Intermitent Voltage Drop - '88 Mercury 135HP

[Zteven]

Here's my problem:

Voltage at the control box drops intermitenttly from the full 12V to ~5V. This is a problem because the control box does not have enough power to activate the starting solenoid when turning the key to crank and start the engine.

If I hotwire the engine (key in on position - manually bridge the starter solenoid with jumper cables) it starts and runs, but the control box still reads at 5V.

This does not always happen though; When the boat is on the trailer or when I put it in the water for the first time the control box has 12V and starts the engine up - no problems.

Here's what I know:

• Battery reads 12V at the terminals
• Battery Cables read 12V at the starter solenoid (this reading is consistent with the control box when there is the full 12V at the control box and when there is only 5V at the control box)
• Rectifier output is 3.6V when in neutral (800RMP) - Control box reads ~5V (wasn't able to get a reading when the control box had 12V)

I cannot seem to replicate this problem on the trailer - the control box always reads 12V.

Is this a symptom of a bad switch box? rectifier? or is the problem in my control box?

Thanks for any responses.

 

[Texasmark]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Man, I don't know where to start to help you on this but this is a place to start and won't hurt anything. Start at your battery terminals, both + and -. remove the lugs and clean them and the terminals on the wiring. A little WD 40 won't hurt before you reassy. Reassy and get everything nice and tight. Then go all over your wiring, internal to the engine, and external, wherever you have an exposed terminal (not one in a rubber sealed plastic housing), and do the same; I mean all over engine, dash board, trolling motor, wherever.

Once finished, look at the problem again and get back with us.

You may return with something like......hey guys the problem disappeared. Whadda deal.

Mark

 

[CharlieB]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Most likely a poor connection. On the starter solenoid battery connection there is a red wire that leads to the fuse, make sure the fuse connectors are clean. Then check the sockets and pins in the large Cannon plug that connects the engine wiring harness to the control wiring harness. The pins can be enlarged using an exacto-knife blade, carefully slide the blade down into the slot on each pin to 'fatten' each one just a small amount, enough that they snuggle fit and make a good connection.

WARNING - Disconnect the battery cables BEFORE attempting to widen and of the pins, it is too easy to cause a short by touching two pins with the blade.

 

[Zteven]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Thanks for the reply's. I've already gone through all the wiring. I found that the 20A fuse for constant 12V to the control box was frayed on one end of the boot - only 3 copper strands left making contact at all times. I disassembled, cut out oxidized wire, and soldered back in place. Thought that was the problem / solve, but when I took the boat out last night the gauges dropped to 5V as soon as I left the ramp.

Looking through the wiring diagrams my thought is either poor connection somewhere (that means I'll go through everything again to double check), bad switch box on the engine, or bad key module on the control box.

If anyone has other ideas about where to look, feedback is much appreciated! Thanks again guys.

 

[Texasmark]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Thanks for the reply's. I've already gone through all the wiring. I found that the 20A fuse for constant 12V to the control box was frayed on one end of the boot - only 3 copper strands left making contact at all times. I disassembled, cut out oxidized wire, and soldered back in place. Thought that was the problem / solve, but when I took the boat out last night the gauges dropped to 5V as soon as I left the ramp.

Looking through the wiring diagrams my thought is either poor connection somewhere (that means I'll go through everything again to double check), bad switch box on the engine, or bad key module on the control box.

If anyone has other ideas about where to look, feedback is much appreciated! Thanks again guys.

Go thru the wiring as suggested. Your problem really smells of a bad ground. Your tests make it seem to be low voltage, but a bad ground can just as easily make you think the problem is power (+12v) related. I spent my career in electric/electronic circuits and components and worked with a wide variety of awesome machines. Will not bore you with facts, but your problem smells of a bad connection, as said, a bad ground (reference) connection.

Mark

 

[CharlieB]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

It is NOT the ignition switchbox on the motor, the ignition is a totally separate electrical system, the ignition key switch is actually two separate switches within the same housing and simply grounds the black w/yellow to the solid black ground to kill the ignition.

