabilizer and charging light that were on the same fuse as the tach and quite by accident I did not put the connector for those components back on the fuse box connection after I connected the tach.

Went out for a drive and found that the tach was solid as a rock at all rpms. Noticed I had no gas or oil temp gauge (the connection I forgot to put back) and strangely ( for my car anyway!), the charging light was on. Stopped to fix that problem and applied the parking brake and even w/o power the warning light glowed very weakly. Reconnected the forgotten wire to the fuse box and the tach went crazy again but the charging light went out and brake lamp lit up to full brightness when I applied the brake at next stop.

Sounds like I have some type of voltage feedback from the charging light that has been disrupting the tach. I may try a diode in the line between the tach and the charging light so power can only go one way.



Any comments on the feedback diagnosis and/or the diode idea?


Thanks

Craig
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Craig 73ES

Craig;

This is an interesting electronic problem...and all of that is good symptom info, but not nearly enough for me to give specific help...I will try to help with what we have sofar...let's start with generals and basics...the fact that tach acts normally when those other loads are disconnected (and not drawing current) suggests that their current is influencing the tach...that, I'm fairly certain of from the symptoms...this can be due to two things...(again, only in general terms):

1. Poor connection at power side (due to resistance at FuseBlock (connections, staked terminals, or fuse conical ends. Reference: http://www.sw-em.com/gastight.htm ,because ES FB construction is similar to that of 122) resulting in a current dependent voltage drop on power line of tach, which tach is interpreting as an ignition pulse input...normally tach ONLY gets its pulses on its sensing line, but under this unusual condition, it is also getting pulses on its power line and these are not adequately filtered so they tach reacts to them and they cause deflection to sporadic action. OR

2. Poor connection at low (chassis return) side (bad ground on one of those loads) which effectively connects loads which normally don't have anything to do with each other...this can add pulses onto the ground connection of tach... also a condition which could easily result in tach acting strangely...

You will notice that the common element to both is a poor connection, so you might just want to equip yourself with some emery cloth and ACZP (Ref: http://www.sw-em.com/anti_corrosive_paste.htm ) and inspect, clean to shiny metal and reconnect with a film of ACZP, every connection having the slightest thing to do with tach, brake failure light, voltage stab, etc.

Alternately, if you have access to an oscilloscope, monitor tach power quality including ground as you disconnect other loads until it clears up... load disconnected which results in clean power should be a big hint.

A sneak (unusual) path for current can also be explained by a poor connection, but adding a blocking diode will not cure it...again, the proper cure is to locate and correct what is most likely a poor connection...besides, the sneak path is only apparent during an abnormal condition of not having its (normal) power connected.

This is a good start...please report any additional findings with more specifics...I will try to help with more specific suggestions.

Good Hunting!

Great response Ron!I have cleaned all connections to the tach but have avoided the hard to get to ones (voltage stabilizer!).
I did isolate the tach with it's own power (Using the spare fuse location jumpered to ignition power)and ground but reconnecting the brake warning circuit to ANY fuse on the ignition side of the fuse box affected the tach. I did get slight relieve by using the farthest apart fuses possible. Pulses were less but still there when pushing the rpm.

Frame to engine ground strap is new and clean and battery ground is clean.

How does that"sneak power" thing happen?

Thanks for the solid help so far! I'll report back after further cleaning

Craig
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Craig 73ES

Craig;

Thanks for the kind words, but this is only a start...I hope to help you locate and remedy root cause and understand it well so that other ES owners (probably also E because harnessing is quite similar) can benefit from the info...what fails on one vehicle often fails on another, so learning from first occurrence is quite useful!

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Questions:

How did you "isolate the tach with it's own power"? Did you separate normal harness power connection to Tach, then run a separate power wire from Fuse to Tach...or how?

Using "farthest fuse" removed from F4 may help if it gives troubleshooting info but it doesn't really address the root cause...

You might describe in greater detail the faulty action of the Tach. At idle? high RPMs? With lights ON? ETC.

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Additional Thoughts:

I have studied this a bit more and I believe I understand a POSSIBLE failure mechanism. [Reference Marked-up Wiring Diagram at (use Ctrl + and - to vary zoom): http://www.sw-em.com/1800ES_Wiring_Diargam_Markup_for_Spurious_Tach_Apr_2014.jpg ]

Wiring Diagram shows two W-GN (White-Green) and one BL-Y (Blue-Yellow) wires departing from Fuse 4 load-side.

