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MG MGB Technical - Help? Electrical/transmission

I recently purchased a '76 mgb roadster and I'm looking for tips you might have about the following.

First, the turn signals and fuel gauge aren't working. Everything else electrically works fine. I've checked the fuses and they all look good. Any thoughts?

Secondly, the vehicle shifts fine going from 1st to 4th. But.. if I try to dowshift the gears grind. Am I doing something wrong? The lady I purchased the vehicle from was elderly and she did mention that I needed to "double clutch". Any tips here?

thank you!!!!

Marc Welker
Canby, OR
USA
marc welker

Marc,

check all connections under the dash. Check that the ground wires (black) are secure. Perhaps you need a new turn signal flasher unit. Thats the little gizmo under the passenger side dash that makes the clicking sound. Maybe the bulbs are burnt out. I believe if one of the bulbs are inop then the flashers may not work. As far as the gauge, the same applies. Check connections both at the gauge and at the sender unit at the tank. Sign on as a member (free) and check the archives for more on electrical issues. There's test you can run to determine specific faults or stay tuned here, always lots of help.

Double clutching grandma huh? First thing to verify is that the hydraulics are working but more than likely is a worn syncro issue which requires removal of the engine and gearbox for a rebuild. I'd live with it till then or learn to double clutch. You need not do a proper double clutch. When you want to downshift, depress the clutch, rev the engine with the clutch still depressed(if you rev it twice-zoom zoom, people will think you know what you're doing!)and with the higher rpm's, downshift and release clutch pedal.

Hope this helps

Paul
Paul Hanley

Marc - The first thing I would do regarding the transmission and the "need" to double clutch, is to change the transmission lubricant. When you drain it, smell it to see if it has a strong sulpher smell. If so, someone put 90W gear lube in it. Refill the transmission with the same oil you are using in the engine, drive the car for awhile and then drain the transmission and refill again using engine oil. The 76 transmission has syncromesh on all 4 gears, so there should be no need to double clutch, even to go into 1st gear while moving forward. If the correct oil does not correct the situation, then there is a more deep seated problem that is beyond my expertise.

When checking fuses, "looks good" is a good way to get led down the garden path. A fues can open at the end, under the metal end pieces and still look good. The only way to check a fuse is with an ohmeter, a multimeter set to the resistance scale or by substitution of a known good fuse.

Turn signals are not on the same fuse as fuel gauge. The turn signals get their power from the fuse with the purple wires on it. Does the curtesy light work - it is on the same fuse. Try the hazard lights - if they work then the "purple" fues is good. The hazard light switch itself could be the culprit, not being turned off firmly.

Fuel gauge - does the tach work? If so, then all is well with the "green" fuse (the fuse with the green wires on it). Does the temperature gauge work? If so, then the voltage stabalizer is good. This leaves you with the ground at the sending unit or the sending unit is bad.
Good luck - Dave

David DuBois

Sage advise David. Forgot about the oil in the gearbox!
Paul Hanley

Marc. All that David says. The only thing I would add is that I use a test light, a simple probe with a light in the end and a ground wire, for checking to see if there is voltage present. You can pick these up for as little as $2 each (and I picked up one for each car's tool box). Very easy to use. Turn on the current to the circuit, then ground the test light's ground wire to a good surface and touch the probe to various connectors to see if the light illuminates. If so, you have current flowing. A mulit-meter is better for through diagnostic use, but the test light is great for "quick and dirty" checks, particularly on the road. It is also excellent for testing fuses. Check the front of the fuse (power input), then the back of the fuse (power output), then the metal tab for that fuse at the back of the fuse box. In about three seconds you can tell if you have power into the fuse, through the fuse and if the metal holder is making good contact to allow the power to flow. Les
Les Bengtson

If all gears grind on a downward shift it is highly unlikely to be a gearbox problem.
Check to ensure that the clutch is fully disengaging by selecting reverse from idle and with car stationary.
If it is selected silently then fine but if it grinds that indicates that the gearbox is still turning and therefore the clutch not fully disengaging. This could be caused by a fault in the clutch master cylinder or lack of stroke due to the need to bleed system. Otherwise it's engine out time for a new clutch.

Get the prayer mat out!!
Iain MacKintosh

IME clutch disengagement problems will crunch in reverse but not in forward gears with working synchro.

a 76 North American spec car should have electric temp gauge as well as fuel gauge, so if the temp guage is working OK you can forget the instrument voltage stabiliser. Turn the ignition on, get a good ground from somewhere, and connect it to the green/black wire on the tank sender (right rear corner). Just connect it for long enough to see if the fuel guage starts rising smartly towards F. If so, then connect the ground to the body of the sender. If the gauge now registers a fuel level (assuming there is more than a gallon or so in the tank) then the tank grounding is at fault. Your car probably has the one-piece sender and fuel outlet pipe and no wired ground, it relies on various physical fixings which can rust. If grounding the green/black moved the gauge but grounding the sender body didn't then the sender is broken. If grounding the green/black didn't move the gauge then try grounding that wire where the rear loom joins the main loom near the fusebox, and again at the back of the gauge. If still no go check for 12v switching on and off about once per second on the light-green/green at the back of the gauge, if so the gauge is faulty.

There are no ground wires involved in the fuel gauge circuit, and in the turn flasher circuit only to the headlamps.

The turn signals *do* get their power from the same fuse as the fuel gauge, but so do the reverse lights, stop lights, tach, washers, wipers, heater fan etc., so if most of these work it isn't the fuse. This is the green circuit i.e. fused ignition, the purple circuit is always hot but fused and feeds things like the horns and courtesy lights. The hazards don't get their power from the purple circuit but from their own in-line fuse off the brown (always hot unfused) circuit.

If the turn signals light but don't flash check if one of the bulbs is out on the appropriate side. If both are lit either the flasher unit is faulty or there is low voltage/bad connections in the circuit. If they don't even light flick the hazard flasher switch on and off a few times and see if that helps. The turn flasher power supply goes through the hazard switch so they are disconnected when the hazards are on, this switch is rarely used and the contacts can gum up.
Paul Hunt

This thread was discussed on 12/01/2004

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