You still have a poor contact within the 12 volt system somewhere between the battery cable at the starter solenoid and your volt meter.

It is possible to have a poor connection within the ignition switch, test the red wire comin into the ignition switch when the condition occurs, if you see 12 V here yet only 5 V on the boats gauge try wiggling the ignition switch, if it changes that should confirm the switch is going bad.

 

[Zteven]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Over the weekend I went through the wiring on the engine and couldn't really find anything. Next I checked the wiring on the control box.

When testing with a voltmeter I found that the keyed 12V (purple) lead going to gauges (voltmeter, tach, temp), read 12V when disconnected and 5V when connected. This, however, was still only intermittent.

I started the engine on the hose with purple wire disconnected (couldn't get started while connected). The engine started up and ran fine. Plugged purple in, still ran fine. Tested the purple lead again while connected, but now it was back to 12V. I ran the engine for a bit, connecting and disconnecting purple a few times, but couldn't get it to read 5V again. At some point during this process the tach stopped working. Voltage and temp still worked.

I tested grey at the tachometer for Volts and got readings of ~.65 at idle (~800RPM) and ~.70 in fast idle (what I would guess to be 1100 RPM or so). The power terminal on the tach was reading 12V at this point. The tach still would not work.

I eventually got the boat out on the water and the tach started working again until it cut out completely around 2000RPM. I could not replicate the 5V problem again and now the tach does not seem to work.

As a guess, I am thinking that the tach is going bad and causing a short in the system. When I get time my next test will be to pull the tach from the dash and wire it directly to the engine to see if it will work. Are there any other tests I can run on the tach to see if it is going bad?

Any additional insight would be appreciated.

Edit: I tested Red on one switchbox when the 5V issue was happening. It consistently read 12V even when wiggling the wire.

 

[phebus1]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Sounds like the tach is the issue. I have had them go bad before and ground the system out.

 

[CharlieB]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Edit: I tested Red on one switchbox when the 5V issue was happening. It consistently read 12V even when wiggling the wire.

Grey is the tach trigger which comes off the rectifier. Double check that you have a good ground to the tach, otherwise it looks like your tach is about finished.

Curious what 'Red' wire you found on the switchbox with 12 V as this switchbox ignition system is not battery driven and is totally seperate from the battey system with the exception that the ignition switch itself is two totally seperate switches sharing the same housing.

The stator does have s red and blue wire to each switchbox but voltages here are AC and vary with engine RPM. With the key ON but the motor NOT running, there should be NO VOLTAGE on ANY wire to a switchbox, Unless this motor is so old as to have a distributor.

 

[Zteven]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Sorry, meant red to the controlbox from the engine. Reading back, I probably did that test wrong anyway as I tested at the engine, not the control box.

If the problem comes up again I'll disassemble the control box and do the test properly. Hopefully that doesn't happen though.

Are there any potential complications in hooking up a bad tach to the rectifier? IE will it damage the rectifier?

 

[Zteven]

Re: Intermitent Voltage Drop - '88 Mercury 135HP

Had the motor on the hose again today to test the tach.

I connected to the tach signal wire to the yellow stator wire that did not already have the grey tach signal going to the control box. When hooked up to this wire the tach worked. Moving the signal wire to the other stator yellow resulted in the tach not working.

I tested the rectifier and it shows that it is bad (continuity from yellow stator lead to rectified +12V and vice versa). The catch is, that the bad terminal is the only terminal that gives the tach signal.

Through this process I scoured the engine a third time and noticed that the battery negative was not hooked up to the correct ground point as noted in the service manual. This has been changed to the correct location, but now I have a bad rectifier.

Two questions:
1. Do you think the rectifier was damaged as a result of the ground cable being hooked up to a different ground point than indicated in the manual?
2. Can I take the boat out to the lake for a day trip before I get a new rectifier without worrying about getting stuck out on the water? (note: I have a dual battery system and a jumper box in case the starter battery does not get charged enough - just want to know if a bad rectifier can damage something else or prevent a start on its own).