The daisy chain of particular interest (colored Red on diagram) is one of the W-GN wires...which runs off to (Item 97) Brake Warning Lamp, Tachometer (Item 82), then Voltage Stabilizer (Item 28), then continues on to various other Ignition Powered Loads. With this configuration, with the Tach located upstream of the Voltage Stabilizer (VS), the Tach would indeed be subjected to voltage step pulses on its power input, as the VS switched, and more so, if any in-line resistance (for instance due to less than perfect connections) increased the I2R voltage across the poor connection. Sensitivity to this could be exacerbated due to age of the electrolytic filter capacitors internal to the Tach. As these dry out with age, they lose capacitance and become less able to filter out power line fluctuations...Results: Tach sensitivity to power line noise increases to the point of spurious operation when step pulses occur on the line. So much for the theoretical...

To practically confirm it is the VS switching pulse which is disrupting the Tach, if possible, locate the VS (mounted on backside of Tach) and place a fingertip on it with engine running normally (yes, I know this might require some contortions if doable at all…welcome to 1800 ownership!). You MIGHT be able to sense when the internal element switches…if this coincides with the sporadic operation or maybe simply a single spastic blip of the Tach needle, there's a good chance my theoretical explanation hit the spot.

My recommendation therefore is to clean, check for snugness and treat with ACZP all connection upstream of the Tach supply, starting with all connections (both sides) of Fuse 4, also Fuse 4 itself.

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In general terms, sneak paths for power occur typically during an abnormal operating condition...when power is applied to a point through a path other than by which it is normally supplied...I'll see if I can illustrate a decent example...later...
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Good Hunting!

for isolated power to the tach I disconnected the w gn wire at the tach which is part of the daisey chain and ran a seperate line from that spade to a new 5 amp fuse I installed in the empty fuse holder f7. I then connected a jumper line to the new 5 amp fuse in F7 from an ignition power input at F5. The rest of the components in that circuit still get power from the joined wire that used to be connected to the tach so I also diconnected the w gn wire from F4 and reconnected to F2 just to complete the "isolation" of he tach. Seemed to help a bit but I will still get some instability and crazy depending on how rapidly I increase RPM

The tach acts up at higher rpm(over 3000) and will get more erratic when there is a rapid increase in rpm. All of this only happens when the lights get turned on. Parking lights start a small bounce and headlights increase the needle bounce. Emergency flashers (even w/o lights) will cause the tach to bounce also.

Cleaning will begin over the next few of days. Sounds like it's finally time for me to get the fuse box out and spring clean! Don't like disconnecting all that wire however.

Oh yeah..think I'll skip the finger on the stabilizer move, To much traffic to deal with at the speed I need to do the test!

THe hunt continues

Craig
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Craig 73ES

Mini update... I know you did not like my diode idea but I had to try it as a troubleshooting method only...Installed it between the brake warning light circuit and the F4 fuse leaving the tach rewired as before. Power can only go to the brake light circuit no feedback allowed!

Tach does not bounce at any rpm w/lights on. RPM does drop about 500 (only at 3000 rpm or greater) when lights are turned off and goes up 500 rpm when lights turned back on. Still seems to be something bothering the tach but cleaning fuse box contacts may help calm things down another notch.

Next I will try putting all connections back to original but still leave the diode on F4.

Craig
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Craig 73ES

Craig;

Thanks for update...I need to digest update info...and I have yet to understand how turning on the light plays with this...that's also telling us something, but I have yet to figure out what that is...

Again, diode should not be necessary if we locate and correct root cause.

Remember also ...diode will only conduct (allow current to flow) in the forward direction, and when it does, it will have about 1-1.5V drop (for a 1N4002 or equivalent power diode which I hope you're using) which is voltage not available to the load, but this shouldn't be a problem.

Good Hunting!

I'm with you on the diode. I just hated to see my tach take such a beating!

The diode I am using is a Painless Wiring part #80111 that comes with spade connectors on each end...made it easy. No idea what the specs are. Only know that it is sold by all the performance guys... probably for some add on equipment they sell. So far all components downstream of the diode are working fine.

Hope digesting this wierd challenge won't give you heartburn!!

Craig
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Craig 73ES

OK...nothing has seemed to improve my trouble except isolating the tach power source and using a temporary diode to prevent feedback from the emergency brake light circuit. Turning on the lights still causes the tach to become unstable but it is less jumpy with the diode in place. As a side note the idle also drops 100 -150 rpm when I turn the lights on.

I have been focusing on the fuse box output side but could there be some trouble with the input power from the ignition switch? I thought about trying to power the tach from the directly from the battery side of the fuse box so ignition switch is out of the picture temporarily but only as a troubleshooting measure. Not sure if that is wise....don't want to mess up my tach!

Any opinions on this course of action?

Thanks

Craig
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Craig 73ES

Craig;

Did you ever resolve this issue? Please let us know.

I did put together some notes on inadvertent interaction of circuits...see: http://www.sw-em.com/electical_circuit_interaction.htm

Cheers

Sorry I meant bounce around "1000rpm or so"
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Craig 73ES