4500 RPM Dash Indicator Light Stays On

[LSU Pinz]

I'm a new member and have an '81 712 AMB-S that I just got. It was in Jennings, LA if anyone is familiar with this 712.

When running/ driving the 4500 indicator light on the dash stays on all the time. When idling it kind of flutters, then with increased rpm while driving it stays on bright. I am just driving slowly around the property. Everything seems to be running fine and the batteries are charging. Could this be a 4500 module fault? Thanks

[Jimm391730]

The 4500 rpm modules are known to die, eventually. Most just disconnect them by removing the 6 pin plug and all that is needed is to connect the middle pin of one row with the middle pin of the other row. Without this jumper the coil will not get power and you won't have any spark. A common module failure is to reduce the power to the coil, resulting in weak spark and poor performance -- mine did this, but my 4500 light never came on.

Of course this means that you won't have any over rev protection and no 4500 rpm light if you remove the module; but yours isn't working correctly as it is.

[audiocontr]

I'd try the bypass Jimm recommended and see what happens. REALLY simple check

[stark311]

When it's cold, mine comes on just above idle, but behaves normally once it warms up. Same problem?

[krick3tt]

Just tonight the light comes on at about 1800 RPM then goes out. It is very cold, about 9 deg. Never had the issue before.
Also comes on intermittently after 2300 RPM in a random pattern which is really no pattern at all.

The vehicle is over 40 years old, I expect some issues. Seemed to make no difference in the performance in the pinz. Just a bit
disconcerting to have the flashing light.

Apparently a rebuilt one is $95 from SAV. I really don't want to just jump it so a replacement seems to be the route I will take.

Any advise?

[krick3tt]

Got a used module from SAV and after install everything was fine. Now the same issue has returned, although this time the RPM gage jumps from 2200 to over 6000 and then drops back but there is no increase in engine RPM. Light comes on at about 2200 RPM when accelerating through the gears.
I wonder if it is the module or another gremlin in the electrical system. I am going to try the bypass and see what happens, it seems to be the most recommended course of action.
Any other recommendations? As the vehicle has no other issues at this time except the starter does not seem to start the vehicle without using the choke. Often spins without engaging the flywheel. Hard start.

[Jimm391730]

So you have a tach? Getting the tach input from the coil negative terminal (-)? Since the tach is completely separate from the 4500rpm module, but the tach is affected, I'm thinking that your new module is iffy at best.

Everytime the points close, current flows through the coil primary (and this also makes the coil negative terminal drop to almost zero volts, referenced to chassis). When the points open, the current stops flowing (and the coil negative terminal jumps to a relatively high voltage). This changing current flow is detected by the rpm box. The box determines the frequency of the current pulses and if they happen too rapidly it is supposed to light the dash indicator; at some point the box starts to reduce power (maybe it was supposed to kill power) to the coil so the engine won't overspeed.

Of course the tach is detecting the changing voltage at the coil negative terminal. So if the tach is getting confused, it is pretty likely that this is because the voltage/pulses at the coil is not what it is supposed to be. If the points bounce then the current will start and stop more often, resulting in an artificially high tach reading and the box may be thinking the same; but I'd guess that the most likely culprit is still the rpm box as it is "old school" discrete transistors and other components on a PCB and is subject to component failure, soldering issues, contamination, etc. and it has a known history of causing problems and could also cause the same tach symptoms: If the box starts killing power, then quickly restores it, the coil terminal voltage will be effected and result in the increased tach reading. Try jumpering out the box and see if your tach responds as it should; if so it is definitely the rpm box.

[krick3tt]

Jim, thanks for your detailed and rapid response explanation. As the RPM module is a refurbished one from SAV, it may very well be at issue. No new ones were available at the time I ordered this one.
I am not going to invest any more money in this issue, as in getting another module to solve the problem. I will jump the plug and see if this will solve the issue.
It doesn't seem to be noticeable during the day, only at night with driving lights on and at regular road (city driving, not highway) speeds. Mostly during acceleration and not as the vehicle slows down. Only during shifting from lower to upper gears as rpm's change.
It may not be not a daytime issue or that I don't notice the flashing light in the daytime. Although it is rather bright red light. At night the only lights in the cab are the tach and the gas gage so that flash from the module light is very noticeable.

[Jimm391730]

And it may well be that the temperature difference between day and night affects the rpm module. Just another part of the problem.

As for your starter, I had similar issues until I replaced the Bendix in the starter (the one-way clutch). Once they start to slip there is not enough torque to turn the engine over; and as soon as they start to slip they slip really easily. So it sounds like the starter is just spinning (which it is) even though the gears are engaged.

[krick3tt]

I have purchased one of the newer starters that EI sells. All that remains is to install it and then decide if the old one is worth rebuilding or just selling outright.
Just not sure if the starter is as easy as it sounds...remove and replace. I will have to do a bit of research, although several on the site have done the procedure
and it seems that there were no real issues except maybe the rubber mount possibly needing replacement as well. Won't know till it's off.

[Jimm391730]

IIRC the upper nut is blocked by the solenoid. A 19mm crows foot will help you get around the blocking part. Or do as I did and take a relatively cheap 19mm box wrench, heat the handle about an inch from the box portion, and bend the handle almost 90 degrees. This worked for me but I ended up twisting the handle since it was softened by the torch that allowed the bend to be made. So I bought the crows foot after I fixed the starter. Of course I've never needed it since! But maybe that's a good thing!

[BobbyD]

I have had the new EI starter for a while now. It just rocks.